XLIX. THE SUBSIDENCES AND DISLOCATIONS OF THE PLATEAUX…

XLIX. THE SUBSIDENCES AND DISLOCATIONS OF THE PLATEAUX

There can be no doubt that considerable alterations of level have taken place over the volcanic areas of North-Western Europe since the eruptions that produced the basalt-plateaux, These alterations embrace general and local subsidences, and also dislocations by which considerable displacements of the crust either in a downward or upward direction have been effected.

i. SUBSIDENCES

The mere fact that in many places the lower members of the series of terrestrial lavas have been submerged under the sea may be taken to prove a subsidence since older Tertiary time. Along the west coast of Skye this depression is well shown by the almost entire concealment of the bottom of the plateau under the Atlantic. In the Faroe Isles the subsidence has advanced still further, for not a trace of the underlying platform on which the basalts rest remains above water. In Iceland, too, the complete submergence of the base of the Tertiary volcanic sheets points to a widespread subsidence of that region.

Another strong argument in favour of considerable depression may be derived from a comparison of the submarine topography with that of the tracts above sea-level. It is obvious that the same forms of contour which are conspicuous on the land are prolonged under the Atlantic. If we are correct in regarding the valleys as great lines of subærial erosion, their prolongations as fjords and submarine troughs must be considered as having had a similar origin. We can thus carry down the surface of erosion several hundred feet lower than the line along which it disappears under the waves.

I know no locality where this kind of reasoning is so impressively enforced upon the mind as the west end of the Scuir of Eigg. The old river-bed and its pitchstone terminate abruptly at the top of a great precipice. Assuredly they must once have continued much further westward, as well as the sheets of basalt that form the main part of the cliff. Yet the sea in front of this truncated face of rock rapidly deepens to fully 500 feet in some places. Had any such hollow existed in the volcanic period it would have been filled up by the long-continued outflowings of basalt. Making every allowance for concealed faults and local subsidences, we can only account for this submarine topography by regarding it as having been carved out, together with the topography of the land, at a time when the level of the latter was at least 500 feet higher than it is now.

The subsidence which is thus indicated along the whole of the North-West of Europe probably varied in amount from one region to another. We seem to have traces of such inequalities in the varying inclinations of different segments of the basalt-plateaux. The angles of inclination are almost always gentle, but they differ so much in direction from island to island, and even among the several districts of the same island, as to indicate that certain portions of the volcanic plain have sunk rather more than other portions.

Thus in the Faroe Islands, where the bare cliffs allow the varying angles of inclination to be easily determined, a general gentle dip of the basalts in a south-easterly direction has been noted among the central and northern islands by previous observers. This inclination, however, is replaced among the southern islands by an equally gentle dip towards the north-east. The centre of depression would thus seem to lie somewhere about Sandö and Skuö. The highest angle of inclination which I noticed anywhere was at Myggenaes, where the basalts dip E.S.E. at about 15°.

Among the Western Isles, also, where similar variations in the inclination of the basalt-sheets are observable, it might be possible by careful survey to ascertain the probable position of the areas of maximum depression, and to show to what extent differential movements have affected the originally nearly level volcanic floor. It would doubtless be found that everywhere the dominant movement has been one of subsidence. The vast outpourings of lava would tend to leave the overlying crust unsupported, and to cause it to sink into the cavities thus produced.

Perhaps the most extensive subsidence of this kind, at least that which admits of most satisfactory investigation, because it still remains above sea-level, is displayed by the vast hollow in the Antrim plateau, which embraces the basin of Lough Neagh and the valley of the Lower Bann. This depression measures about 60 miles in length by about 20 in breadth. Its axis follows the N.N.W. trend so characteristic of the volcanic features of Tertiary time. The depression may be said to involve the entire basaltic plateau of Antrim, for with the exception of a few insignificant areas along the borders, especially on the east side between Larne and Cushendall, the whole region slopes inward from its marginal line of escarpments, which reach heights of 1800 feet and upwards, towards the great hollow in its centre (see Map VII.).

Lough Neagh, which occupies the deepest part of this hollow, and covers about one-eighth of the whole area of subsidence, is the largest sheet of fresh water in the British Isles, for it exceeds 150 square miles in extent of surface. Yet, for its size, it is one of the shallowest of our lakes, its average depth being less than 40 feet. Its shallowness, compared with its wide area, marks it out in strong contrast to most of the larger British lakes. Its surface is only 48 feet above the level of the sea.

The origin of Lough Neagh, the theme of various legends, has been seriously discussed by different writers, but most exhaustively by the late E. T. Hardman of the Geological Survey.[434] This author connected the formation of the lake-basin with a series of large faults which are found intersecting the rocks around the basin, and passing under the water in a general north-easterly direction. He showed that these faults have produced serious displacements of the strata, amounting sometimes to as much as 2000 feet, and he believed that it was by the concurrent effect of such dislocations that the depression of Lough Neagh had been caused.

[Footnote 434: "On the Age and Formation of Lough Neagh," Journ. Roy. Geol. Soc. Ireland, vol. iv. (1875-76), p. 170; also Explanation of Sheet 35 of the Geol. Surv. Ireland (1877), p. 72.]

It is possible that these displacements may have contributed to at least the earlier stages in the history of the Antrim subsidence. They have undoubtedly taken place after the outpouring of the basalts, for these rocks are involved in their effects. But in the hollow of the Bann valley north of Lough Neagh the faults which have been detected in the basaltic plateau are few and trifling. The bold and bare escarpments, that so clearly display the relations of the rocks, reveal few traces of any important transverse dislocations. Nor has any proof of large longitudinal faults parallel with the axis of depression been obtained within the area of the Bann valley.

The earliest evidence for the existence of a lake on the site of the present Lough Neagh has been supposed to be furnished by certain fine clays, sands, seams of lignite and clay-ironstone, which have been referred to the Pliocene period. These deposits have been regarded as indicating the accumulation of fine sediment with drift vegetation brought down into a quiet lake by streams entering from the south. Their fresh-water origin was believed to be further corroborated by the occurrence of shells belonging to the lacustrine or fluviatile genus, Unio.[435]

[Footnote 435: These shells were regarded as forms of Unio by the late W. H. Baily; but Dr. Henry Woodward assigned them to Mytilus. See Prof. Hull's Physical Geology and Geography of Ireland, 2nd edit. p. 101. The shells have been more recently dug out by Mr. Clement Reid, who has found them to be the common Mytilus edulis.]

The thickness of this series of strata, their position above sea-level, and their distribution are important parts of the evidence for the geological history of the locality. At one place the deposits are said to have been bored through to a depth of 294 feet, and Mr. Hardman believed them to be not less than 500 feet deep. The same observer found that they certainly reach a height of 120 feet above the sea, and he was of opinion that in some places their height was not less than 140 feet. The deposition of strata to the depth of 300 feet below a level of 120 feet above the sea would, of course, entirely fill up Lough Neagh, and spread over a large tract of low ground around it. The pottery-clays and lignites, however, appear to be confined to the southern half of the lake, from which they rise gently into the low country around.

The distribution of these deposits and their extraordinary variations in altitude, as described by Mr. Hardman, present great difficulties in the attempt to regard them as the sediments of a Pliocene lake. A more recent examination of the ground by Mr. Clement Reid of the Geological Survey has led that able observer to believe that two totally different groups of strata at Lough Neagh have been confounded. He noticed the Mytilus-clay to be a dark blue mass full of derived boulder-clay stones, and yielding Mytilus edulis and seeds of a sedge. This deposit cannot be Pliocene, but must be of Glacial or post-Glacial age, possibly contemporary with the Clyde beds. The junction of this clay with the pipe-clays is not at present seen, but the lithological contrast between the two groups of strata is so strong as to indicate their independence of each other. Mr. Reid found the white, red and mottled pipe-clays with their masses of lignite to present a strong resemblance to the Bagshot group in the Tertiary series. It is possible, as already suggested, that the pipe-clays and lignites may belong to the sedimentary zone that separates the lower and upper basalts of Antrim. At all events they furnish no proof of any Pliocene lake, and may not indicate more than a deeper part of the depression in which the tuffs, lignites and iron-ore were laid down.

The existence of the Mytilus-clay shows that in Glacial or post-Glacial times the valley of the Bann was a strait or fjord into which the sea entered. Thick masses of drift have been laid down all round and over the depression now occupied by Lough Neagh, insomuch that had any older lake existed here in Glacial times, it could hardly have escaped being filled up.

The observer, who from one of the basalt-heights looks down upon the expanse of Lough Neagh and the broad peat-covered plain that continues the level platform of the lake-surface down the valley of the Bann, cannot but be impressed with the size of this wide hollow in the heart of the Antrim plateau, and with the evident continuity of the whole depression from the lake to the sea. If he be a geologist, he will be further struck by the fact that while the Chalk and other older rocks appear from under the basalt-escarpments all round the plateau, at heights of many hundred feet above the sea, the floor of this wide hollow is entirely covered with basalt. Had the depression been merely due to denudation, the rocks that underlie the volcanic series would have been exposed to view. The base of the basalts which, on either side of the depression, is often more than 1000 feet above the sea-level, sinks below that level in the hollow of the Bann and Lough Neagh.

This inequality of position may have been partially brought about by faults like those around Lough Neagh, and may thus have been begun long before the Glacial period. But it appears to me to be mainly due to a wide subsidence, of which the axis ran in a N.N.W. and S.S.E. direction from the present coast up the valley of the Bann and the basin of Lough Neagh to beyond Portadown.

We may conceive that after the cessation of the outflows of basalt, the territory overlying the lava-reservoir that had been emptied would tend to subside, partly by ruptures of the crust producing faults and partly by a downward movement of a more general kind. In course of time, these disturbances turned the drainage into the hollow now traversed by the Bann. Denudation would necessarily accompany them, and the surface of the country would be continually eroded and lowered.

Lough Neagh has been carefully sounded by the Admiralty, and its chart affords much suggestive material for the consideration of the geologist.[436] From the soundings there given it has long been known that the lake deepens towards its northern end, and attains a maximum depth of 102 feet. But it is not until we trace on the chart a series of contour-lines for successive depths, as shown by the soundings, that we realize the remarkable form of the lake bottom. We then discover that below a depth of 50 feet a well-defined channel extends for rather more than half the length of the lake. This channel begins to be distinctly perceptible between Kiltagh Point and Langford Lodge. It first runs in a northerly course on the west side of the centre of the Lough, but when it comes into a line with Saltera Castle on the western shore, it wheels round so as to conform to the curve of the Antrim coast-line, which it follows northward until, about two miles from the exit of the lake, its outline ceases to be traceable on the gently shelving bottom. Its total length is thus about 12 miles.

[Footnote 436: Lough Neagh surveyed and sounded by Lieut. Thomas Graves, R.N.]

There can hardly be any doubt that this channel is a former bed of the River Bann. It occupies exactly the position which that stream would take if the lake were drained, and its depth and breadth correspond to those of the valley-bottom of the present river. If this conclusion be accepted, some important conclusions may be further deduced from it.

1. The presence of a former course of the Bann on the bottom of Lough Neagh proves the lake to be much younger than the Ice Age. The thick boulder-clays and Glacial gravels which so encumber the country around and descend under the lake, would assuredly have filled up the river-channel had it existed at the time of their deposition. The channel has obviously been cut out of these drifts since the Glacial period. When the erosion took place, the present Lough Neagh could not have existed, but the Bann followed a continuous course across the plain which the lake now covers. The river probably maintained its place for a long period, so as to be able to excavate so wide and deep a bed in the drifts, if, indeed, it did not to some extent slowly carve its bed out of the underlying basalts. It must be remembered that sediment is being continually poured into Lough Neagh, and that some of the silt must have accumulated in the submerged river-course, thus lessening its depth and width. That the channel should still be so marked may be used as an argument for the comparatively late date of the subsidence.

2. The submerged river-course is a clear proof of subsidence. The present Lough Neagh cannot be looked upon as a glacial lake formed by rock-erosion or by irregular deposition of drift. Its floor must have been a land surface when the Bann cut out its bed upon it. The whole area has sunk down, the drainage has been arrested, and some 20 miles of the course of the Bann are now under a sheet of shallow water. This subsidence was not brought about by faults. It seems rather to have resulted from a general sinking of the ground. The movement was probably comparatively rapid, otherwise the river-course would hardly have survived so well.

3. These inferences, based upon purely geological considerations, have an interesting bearing upon the allusions to the origin of Lough Neagh contained in some ancient historical documents. Various legends have from an early period been handed down as to the first appearance of this sheet of water. These myths, though differing in details, agree in describing such a sudden or rapid accumulation of water as destroyed human life, in a district which had previously been inhabited by man. The earliest records indicate that the alleged catastrophe took place in the first century of the Christian era.[437] It appears to me not improbable that the tradition,thus preserved in these legends, may have had its basis in the actual disturbance which, on geological grounds, can be shown to have determined the existence of Lough Neagh. Though the event may go back far beyond the first century, there can be no doubt that, in a geological sense, it was one of the most recent topographical changes which the British Isles have undergone.

[Footnote 437: For versions of the legends, see Dr. Todd's "Irish Version of the Historia Britonum of Nennius," Roy. Hist, and Archæol. Assoc. Ireland; Dr. Reeves' "Ecclesiastical Antiquities of Down," etc., p. 370; Mr. J. O'Beirne Crowe's "Ancient Lake Legends of Ireland," No. 1 in Journ. Roy. Hist. and Archæol. Assoc. Ireland, vol. i. (1870-71), p. 94; Giraldus Cambrensis, vol. v. cap. ix. p. 91--"de lacu magno miram originem habente." Moore's well-known lines embody the popular belief that round towers and other buildings were submerged by the inundation.]

Thus the Antrim basalt-plateau, in addition to the high interest of its volcanic history, has the additional claim to our attention that it has preserved, more fully and clearly than any other of the plateaux, the evidence for the latest subterranean movements that followed the long series of volcanic eruptions during Tertiary time. It contains the record of a post-Glacial subsidence that gave birth to the largest lake in Britain.

ii. DISLOCATIONS

Though I have not observed any features among the Tertiary basalt-plateaux of the British Isles that can be compared to the remarkable rifts and subsidences of Iceland, it can be shown that these piles of volcanic material have undoubtedly been fractured, and that portions of them have subsided along the lines of dislocation.

Careful examination of the basalt-escarpments of the Inner Hebrides discloses the existence of numerous faults which, though generally of small displacement, nevertheless completely break the continuity of all the rocks in a precipice of 700 or 1000 feet in height. Not infrequently such dislocations give rise to clefts in the cliffs. Some good illustrations of this feature may be noticed on the north side of the island of Canna, where the precipice has been fissured by a series of dislocations, having a hade towards the west and a throw which may in some cases amount to about 20 or 25 feet. The cumulative effect of this system of faulting, combined with a gentle westerly dip, is to bring down to the sea-level the upper band of conglomerate which further to the east lies at the top of the cliff. Again, the basalt-escarpment on the west side of Skye, from Dunvegan Head to Loch Eynort, is traversed by a series of small faults. On the east side of Skye and in Raasay, a number of faults, some of them having perhaps a throw of several hundred feet, has been mapped by Mr. H. B. Woodward.

The largest dislocation observed by me among the basalt-plateaux of the Inner Hebrides is that already referred to (p. 209), which runs at the back of the Morven outlier, in the west of Argyllshire, from the Sound of Mull by the head of Loch Aline to the mouth of Loch Sunart, along the line of valley that contains the salt-water fjord Loch Teacus and the fresh-water lakes Loch Durinemast and Loch Arienas. While the Cretaceous deposits and the bottom of their overlying basalts rise but little above the sea-level on the south-west side of this line, they are perched as outliers on hill-tops on the north-east side, where they rise to 1300 feet above the sea. The amount of vertical displacement here probably exceeds 1000 feet. The fault runs in a north-westerly direction, and has obviously been the guiding influence in the erosion of the broad and deep valley which marks its course at the surface.

This dislocation is only the largest of a number by which the basalt-plateau has been broken in the district of Morven. Their effects are well shown in the outlier of basalt which caps Ben Iadain, where two parallel faults bring down the lavas against the platform of schists on which they lie (see Fig. 266).

Many faults have been traced in the Antrim plateau, and are represented on the Geological Survey Maps. In general they are of comparatively trifling displacement. Occasionally, however, they amount to several hundred feet, as in those already referred to as occurring near Ballycastle and around the southern part of the basin of Lough Neagh.

To what extent the dislocations that traverse the British Tertiary basalts are to be regarded as comparable to those which in Iceland have been referred to subsidence caused by the tapping and outflow of the lower still liquid parts of lava-sheets must be matter for further inquiry. So far as my own observations have yet gone, the faults do not seem explicable by any mere superficial action of the kind supposed. Where they descend through many hundreds of feet of successive sheets of basalt, and dislocate the Secondary formations underneath, they must obviously have been produced by much more general and deep-seated causes.

It is conceivable that, if these dislocations took place during the volcanic period, they broke up the lava-plains into sections, some of which sank down so as to leave a vertical wall at the surface on one side of the rent, or even to form open "gjás," like those of Iceland. But it is noteworthy that the fissures, which have been filled with basalt and now appear as dykes, comparatively seldom show any displacement in the relative levels of their two sides. In Iceland, also, the great lava-emitting fissures seem to be in general free from marked displacements of that kind.

The faults in the Inner Hebrides, so far as I have observed, are all normal, and indicate nothing more than gentle subsidence. But among the Faroe Islands I have come upon several instances of reversed faults, which, in spite of the usually gentle inclinations of the basalts, probably point to more vigorous displacement within the terrestrial crust.

On the east side of Svinö a fault with a low hade runs from sea-level up to the top of the cliff, a height of several hundred feet. It has a down-throw of a few yards, but is a reversed fault, as will be seen from Fig. 382. Another similar instance may be noticed on the north-east headland of Sandö, where, however, on the upcast side, the basalts appear as if they had been driven upward, a portion of them having been pushed up into a low arch (Fig. 383).

When the Tertiary basalt-plateaux of the Hebrides and the Faroe Isles come to be worked out in detail, many examples of dislocation will doubtless be discovered. We shall then learn more of the amount and effects of the terrestrial disturbances which have affected North-Western Europe since older Tertiary time. In the meantime evidence enough has been adduced to prepare us for proofs of very considerable recent displacements even among regions of crystalline schists, like that which has been disrupted by the Morven faults above alluded to. While the study of the Tertiary volcanic rocks demonstrates the vast general denudation of the country since older Tertiary time, the proofs that these rocks have been faulted acquire a special interest in relation to the origin and evolution of the topography of the region.

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L. EFFECTS OF DENUDATION

Among the more impressive lessons which the basalt-plateaux of North-Western Europe teach the geologist, the enormous erosion of the surface of this part of the continental area since older Tertiary time takes a foremost place. He may be ready almost without question to accept the evidence adduced in favour of a vast amount of denudation among such soft and incoherent strata as those of the older Tertiary formations of the south-east of England or the north-west of France. But he is hardly prepared for the proofs which meet him among the north-western isles that such thick masses of solid volcanic rocks have been removed during the same geological interval.

To gain some idea of the amount of this waste we must, in the first place, picture to our minds the extent of ground over which the lavas were poured, and the depth to which they were piled upon it. Though we may never be able to ascertain whether the now isolated basalt-plateaux of Britain were once united into a continuous plain of lava, we can be quite certain that every one of these plateaux was formerly more extensive than it is now, for each of them presents, as its terminal edge, a line of wall formed by the truncated ends of horizontal basalt-sheets. And there seems no improbability in the assumption that the whole of the great hollow from the centre of Antrim up to the Minch was flooded with lavas which flowed from many vents between the hills of ancient crystalline rocks forming the line of the Outer Hebrides on the west, and those of the mainland of Scotland on the east.

It is certain that the depth to which some parts of this long hollow were overflowed with lava exceeded 3000 feet, for more than that depth of rock can be shown to have been in some places removed. The original inequalities of surface were buried under the volcanic materials which were spread out in a vast plain or series of plains, like those that have been deluged by modern eruptions in Iceland. Owing, however, to a general but unequal movement of subsidence, the lava-fields sank down here and there to, perhaps, an extent of several hundred feet, so that the old land-surface on which they began to be poured out now lies in those places below the level of the sea.

I have shown that even during the volcanic period, while the lavas were still flowing from time to time, erosion was in active progress over the surface of the volcanic plain. The records of river-action in Canna and Sanday, and the buried river-channel of the Scuir of Eigg, prove that, while eruptions still continued, rivers descending from the mountains of the Western Highlands carried the detritus of these uplands for many miles across the lava-fields, swept away the loose material of volcanic cones, and cut channels for themselves out of the black rugged floor of basalt.

The erosion thus early begun has probably been carried on continuously ever since. The present streams may be looked upon as practically the same as those which were flowing in the Tertiary period. There may have been slight changes of level, oscillations both upward and downward in the relative positions of land and sea, and shiftings of the water-courses to one side or other; but there seems no reason to doubt that the existing basalt-plateaux, which were built up as terrestrial areas, have remained land-surfaces with little intermission ever since, although their lower portions may have been in large measure submerged.

In the existing valleys, fjords and sea-straits by which these plateaux have been so deeply and abundantly trenched, we may recognize some of the drainage-lines traced out by the rivers which flowed across the volcanic plains. The results achieved by this prolonged denudation are of the most stupendous kind. The original lava-floor has been cut down into a fragmentary tableland. Hundreds of feet of solid rock have been removed from its general surface. Outliers of it may be seen scattered over the mountains of Morven, whence they look into the heart of the Highlands. Others cap the hills of Rum, where they face the open Atlantic. Several miles from the main body of the plateau in Skye, a solitary remnant, perched on the highest summit of Raasay, bears eloquent witness that the basaltic tableland once stretched far to the east of its present limits.

Two lines of observation and of argument may be followed in the effort to demonstrate how great the denudation has been since older Tertiary time. In the first place, there is the evidence of the level or nearly level sheets of basalt that form the plateaux, and, in the second place, there is the testimony of the dykes, sills and bosses by which these lavas have been disrupted.

1. The study of the denudation of the Tertiary volcanic rocks of North-Western Europe is most satisfactorily begun by an attempt to measure the minimum amount of waste which in certain places the basalt-plateaux can be proved to have undergone. For the purposes of this study, the stratification of the lavas and their nearly horizontal, or at least very slightly disturbed, position afford exceptional facilities. Amorphous rocks, such as granites and gabbros, or even foliated masses like the old gneisses and schists, may have been enormously denuded. Their mere presence at the existing surface may be taken as proof of such waste, yet they furnish in themselves no criterion by which the amount of removed material may be estimated.

But in the case of the basalt-plateaux, as in that of horizontal sedimentary formations, the successive lines of superposition of the component beds of the whole stratigraphical series supply admirable datum-lines which, on the one hand, vividly impress the imagination by the demonstration which they afford of the reality and magnitude of the denudation, and, on the other hand, furnish a measure by which the minimum amount of this denudation may be actually computed.

Availing ourselves of this kind of evidence it is easy to show that valleys many miles long, several miles broad, and from crest to bottom several thousand feet deep, have been excavated out of the basalt-plateaux since the close of the volcanic period. And if this conclusion can be demonstrated for these plateaux, it must obviously apply equally to the rest of the country. We thus obtain a most important contribution to the investigation of the origin and relative age of the present topographical features of the surface of the land.

Let me give a few illustrations of the nature of the investigation and of the results to which it leads. Throughout the Western and Faroe Islands the level bars of basalt present their truncated ends in the great escarpment-cliffs which wind mile after mile along their picturesque coasts. Where they front the open sea, it is obviously impossible to say how much further seaward they once extended. But where they retire in fjords or sea-lochs, and sweep inland into glens, it is easy to measure the distance from the bottom of the eroded hollow to its bounding watersheds, and to estimate the amount of material that has been worn out of it. The only uncertainty in this computation arises from our inability to determine to what extent movements of subsidence may have come into play to aid in the disappearance of the basalts. Where the bottom of the lavas can be seen at the same level on either side of an inlet, with no evidence of faulting, or where a definite horizon in the volcanic series can be traced round the head of a glen or sea-loch, the influence of underground movements may be eliminated. The evidence of vast denudation is always visible, the proofs of subsidence are much less frequently observable.

The island of Mull supplies many striking examples of the enormous waste of the basalt-plateau. The Sound of Mull, for instance, has been eroded out of the volcanic series for a distance of 20 miles, with a mean breadth of about two miles. From the deepest part of this fjord to the summit of the Mull plateau is a vertical height of 3600 feet. The whole of this vast excavation has taken place since older Tertiary time. On the opposite side of Mull the hollow of Loch Scridain has been eroded to a mean depth of at least 1200 feet below the average level of the surrounding plateau, with a breadth of rather more than a mile.

The scattered islands which lie to the west of Mull tell the same tale. They are all outliers of the same basalt-plateau, and have not only been greatly lowered by the removal of their upper lavas, but have been separated by the erosion of long and deep hollows between them. Thus from the summit of the Gribon cliffs in Mull to the deepest part of the sea-floor between that precipice and the Treshnish Isles a vertical depth of at least 2000 feet of rock has been removed since the basalts ceased to be erupted.

I have referred to the impressive evidence of denudation displayed on the west side of the island of Eigg. The vertical distance from the summit of the Eigg plateau to the bottom of the submarine valley between this island and Rum is about 1500 feet, but as that summit lies below the original surface of the lava-field, the depth of rock which has been removed must exceed 1500 feet. We thus learn that since the close of the volcanic period the hollow between the islands of Eigg and Rum has been eroded to this great depth.

Still more striking is the evidence of enormous waste presented by the Faroe Islands. The cliffs there are loftier and barer, and the fjords have been cut more deeply and precipitously out of the basalt-plateau. I shall never forget the first impression made on my mind when the dense curtain of mist within which I had approached the southern end of the archipelago rapidly cleared away, and the sunlit slopes and precipices of Suderö, the two Dimons, Skuö and Sandö, rose out of a deep blue sea. Each island showed its prolongation of the same long level lines of rock-terrace. The eye at once seized on these features as the dominant element in the geology and the topography, for they revealed at a glance the true structure of the islands, and gave a measure of the amount and irregularity of the erosion of the original basalt-plateau. And this first impression of stupendous degradation only deepened as one advanced further north into the more mountainous group of islands. Probably nowhere else in Europe is the potency of denudation as a factor in the evolution of topographical features so marvellously and instructively displayed as among the north-eastern members of the Faroe group.

Availing ourselves of the datum-lines supplied by the nearly level bars of basalt, we easily perceive that in many parts of the Faroe Isles the amount of volcanic material left behind, stupendous though it be, is less than the amount which has been removed. Thus the island of Kalsö is merely a long narrow ridge separating two broad valleys which are now occupied by fjords. The material carved out of these valleys would make several islands as large as Kalsö. Again, the lofty precipice of Myling Head, 2260 feet high, built up of bedded basalts from the summit to below sea-level, faces the north-western Atlantic, and the sea rapidly deepens in front of it to the surface of the submarine ridge 200 to 300 feet below. The truncated ends of the vast pile of basalt-sheets which form that loftiest sea-wall of Europe bears testimony to the colossal denudation which has swept away all of the volcanic plateau that once extended further towards the west.

Nevertheless, enormous as has been the waste of this plateau of the Faroe Islands, we may still trace some of its terrestrial features that date back probably to the volcanic period. Even more distinctly, perhaps, than among the Western Isles of Scotland, we may recognize the position of the original valleys, and trace some of the main drainage lines of the area when it formed a wide and continuous tract of land.

A line of watershed can be followed in a south-westerly direction from the east side of Viderö, across Borö to the centre of Osterö, and thence by the Sund across Stromö and Vaagö. From this line the fjords and valleys diverge towards the north-west and south-east. There can hardly be any doubt that on the whole this line corresponds with the general trend of the water-parting at the time when the Tertiary streams were flowing over the still continuous volcanic plain. Considerable depression of the whole region has since then sent the sea up the lower and wider valleys, converting them into fjords, and isolating their intervening ridges into islands.

The topography of the Faroe Islands seems to me eminently deserving of careful study in the light of its geological origin. There is assuredly no other region in Europe where the interesting problems presented by this subject could be studied so easily, where the geological structure is throughout so simple, where the combined influences of the atmosphere and of the sea could be so admirably worked out and distinguished, and where the imagination, kindled to enthusiasm by the contemplation of noble scenery, could be so constantly and imperiously controlled by the accurate observation of ascertainable fact.

2. Impressive and easily comprehended as are the proofs of denudation supplied by the basalts of the plateaux, they are perhaps to a geological eye less overwhelming than those furnished by the eruptive rocks which have been injected into these plateaux. In the case of at least the basic intrusions, we may reasonably infer that they assumed their present position under a greater or less depth of overlying rock which has since been removed. When, therefore, they are found at or above the summits of the plateaux, they demonstrate that a vast amount of material has been removed from these summits.

The argument from the position of the dykes has already been enforced. It is absolutely certain that valleys several thousand feet deep must have been excavated since these dykes were erupted, for had such valleys existed at the time when the dykes were injected across their site, the molten rock, instead of ascending to the tops of the surrounding mountains, would obviously have rushed forth over the valley-bottoms. I have shown that this reasoning applies not merely to the volcanic districts, but to the whole surface of the country within the region of dykes. Thus the uplands of Southern Scotland, and wide areas in the Southern and Western Highlands, can be proved to have had glens cut out of their mass to a depth of hundreds of feet since the Tertiary volcanic period.

Not less convincing is the evidence afforded by the great eruptive masses of gabbro. We have seen that these complex accumulations of sills, dykes, and bosses include rocks so coarse in grain as to show that they must have consolidated at some considerable depth, but that they now appear in hill-groups 2000 to 3000 feet in height, the whole of the original basaltic cover having been stripped off from them. But these gabbro hills have been in turn traversed up to the very crests by later basalt-dykes, which thus supply additional proof that the erosion here has been stupendous.

The granophyre bosses tell the same tale. Though, like the domite Puys of Auvergne, they may still retain, in their conical forms, indications of the original shapes which their component material assumed at the time of its protrusion, we may be confident that their existing surfaces have been reached after the removal of much rock which once lay above them. This inference is confirmed by the fact that these eruptive bosses have been invaded by a younger system of dykes. The black ribs of basalt which may be traced along their pale declivities, which cross the glens that have been eroded in them and which mount up to their very crests, prove that since the latest manifestations of volcanic energy in the West of Scotland, extensive changes in the topography of the land have been effected by the operation of the subærial agents of degradation.

So much for what can be demonstrated. But how much more may, with the highest probability, be inferred! The original limits of the plateaux are unknown. The waves of the wide Atlantic now roll over many a square league of the old lava-plains, and wide tracts of the islands and the mainland from which the basalt has been entirely stripped, or where it remains only in scattered outliers, were once deeply buried under piles of lava-sheets. It would probably be no exaggeration to affirm that over the British area, as well as over the Faroe Isles, the amount of Tertiary volcanic rock that now remains, large as it is, falls short in amount of what has been removed. The geologist who has made himself familiar with the effects of denudation in other Tertiary volcanic districts, such as Central France, Saxony and Bohemia, will be prepared for almost any conceivable amount of erosion among the far older volcanic series of the north-west of Europe.

To the student of the origin of the existing topography of the land there is a profound interest in the demonstration which these volcanic rocks supply of the vast changes which the terrestrial surface has undergone within a period geologically so recent as older Tertiary time. When, on the one hand, he finds himself more and more restricted in his demands for time by the confident assertions of the physicist that all the phenomena of geological history must have been comprised within a few millions of years, and when, on the other hand, he watches the seemingly feeble and tardy operations of the forces of denudation and sedimentation which have played the chief parts in that history, he may well be excused if sometimes he is apt to despair of ever reconciling the facts which he observes with the physical deductions that are somewhat dogmatically brought forward in opposition to his interpretation of them. He may feel sure that his facts cannot be gainsaid, and he may be unable to find any other way of comprehending them save by the admission that they necessitate a liberal allowance of time. Yet he may not feel himself to be in a position to offer any valid objections to the arguments from physical considerations that would so seriously abridge the length of time which geology requires.

In these circumstances it is some satisfaction to be provided with definite measurements of the amount of geological change which has been effected within a limited and relatively recent period of time. This change has resulted from the operation of the same agents by which it is still being carried on. No break in the history can be detected. There is not the least reason to suppose that the agents of denudation and sedimentation have, during the period in question, differed in their rate of working. Their activity at the present time is probably neither greater nor less than it was then. If, therefore, during so recent an interval such a stupendous amount of material has been worn away from the surface of the land and deposited on the sea-floor as the Tertiary volcanic rocks demonstrate, the geologist may surely contemplate without misgiving the lapse of time required for the completion of older geological revolutions. He may oppose to the arguments of the physicist the measurements and computations which he himself makes from data which are at least as reliable as the postulates whereon these arguments are based. The rate at which denudation and sedimentation are now taking place has been measured with tolerable accuracy, and a fair average for it has been obtained. Whatever may be maintained as to this rate in early geological ages, there can be no serious opposition to its being taken as fairly constant since older Tertiary time. We are thus provided with data for estimating the minimum amount of time that can have elapsed since the volcanic plateaux began to be denuded. But as no relic remains of the original upper surface of those plateaux, and as we are consequently ignorant of how much rock has been removed from their highest surviving outliers, it is obvious that such estimates are more likely to err in understating than overstating the amount of time required.

It would be beyond the scope of the present volume to enter fully into the measurements and calculations required for the adequate treatment of this subject. I will merely illustrate my argument by again taking a few data from the plateau of Mull. The original height of this plateau is shown by the outlier of Ben More to have been at least 3200 feet. If to this figure we add the portion of the basalt-group submerged under the sea the height will probably be increased by several hundred feet. But let us take 3000 feet as a moderate computation for the average thickness of the volcanic series here at the close of the plateau-period. Until a number of sections have been carefully plotted from the Ordnance Maps, in order to ascertain with approximate accuracy the average height of the present surface of the Mull basaltic plateau, making due allowance for the vast erosion of the Sound of Mull and the numerous glens and sea-lochs that traverse the island, any estimate which may be offered as to this average must be merely provisional. If, in the meantime, we suppose the present mean level of the plateau to be 1000 feet above the sea, the difference between this amount and the assumed original height will be 2000 feet. If, further, we take the present average rate of degradation of the Mull plateau to be 1/6000 of a foot in a year, which has been shown to be probably a fair estimate, then the time required for the lowering of the Mull plateau from its original to its present average level amounts to twelve millions of years. Yet this period, vast though it be, does not carry us back even as far as the beginning of Tertiary time.

* * * * *

In concluding this lengthened discussion of the Tertiary volcanic history of Britain, I may, perhaps, usefully add a brief summary of the leading features of the long record.

The region within which volcanic activity displayed itself during older Tertiary time in the British Isles, if our estimate of its area is restricted to those parts of the country where igneous rocks, probably of that age, now appear at the surface, embraces the North of England and of Ireland, the southern half and the west coast of Scotland--a total area of more than 40,000 square miles. Over that extensive region volcanic phenomena were displayed during an enormously protracted interval of geological time. The earliest beginnings of disturbance may possibly have started in the Eocene, and the final manifestations may not have ceased until the Miocene period. So prolonged was the duration of the eruptions, that enormous topographical changes from denudation, and probably also considerable variation in the fauna and flora, alike of land and sea, may have been effected.

Owing to some cause which has not yet in this relation been investigated, but which is probably referable to secular terrestrial contraction, the volcanic region underwent elevation, while, at the same time, a vast subterranean lake or sea of molten rock existed underneath it. Enormous horizontal tension thus arose, and at last the stretched terrestrial crust gave way. A system of approximately parallel fissures opened in it, having a general direction towards north-west. The rapid and simultaneous production of such a gigantic series of rents must have given rise to earthquakes of enormous magnitude and destructive force. The great majority of the fractures, doubtless, did not reach to the surface of the ground, though probably not a few did so. Such was the potency of this development of terrestrial energy, that the fissures ran through the most varied kinds of rocks and the most complicated geological structures, crossing even earlier lines of powerful dislocation, and yet retaining their direction and parallelism for sometimes 50 or 100 miles.

Into the fissures thus formed the molten magma from underneath was forced for many hundreds or even thousands of feet above the surface of the subterranean lava-reservoir. Solidifying between the fissure walls, it formed the crowd of basic dykes that stand out as the most widespread and distinctive feature of the volcanic region.

Where the fissures reached the surface or near to it, the molten rock would seek relief by egress in streams of lava. This probably occurred in many places from which subsequent denudation has removed all vestige of superficial volcanic manifestations. But, in the great range of basalt-plateaux, from Antrim northwards through the chain of the Inner Hebrides, there are still left abundant remains of the surface-outflows. Like the modern lavas of Iceland, the molten material probably flowed out sometimes from the open fissures, sometimes from vents formed along the chasms. After the convulsions ceased which produced the earliest dykes, the communication that had been established between the magma-reservoir underneath and the air above would be maintained, and repeated eruptions might take place, either from the original fissures and vents or from others afterwards opened by the volcanic energy.

As in the modern eruptions of Iceland, new fissures are successively opened through the older lava-sheets, so in the Tertiary volcanic areas, renewed ruptures of the earth's crust allowed later dykes to be formed. The basalt-plateaux are traversed by such dykes, even up to their highest sheets. It is impossible to say how often the process of dyke-making may have been repeated. Not improbably it recurred again and again during the building of the basalt-plateaux, and we know that it was renewed even after the protrusion of the granophyre bosses which mark one of the latest phases of volcanism in the region.

For a protracted geological period, with long intervals of quiescence, various basic lavas (basalts, dolerites, etc.), with occasionally some of intermediate composition (andesites, trachytes), and perhaps in Antrim acid rhyolites, flowed out from fissures and vents until they had filled up the hollows of the great valley, which then stretched from the south of Antrim northwards between the west coast of Scotland and the chain of the Outer Hebrides. In some places the accumulated pile of these ejections even now exceeds 3000 feet in thickness, but we cannot tell how much material has been bared away from its top by denudation. The volcanic discharges consisted mostly of lava, fragmentary materials being comparatively insignificant in amount and local in origin, though layers of fine tuff and basalt-breccias occur in all the plateaux. None of the erupted materials thicken towards any centres that might be taken to mark volcanoes of the type of Vesuvius or Etna. On the contrary, the persistent flatness and uniformity of the volcanic series, and the thinning out of the separate beds in different directions, show that the lavas issued from many points all over the region. The positions of some of the actual vents can still be ascertained. They are now filled sometimes with dolerite, sometimes with coarse agglomerate.

The surface over which the lava flowed seems to have been mainly terrestrial. Here and there, between the successive sheets of basalt, the leaves, stems, and fruit of land-plants, sometimes in most perfect preservation, may be observed, together with the remains of insects and fresh-water fish. Distinct relics of old river-channels can be recognized which have been buried under streams of lava. Among the deposits left by these streams the uppermost layers are commonly dark with decayed vegetation, while layers of coal are found here and there between the basalts.

As the pile of erupted materials gradually thickened, and the subterranean energy possibly grew feebler, the ascending magma was forced between the layers of sedimentary strata underneath the basalts, or between these strata and the overlying volcanic series, or along any other plane of weakness in the terrestrial crust. In this way arose the multitudinous sills or intrusive sheets.

When the great volcanic plateaux had been built up to a thickness of several thousand feet, another remarkable episode in the history occurred. At certain points large bodies of coarsely crystalline basic rocks were pushed into and through the plateaux-basalts, upraising them in dome-shaped elevations, and ultimately solidifying as dolerites, gabbros, troctolites, picrites, etc. There is reason to believe that the points of extravasation of these materials were mainly determined by the positions of the larger or more closely clustered vents of the plateau-period, where points of weakness consequently existed in the terrestrial crust. Rising as huge bosses through such weak places, the gabbros and associated rocks raised up the overlying bedded basalts, and forced themselves between them, forming thus a fringe of finer-grained intrusive sills and veins around the central banded and amorphous masses of more coarsely crystalline material. Whether, in any of these vast domes of upheaval, the summit was disrupted, so as to allow the basic intrusion to flow out as lava at the surface, cannot now be told, owing to enormous subsequent denudation.

The next chapter in the chronicle shows us that probably long after the eruption of the gabbros, when possibly all outward symptom of volcanic action had ceased, a renewed outbreak of subterranean activity gave rise to the protrusion of another and wholly different class of materials. This time the rocks were of a markedly acid type. They included varieties that range from obsidians, pitchstones, flinty felsites and rhyolites, through porphyries and granophyres, into compounds which cannot be classed under any other name than granite. These masses likewise availed themselves of older vents in the plateaux, and broke through them. They now form huge conical hills, which, in their outer aspect, and even to some extent in their inner structure, recall the trachytic puys of Auvergne. But the granophyres not only ascended through the basalt-plateaux and the gabbro-bosses; they sent into these rocks a network of veins, pushed their way in huge sheets or sills between the strata below, and actually incorporated a considerable proportion of the basic materials into their own substance. Around the bosses of gabbro and granophyre, the bedded basalts have undergone considerable contact-metamorphism.

The gabbro and granophyre bosses of the Inner Hebrides demonstrate with singular force how unreliable petrographical characters are as a test of the relative age of rocks. No one, looking at hand-specimens of these rocks, or even studying them in the field, would at first suspect them to be of Tertiary date. They closely resemble rocks of similar kinds in Palæozoic and even Archæan formations. Yet, of their late appearance in geological time, there cannot be any possibility of doubt.

After the uprise of the granophyre, and the injection of the network of felsitic veins, there came once more a period of terrestrial convulsion, like that of the earliest basic dykes, but of less intensity. Again, the crust of the earth over the volcanic region was pushed upward and rent open by another system of parallel fissures. Again, from a reservoir or basin of basic lava underneath, molten rock was forced upwards into the rents, and thus another system of basic dykes was formed. These dykes are found crossing those of earlier date, and rising through the other volcanic rocks. They traverse the plateau-basalts from bottom to top; they climb to the summits of the gabbro mountains, and they even pursue their undeviating course over the huge domes of granophyre. No proof has yet been found that from any of these dykes there was a superficial outflow of lava. But so great has been the subsequent denudation of the areas, that such outflows might quite well have taken place, and have subsequently been destroyed.

Whether these basic dykes were the last manifestation of volcanic energy in our region cannot yet be decidedly affirmed. So far as the evidence at present goes, they are possibly older than another series of acid veins and dykes (pitchstone, felsite, and granophyre), which are found at many points from Antrim to the far end of the Inner Hebrides. These protrusions traverse every other member of the volcanic series, except some of the youngest basic dykes, and do not appear to be themselves cut by any.

Since the close of the volcanic period considerable disturbance of the basalt-plateaux has taken place. The whole volcanic region has subsided, some districts having sunk more than others. In Britain the most striking evidence of such depression is supplied by the basin of Lough Neagh. But throughout the Inner Hebrides much of the lower portion of the terrestrial lava-plateaux is now below sea-level. In the Faroe Islands and in Iceland the subsidence has been still more marked. Dislocations, also, sometimes amounting to more than a thousand feet of displacement, have occurred among the volcanic masses. The bedded basalts, originally on the whole nearly flat, have thus been broken up into large blocks of country wherein the sheets are now inclined in various directions.

One of the most important lessons taught by the Tertiary volcanic series of the north-west of Europe is the extent of the denudation of the land since the close of the volcanic period. The horizontal or gently inclined layers of bedding among the basalts afford datum-lines from which the minimum amount of material removed may be measured. As a reasonable estimate it may be inferred that in the case of the Mull plateau, for example, the average amount by which its surface has been lowered since the close of the volcanic period cannot be less than 2000 feet. If the rate of lowering of the land-surface in western Europe by subærial denudation be taken as 1/6000 of a foot in a year, then the lapse of time required for the degradation of the Mull plateau must amount to about twelve millions of years. Some such interval has therefore elapsed since the last Tertiary volcanoes became extinct.

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LI. SUMMARY AND GENERAL DEDUCTIONS

The foregoing chapters comprise a connected narrative of the history of volcanic action in the area of the British Isles during the vast succession of ages from the early Archæan dawn down to the latest eruptions of Tertiary time. In this final chapter I propose to present a brief summary of the facts of largest import and widest interest which this protracted history has placed before us, together with a statement of deductions which may be drawn from them regarding the nature and progress of volcanism in the evolution of the globe.

1. Among the broad features which soonest arrest attention in such a survey is the geographical position of the theatre of this volcanic activity. In the distribution of volcanoes at the present time we are familiar with their tendency to range themselves along continental borders or in oceanic islands. The volcanic energy so conspicuous in the geological history of Britain has shown itself along the western or Atlantic margin of the European continent. When the eruptions have not been actually on the land itself, they have taken place within the shallow tracts near the land, where the lavas and tuffs have been interstratified with sediments derived from the adjacent coasts.

Moreover the volcanic rocks in Britain are ranged along the greatest length of the group of islands, in a general north and south line, from the south of Devonshire to the far Shetlands. It is on the western side of the country that they occur. East of a line drawn from Berwick by Leicester to Exeter, although the geological formations, ranging from the Carboniferous Limestone to the latest Pleistocene deposits, are there abundantly exposed to view, they include no contemporaneous volcanic rocks.

2. A second and still more remarkable feature in the geological history of Western Europe is the persistence of volcanic activity along the site of the British Isles. Evidence has been brought forward in these volumes that from the primeval time vaguely termed Archæan, onward to that of the older Tertiary clays and sands of the south-east of England--that is to say, through by far the largest part of geological history, as chronicled in the stratified crust of the globe--this long strip of territory continued to be intermittently a theatre of volcanic action. Every great division of Palæozoic time was marked by volcanic eruptions, sometimes over tracts hundreds of square miles in area and on a colossal scale. After a long period of quiescence during the Mesozoic ages, the renewed outbreak of volcanic energy in older Tertiary time, so marked over the western half of Europe, reached its maximum of development along the Atlantic border, from the north of England and Ireland through the chain of the Inner Hebrides to the Faroe Islands, Iceland and Greenland.

3. Not only has there been a remarkable persistence of volcanic activity over the comparatively limited area of the British Isles, viewed as a whole, but if we examine the different parts of this area we perceive that many of them, of relatively restricted extent, have been the sites of a recrudescence of volcanic action, again and again, through a vast succession of geological periods. While the whole region has been in different quarters and at different times affected, there have been districts where the volcanic fires have been rekindled after long intervals of quiescence, the new vents being opened among or near to the sites of earlier volcanoes. In the south-west of England, for example, the Middle Devonian tuffs and diabases were succeeded in the Carboniferous period by the eruptions of the Culm-measures, and in the very same tracts came last of all the lavas and tuffs of the Permian conglomerates. Still more astonishing is the record of volcanic energy in the south of Scotland, where, within a space of not many hundred square miles, there are the chronicles of the Arenig, Llandeilo and Bala eruptions of the Southern Uplands, the huge piles of lavas and tuffs of the Lower Old Red Sandstone, the long succession of the plateaux and then of the puys of the Carboniferous period, the groups of tuff-cones of the Permian period, and, lastly, the numerous dykes connected with the Tertiary volcanoes.

While some portions of the region have been specially liable to exhibitions of volcanic action, others have continuously escaped. Some of these "horsts," or stationary and unaffected blocks of country, have been surrounded by or have risen close to the borders of this volcanic district, yet have maintained their immunity through a long series of ages. Thus the Central Highlands of Scotland, though they were flanked on the south and south-west by the active volcanoes of the Old Red Sandstone, and again on the south by those of Carboniferous time, had no vents opened on their surface after the metamorphism of their schists. Still more striking perhaps is the immunity of the Southern Uplands. Though they were in large measure surrounded by the volcanoes of the Lower Old Red Sandstone, then by those of the Calciferous Sandstones and Carboniferous Limestone, and though they looked down on the Permian eruptions of Ayrshire and Nithsdale, which spread streams of lava and showers of ash along their flanks, these hills formed a solid block that seems to have resisted perforation by the volcanic funnels. Again, the tracts covered with Carboniferous Limestone in England and Ireland almost entirely escaped from invasion by volcanic eruptions.

We thus learn that even within comparatively restricted regions some portions of the terrestrial crust have been areas of weakness, liable to serve again and again as lines of escape for volcanic energy, while close to them other portions of greater solidity have been persistently left intact.

4. The sites of volcanic vents in all the geological systems wherein they occur in Britain have not usually been determined by any obvious structure in the rocks now visible. They comparatively seldom depend on ascertainable lines of fault, even when faults, probably already existent, occur in their near neighbourhood. This independence, to which, however, there are occasional marked exceptions, comes out more particularly in the coal-fields pierced by vents, for mining operations have there revealed the positions of many more faults than can be traced at the surface. If the sites of the vents have been fixed by dislocations or lines of weakness in the terrestrial crust, these must generally lie below the formations now visible at the surface.

There is one striking connection between the sites of the vents and ancient topographical features to which frequent reference has been made in the foregoing chapters. All through the long volcanic history, as far back as such features can be traced, we see that orifices of discharge for the erupted materials have been opened along low grounds and valleys rather than on ridges and hills. The great central hollow of the Scottish midlands was a depression even as long ago as the time of the Lower Old Red Sandstone, and though it has probably been several times since then filled up, and more or less completely effaced, its ancient features have been partially revealed by extensive denudation. This vast depression, 40 miles broad, between the Highland mountains on the one side and the Southern Uplands on the other, was the chief centre of volcanic activity in western Europe during the latter half of Palæozoic time. The vents of the Old Red Sandstone, Carboniferous and Permian series are scattered all over it, but few or none of them are to be found on the high grounds that bound it. Again, in Tertiary time, the great outpouring of lava took place in the hollow that lay between the ridge of the Outer Hebrides and the mainland of Scotland. This wide and long tract of low ground was buried under upwards of 3000 feet of lava and tuff, but these materials were erupted from fissures and vents within its own border and not from the mountains on either side.

But perhaps the most conspicuous example of any in which the vents keep to the valleys is that supplied by the Permian necks of Nithsdale and the neighbouring glens. These depressions are as old as Permian, and even as Carboniferous time, but they appear to be entirely hollows of erosion; at least they have yielded no evidence that their direction has been determined by lines of fault. The chain of vents can be followed from the lowlands of Ayrshire up to the base of the Southern Uplands, down the wide valley cut by the Nith in these hills and up some of the tributary valleys, and though the volcanoes continued for some time in vigorous eruption, not a trace of any contemporary vent has yet been met with on the surrounding hills.

While the position of volcanic vents in lines of valley may be generally due to guiding lines of fissure in the crust underneath, either within or below the rocks visible at the surface, there may sometimes be conditions in which other dominant causes come into play. The curious coincidence between variations in the upper limit of dykes and inequalities in the configuration of the overlying ground, suggest that where the subterranean magma has ascended to within a comparatively short distance from the surface, a difference of a few hundreds or thousands of feet in the depth of overlying rock, such as the difference of height between the bottom of a valley and the tops of the adjacent hills, may determine the path of escape for the magma through the least thickness of overarching roof.

5. Volcanic phenomena cannot be regarded as a mere isolated and incidental feature in the physics of the globe. During the short time within which man has been observing the operations of existing volcanoes, he has hardly yet had sufficient opportunity of watching how far they can be correlated with other terrestrial movements. Nor, when he endeavours to trace some such connection among the records of the geological past, has he yet collected materials enough to furnish a sufficiently broad and firm basis of comparison. One formidable obstacle is presented by the difficulty in determining chronological equivalents in separated groups of rock. Geologists have tried to discover whether the volcanoes of some particular period or region were in any way connected with such geological changes as extensive plication, dislocations of the crust, or elevation of mountain-chains. In regard to the volcanic history of Britain, various possible relations of this kind obviously suggest themselves. Thus the division of geological time comprised within the Lower Silurian period was undoubtedly an interval of considerable terrestrial disturbance in western Europe. The unconformabilities and overlaps in the series of formations belonging to that period, the frequent conglomerates, the great and often rapid changes in the thickness and lithological characters of the strata, all point to instability of land-surface and sea-floor. During these oscillations a prolonged and widespread series of volcanic eruptions took place. The volcanic manifestations began in Cambrian time and continued in intermittent activity till towards the close of the deposition of the Lower Silurian formations. It is certainly a significant fact that the Upper Silurian deposits, in their lithological characters, present a strong contrast to those that preceded them. They point, on the whole, to quiet sedimentation, during an interval of comparative calm in the terrestrial crust. With this evidence of tranquillity there is, over almost the whole of the British Isles, an entire absence of any trace of renewed volcanic activity. With the exception of the Dingle lavas and tuffs, in the extreme west of Ireland, not a single undoubted instance is yet known of an Upper Silurian volcano.

After the deposition of the Upper Silurian rocks an interval of great terrestrial disturbance ensued, and these rocks over a large part of Britain were intensely plicated and crushed. The movements, continued into the period of the Lower Old Red Sandstone, were, in their later stages, accompanied or, at least, followed by the vast outpourings of lava which now cover so much of the tracts of Old Red Sandstone in Scotland and Ireland.[438]

[Footnote 438: Trans. Geol. Soc. Edin. vol. ii. part iii. (1874).]

In proportion as the volcanic energy was vigorous, widespread and long-continued, we may expect it to have been connected with important terrestrial movements affecting extensive regions of the earth. The Tertiary volcanic history seems to afford a remarkable instance of this connection. A wide area of the European continent is dotted over with old centres of volcanic activity which were in eruption at successive epochs throughout the Tertiary period. Of all these centres the most important was that of the north-western basalt-plateaux, where floods of lava were discharged over many thousand square miles from Ireland to Greenland. The geological date of these outpourings probably coincides with the last great orographic movements that gave to the mountain-chains of Europe their latest elevation and dimensions.

But without entering into what must be for the present a field of speculation, we can be assured of one important fact in the connection of ancient volcanoes with movements of the terrestrial crust. A study of the records of volcanic action in Britain proves beyond dispute that the volcanoes of past time have been active on areas of the earth's surface that were sinking and not rising. We usually associate volcanic action with elevation rather than subsidence, and there are certainly abundant proofs of such elevation around active or recently extinct volcanoes. Many of the active vents of the present time, like Vesuvius and Etna, began with submarine eruptions and have been gradually upraised into land. It may be, however, that such uprise is merely a temporary incident, and that if we could survey the whole geological period of which human history chronicles so small a part, we might find that subsidence, and not upheaval, is ultimately the rule over volcanic areas.

Be this as it may, there can be no question that with the one solitary exception of the Tertiary volcanoes, which were terrestrial and not submarine, all the British vents were carried down and eventually buried under aqueous sediments. Even the Tertiary lava-fields have in many places sunk down below sea-level since their eruptions ceased.

That there are any Palæozoic volcanic rocks now visible at the surface is obviously due to subsequent movements not immediately connected with their original conditions of eruption, and to gigantic denudation. The amount of subsidence which followed on a volcanic episode was sometimes enormous, even within the same geological period, as one may see by observing the prodigious piles of sedimentary material heaped over the lavas and tuffs of Arenig time, or over those of the Lower Old Red Sandstone. I do not wish to maintain that the downward movement was necessarily a consequence of volcanic ejections, for we know that it took place over tracts remote from centres of eruption. But I have sometimes asked myself whether it was not possibly increased as a sequel to vigorous volcanic action; whether, for instance, the great depth of the Palæozoic sedimentary rocks in some regions, as compared with their feeble development in others, may not have been due to an acceleration of subsidence consequent upon volcanic action.

6. A review of the geological history of Britain cannot but impress the geologist with a conviction of the essential uniformity of volcanism in its manifestations since the early beginnings of geological time. The composition and structure of the materials erupted from the interior have remained with but little change. The manner in which these materials have been discharged has likewise persisted from the remotest periods. The three modern types of Vesuvian cones, puys and fissure-eruptions can be seen to have played their parts in the past as they do to-day.

Among the earliest igneous masses of which the relative geological date can be fixed are the dykes which form so striking a system among the Archæan rocks of the north-west, and show how far back the modern type of volcanic fissures and dykes can be traced. No relic, indeed, has survived of any lavas that may have flowed out from these ancient fissures, but so far as regards underground structure, the type is essentially the same as that of the Tertiary and modern Icelandic lava-fields.

The early Palæozoic volcanoes formed cones of lava and tuff comparable to those of such vents as Vesuvius and Etna. In the Lake District the pile of material ejected during Lower Silurian time was at least 8000 or 9000 feet thick. In the Old Red Sandstone basins of Central Scotland there were more than one mass of lavas and tuffs thicker than those of Vesuvius.

The puys of the later half of Palæozoic time closely resembled their Tertiary successors in Central France, the Eifel, and the Phlegræan Fields.

Nor, as regards extent and vigour, did the eruptions of the geological past differ in any important respect from those of the present time. There is assuredly no evidence that volcanic energy has gradually waned since the dawn of geological history. The latest eruptions of North-Western Europe, forming the Tertiary basalt-plateaux, far exceeded in area, and possibly also in bulk of material discharged, all the eruptions that had preceded them in the geological record.

7. Nevertheless, while the Tertiary eruptions showed no diminution of vigour, it is undoubtedly true that the volcanic energy has not manifested itself in a uniform way since the beginning of geological time. There have been periods of maximum activity followed by others of lessened force. Thus if we take a broad view of the general features of volcanic action during the Palæozoic ages in Britain, we see clear evidence of a gradual diminution in its vigour. The widespread outpourings of lava and tuff in the Silurian period in England, Wales, Scotland and Ireland were succeeded by the somewhat diminished, though still important, eruptions of the Lower Old Red Sandstone basins. The latter were followed by the still lessened outflows of the Carboniferous plateaux, which in turn were succeeded by the yet feebler and more localized eruptions of the Carboniferous puys, the whole prolonged volcanic succession ending in the small scattered vents of the Permian period. There were of course oscillations of relative energy during this history, some of the maxima and minima being of considerable moment. But though progress towards extinction was not regular and uniform, it was a dominant feature of the phenomena.

8. The Permian volcanoes were the last of the long Palæozoic series, and, so far as we yet know, the whole of the Mesozoic periods within the area of Britain were absolutely unbroken by a single volcanic eruption. The chronological value of this enormous interval of quiescence may, perhaps, never be ascertainable, but the interval must assuredly cover a large part of geological time. It was an era of geological calm, during which the Triassic, Jurassic and Cretaceous formations were slowly accumulated over the larger part of Europe. The stratigraphical quietude was not indeed unbroken. The widespread subsidence of the sea-bottom was interrupted here and there by important upheavals, and considerable geographical changes were in process of time accomplished. But, save in one or two widely separated areas of Europe, there were no active volcanoes over the whole continent.[439] Here again the scarcity or absence of intercalated volcanic rocks is in harmony with the general stratigraphy of the formations.

[Footnote 439: The Triassic eruptions of Predazzo and Monzoni were important, and traces of others are said to occur in the Cretaceous system in Portugal and Silesia.]

9. After the prodigious interval represented by the whole of the Mesozoic and the earlier part of the Tertiary formations, a time of disturbance arose once more, and the great basalt-floods of the north-west were poured forth. Evidence has been adduced in the foregoing chapters that this latest volcanic period was one of vast duration; that it was marked by long intervals of quiescence, and by repeated renewals of volcanic energy. Yet over the area of Britain the whole of its manifestations were probably comprised within the earlier (Oligocene and perhaps early Miocene) part of older Tertiary time. Since its eruptions ceased, another interval of profound quiescence has succeeded, which still continues. But this interval is almost certainly of less duration than that which elapsed between the Palæozoic and Tertiary outbursts. In other words, remote as the date of these Tertiary volcanoes appears to be from our own day, it comes much nearer to us than did the era of the last Permian eruptions to the earliest of the Tertiary series.

10. By the dissection which prolonged denudation has effected among the old volcanic centres of Britain, materials are supplied for studying the sequence of events from the beginning to the end of a volcanic period. These events have generally followed the same tolerably well-defined order.

In the case of fissure-eruptions, rents formed in the crust of the earth and communicating with the surface have allowed lava to rise and flow out above ground, either from the lips of the fissures or from vents opened along the lines of chasm. The thousands of parallel dykes in Britain remain as evidence of this mode of the ascent of the molten magma. Lines of large cones of the Vesuvian type may be presumed to have risen along guiding fissures in the terrestrial crust.

But it is evident from a study of the British examples that the existence of a fissure in the visible part of the crust is not always necessary for the production of a volcanic vent. In hundreds of instances, communication from the internal magma to the surface was effected by successive explosions, which finally blew out an orifice at the surface with no visible relation to any fissures or dykes. Of course, beneath the formations that now form the surface, and through which the necks rise, there may be lines of fault or weakness in older rocks which we cannot see. But, in what can be actually examined, vents have commonly been drilled through rocks independently of faults.

The discharge of explosive vapours was sometimes the first and only effort of volcanic energy. Generally, however, fragmentary volcanic materials were ejected, or, if the eruption was more vigorous, lava was poured out. In a vast number of cases, especially in the later ages of Palæozoic time, only ashes were projected, and cones of tuff were formed. In the earlier ages, on the other hand, there was a much larger proportion of lava expelled. Towards the close of a volcanic period, the vents were gradually choked up with the fragmentary materials that were ejected from and fell back into them. Occasionally, during the process of extinction, an explosion might still occur and clear the chimney, so as to allow of the uprise of a column of molten rock which solidified there; or the sides of the crater, as well as of the cavernous funnel underneath, fell in and filled up the passage. Heated vapours sometimes continued to ascend through the debris in the vent, and to produce on it a marked metamorphism.

There seems to have been commonly a contraction and subsidence of the materials in the vents, with a consequent dragging down or sagging of the rocks immediately outside, which are thus made to plunge steeply towards the necks.

When the vents were plugged up by the consolidation of fragmentary matter or the uprise of lava in them, the final efforts of the volcanoes led to the intrusion of sills and dykes, not only into the rocks beneath the volcanic sheets, but also, in many instances, into at least the older parts of the sheets themselves. These subterranean manifestations of volcanic action may be recognized in almost every district. They vary greatly in the degree to which they are developed. Sometimes, as in the Cader Idris, Arenig and Snowdon regions, they attain considerable importance, alike as regards the number and thickness of the sheets. In other cases, they are exhibited on so small a scale that they might be overlooked, as in the tract of Carboniferous puy-eruptions in the north of Ayrshire. But they are so generally present as to form a remarkably characteristic feature of the volcanic activity of each geological period from the earliest time to the latest. The basic sheets in the Dalradian series of Scotland display early and colossal examples. All through the successive eruptive periods of Palæozoic time, sills are found as accompaniments of superficial ejections.

The Tertiary basalt-plateaux supply numerous and gigantic examples of intruded sheets. Tertiary cones of Vesuvian type are not found in Britain, but where on the continent they have been sufficiently laid open by denudation, they present sometimes an astonishing series of sills. As a striking illustration of this structure reference may be made to the sheets of trachyte that have been injected between and have marmorized the Cretaceous strata on which Monte Venda stands, among the Euganean Hills.[440]

[Footnote 440: G. vom Rath, Zeitsch. Deutsch. Geol. Gesellsch., xvi. (1864), p. 461. E. Suess, Sitzungsber. k. Akad. Wien., lxxi. (1875), p. 7; Antlitz der Erde, vol. i. p. 193. E. Reyer, Die Euganeen, 1877. This volcano is further referred to, postea, p. 477.]

It is obvious that the time of intrusion of the sills cannot be precisely determined. They were not likely to be injected at an epoch when the volcanic magma could find ready egress to the surface. That they did not arise before such egress was obtained may be inferred from their petrographical characters, which are usually those of the later and not of the earlier outflows of the magma; and from the fact that they not only lie among the rocks below the volcanic series, but intersect the lower parts of that series, sometimes even the higher parts. We may therefore, with every probability, regard the sills as among the closing phases of a volcanic period.

As the lavas and tuffs of each volcanic period are intercalated among the successive geological formations, a definite beginning and end to the period are stratigraphically fixed. We see exactly where in the sedimentary series the first showers of ashes fell, and where the last mingled with the ordinary sand and mud of the sea-door. The same record shows that the volcanic accumulations were finally washed down, that they subsided with the rest of the ground around them, and that usually they were buried under overlying conformable sedimentary deposits. Thus cones of ashes and lava which may have been several thousand feet high completely disappeared.

10. A consideration of the distribution of the volcanic rocks in time shows not only how singularly uniform the course of volcanic activity has been, but that there is no evidence of the cessation of any of the broader petrographical types during geological history. Quite as much variety may be observed among the erupted materials of Tertiary time in Britain as among those of the early ages, when the earth was younger and its volcanic vigour might be supposed to have been greater and more varied than it is now. The table on the following page will make these features at once apparent. From this table it will be seen that while some of the acid rocks have not always been extruded, the basic masses have played their part in every volcanic period.

11. A study of the volcanic products of a long series of eruptions within the same geographical region may be expected to throw light on the changes that take place during the course of ages in the character of the internal molten magma. In a former chapter (vol. i. p. 27) reference was made to the subject of volcanic cycles and to the sequence, observed in various widely separated parts of the world, among the materials erupted from below. Allusion was likewise made in a later chapter (vol. i. p. 90) to the remarkable differences in texture and composition noticeable within some large bodies of eruptive material, and to the evidence which these differences furnish of a segregation or differentiation among the constituents of an eruptive mass after it has been injected into its position within the crust of the earth.

Table of the Periods of Volcanic Action in the British Isles and of the Chronological Distribution of the Volcanic Products.

Key to Columns ==================================== Gr = Granites, Granophyres, etc. Fe = Felsites, Rhyolites, etc. Da = Dacite, "Pitchstone" of Eigg. Tr = Trachytes. An = Andesites (Porphyrites). Ga = Gabbros. Do = Dolerites, Basalts (Diabases). Pi = Picrites and highly basic lavas. Tu = Tuffs, acid or basic.

+--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ | | Gr | Fe | Da | Tr | An | Ga | Do | Pi | Tu | +--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+ | Older Tertiary | | | | | | | | | | | (Plateaux, dykes, | | | | | | | | | | | necks, bosses, | | | | | | | | | | | sills) | * | * | * | * | * | * | * | * | * | | | | | | | | | | | | | Mesozoic | | | | | | | | | | | No volcanic rocks.| | | | | | | | | | | | | | | | | | | | | | Permian | ··· | * | ··· | ··· | * | ··· | * | * | * | | | | | | | | | | | | | Carboniferous | ? | | | | | | | | | | Puy type | ··· | * | ··· | ··· | * | ··· | * | * | * | | Plateau type | ··· | * | ··· | * | * | ··· | * | * | * | | | | | | | | | | | | |{Devonian | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| * | |{ | | | | | | | | | | |{Old Red Sandstone | | | | | | | | | | |{ Upper | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| * | |{ Lower | * | * | ··· | * | * | ··· | * | ···| * | | | | | | | | | | | | | Silurian | | | | | | | | | | | Upper | ··· | * | ··· | ··· | ··· | ··· | ··· | ···| * | | Lower, Bala | * | * | ··· | * | * | * | * | * | * | | " Arenig | * | * | ··· | * | * | * | * | ···| * | | | | | | | | | | | | | Cambrian | ··· | * | ··· | ··· | * | ··· | * | ···| * | | | | | | | | | | | | | Uriconian | ··· | * | ··· | ··· | ··· | ··· | * | ···| * | | | | | | | | | | | | | Dalradian | ··· | ··· | ··· | ··· | ··· | ··· | * | ···| ? | | | | | | | | | | | | | Torridonian | | | | | | | | | | | | | | | | | | | | | | Lewisian | * | ··· | ··· | ··· | ··· | ··· | * | * | ··· | | | | | | | | | | | | +--------------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+

From the history of volcanic action in the British Isles it is clear that differentiation is effected under three distinct conditions.

In the first place, a notable difference may be occasionally observed between two adjacent parts of the same mass of lava which has flowed out at the surface. Thus, in the Carboniferous picrite of Blackburn, there has been a separation of the heavy basic constituents, which have in great part settled down into the lower part of the sheet, while the lighter felspar has mainly come to the top. In this case the gradual transition from top to bottom suggests that the separation occurred after the lava had reached the surface and taken the form of a stream or sheet.

In the second place, segregation has taken place in the magma within the terrestrial crust after intrusion, for it is frequently observable in large bosses and sometimes in sills, the basic elements having tended to mass themselves towards the margins of the rock, leaving more acid material in the centre. The cases of Garabol Hill among the Dalradian schists of Scotland, of Carrock Fell among the Silurian strata of the Lake District, and of the Cramond picrite among the Carboniferous formations of Midlothian, with others that might be cited from various other regions and geological formations in Britain, prove to what a considerable extent a separation of ingredients may take place in a boss, and even sometimes in a comparatively thin sill before the molten mass consolidates.

In the third place, there is good evidence that already before the magma is either intruded or extruded, and while it still lies within the internal reservoir, it may not possess a general uniformity of composition, but may have become more or less heterogeneous. In regard to intrusive rocks, the extraordinarily banded gabbros of the Tertiary series of Skye obviously proceeded from a magma in which the molten material consisted in some parts mainly of felspar, and in others mainly of the ferro-magnesian minerals and iron-ores. Streams from these differently constituted parts of the magma were simultaneously or successively injected as sills into the older portions of the volcanic series, while, as the process of differentiation within the magma proceeded, still more felspathic liquid was left behind, to be thrust into cracks in the sills previously consolidated.

Moreover, the banded basalts of the Tertiary plateaux show that this heterogeneity was not confined to internal intrusions, but maintained its place even when the molten material was ejected to the surface. The differentiation indeed is not so striking there as among the sills of gabbro; but its presence, even in a less degree, proves that the separation of constituent minerals was not due to any general cooling of an erupted body of igneous rock, but was already developed in the reservoir from which the molten material was propelled to the surface.

Attention has been called to the remarkable similarity of structure between these banded intrusive rocks and some of the ancient gneisses. The resemblance is so close that we may with every probability infer that the gneisses acquired their characteristic banding as intrusive masses of igneous rocks, discharged from heterogeneous magmas, like that which supplied the gabbros of the Cuillin Hills. And as these gneisses belong to pre-Cambrian formations, we are thus led to the interesting result that the tendency to develop heterogeneity was already as characteristic of the magma-basins of the earliest geological time as it has been of those of later periods.

The evidence of differentiation presented by superficial lavas, and by intrusive sills and bosses, acquires great interest when considered in connection with the changes which are seen to have occurred in the character of the materials erupted during the course of a definite volcanic period. An attentive examination of the volcanic products of the various ages, so fully recorded in the geological structure of the British Isles, shows that a recognizable sequence in the nature of the materials erupted during a single volcanic period can be traced from the earliest to the latest times, and that, in spite of occasional departures, the normal order remains broadly uniform.

With the important exception of the Snowdonian region and possibly others, we find that the earlier eruptions of each period were generally most basic, and that the later intrusions were most acid. Thus the diabase-lavas and tuffs at the base of the Cambrian series of St. David's are pierced by quartz-porphyry veins. The andesites of the Lower Old Red Sandstone were succeeded by bosses, sills, and dykes of granite, felsite, and lamprophyre. The eruptions of the Carboniferous plateaux began with extremely basic lavas, and ended with trachytes, felsites, and quartz-porphyries. The basalts of the great lava-fields of the Tertiary period are pierced by masses of granophyre and even granite.

There has evidently been, on the whole, a progressive diminution in the quantity of bases and a corresponding increase in the proportion of acid in the lavas erupted during the lapse of one volcanic period. This sequence is so well marked and so common that it cannot be merely accidental. The acid and basic rocks, occurring as they do at each volcanic centre in the same relation to each other, are obviously parts of one connected series of eruptions. We seem to see in this sequence an indication of what was taking place within the subterranean magma. There was first an extensive separation of the more basic constituents, such as the ferro-magnesian minerals and ores, and the lavas which came off at that time were heavy and basic basalts, and even picrites. The removal of these elements left the magma more acid, and such rocks as andesites were poured out, until at last the deeper intrusive sills, dykes and bosses became thoroughly acid rocks, such as felsite, quartz-porphyry and granite, while if any superficial outflow took place it was such a rock as dacite.

In the case of the Tertiary volcanic series there is evidence that after the acid protrusions a final uprise of basic material occurred. No satisfactory proof of any similar return to basic eruptions has been detected among the Palæozoic formations. But it is possible that some of the basic sills and dykes, the precise age of which cannot be fixed, may really mark such a reversion, even in the earlier volcanic periods.

Some illustrative examples of volcanic cycles from other countries were cited in Chapter iii. To these I may add another instance which presents a close analogy to some of the phenomena characteristic of the British examples of Palæozoic as well as of Tertiary age. Monte Venda in the Euganean Hills, already alluded to (p. 474), may be cited as an interesting specimen of an older Tertiary volcano, which has been so dissected by denudation as to show not only the succession of its superficial discharges, but the position and order of its subterranean intrusions. The volcanic eruptions of this neighbourhood, judging from the area which they still cover and the height they reach, may have piled up a mountain rivalling or surpassing Etna in dimensions. In Monte Venda the lowest visible igneous rocks are sills of oligoclase-trachyte that have been thrust between and have highly altered Cretaceous (Tithonian) limestones. Other intrusive sheets of trachyte follow in the overlying Cretaceous strata (Neocomian and Scaglia). It is not until the older Tertiary formations are reached that undoubted tuffs and lavas occur, indicative of truly interstratified volcanic materials. These formations, consisting of nummulitic limestones and other strata together with fossiliferous tuffs, show that the volcano began as a submarine vent. It discharged dark basic dolerites and tuffs. The highest lava, however, crowning the summit of the mountain is a trachyte. There appears to have been a rapid decrease of the bases in the magma, for the later lavas were rhyolites, accompanied with rhyolitic tuffs of Oligocene age, and followed in the end by the black vitreous trachyte of Monte Sieva.

12. From the evidence detailed in these volumes, it appears that the sequence from basic to acid discharges was on the whole characteristic of each eruptive period. It is obvious, however, that as the protrusions of successive periods took place within the same limited geographical area, the internal magma during the interval between two such periods must in some way have been renewed as regards its constitution, for when, after long quiescence, eruptions began once more, basic lavas appeared first and were eventually followed by acid kinds. This cycle of transformation is admirably exhibited in Central Scotland, where the andesites of the Old Red Sandstone with their felsite sills are followed by the limburgites, picrites and other highly basic lavas at the bottom of the Carboniferous plateaux, succeeded in turn by the andesites, trachytes and acid sills of that series. When the puy eruptions ensued, the magma had once more become decidedly basic.

That the true explanation of these alterations is of a complex order may be inferred from the exceptions which occur to the general rule. I have alluded to the Snowdon region, where the acid rhyolites are followed by more basic andesites, and where the sills are also more basic than the superficial lavas. In the Arenig and Cader Idris country the sills are likewise more basic than the bedded lavas. Among the Carboniferous puys of the basin of the Firth of Forth, the sills are not sensibly more acid than many of the superficial basalts, and they even include such rocks as picrite. Possibly in this last-named region we see an arrested sequence, the volcanic protrusions having from some cause ceased before the general uprise of the more acid magma.

INDEX

Aa form of lava in the Sandwich Islands, ii. 187 Abereiddy Bay, i. 206 Abich, H., i. 32 Acid igneous rocks, silica percentage of, i. 14; devitrification of, 19; flow-structure of, 21; occur in thicker sheets than basic, 24; alternations of, with basic, 28, 61, 152, 157, 165, 207, 213, 233, 284, 318; ii. 236, 266, 278; metamorphic action of, i. 95, 96; connection with mountains, ii. 98; scenery of, 102. Acids, mineral, at volcanoes, i. 72 Acland, Mr. H. D., i. 133 Aegean Sea, volcanoes of, i. 1 Agglomerates, i. 31, 57, 58; in dykes, 70; Archæan, 120, 130, 135; Cambrian, 148, 149, 167; Silurian, 178, 180, 181, 184, 185, 194, 199, 206, 214, 237, 241, 244, 247, 253, 255; Old Red Sandstone, 279, 285, 289, 300, 313, 325, 338, 349, 352; Carboniferous, 381, 399, 402, 404, 427, 429, 439, 440; ii. 13, 24, 28, 29; Permian, 62, 64, 99; Tertiary, 194, 277, 278, 281, 289, 292, 293, 384, 400, 423 Allan, T., i. 363 Allotriomorphic minerals, i. 21 Allport, Mr., i. 95, 130, 131, 260, 451; ii. 11, 42, 102, 103, 104, 106, 370 Amber in Tertiary volcanic series, ii. 198 America, Western North, volcanic rocks of, i. 10, 100; ii, 267 Amygdales, origin of, i. 15; ii. 189, 221, 285, 290 Amygdaloidal structure, i. 15, 16, 17, 59, 274, 385; ii. 3, 31, 57, 129, 188 Analyses of Cambrian tuffs, i. 148, 149; of Cambrian diabases, 153; of Old Red Sandstone diabases, 274; of Old Red Sandstone andesites, 275; of Old Red Sandstone trachytes, 276; of Old Red Sandstone felsites, 278; of Carboniferous limburgite, 377; of Carboniferous basalts, 379; of Carboniferous trachytes, 380; of Carboniferous phonolite, 381; of Tertiary trachyte, ii. 139; of Tertiary dacite, 244 Anderson, Dr. Tempest, ii 261, 262, 263 Andesite, i. 24, 131, 136, 164, 165, 167, 178, 180, 184, 189, 190, 204, 212, 213, 214, 215, 229, 230, 245, 246, 247, 252, 274, 275, (analyses), 277, 292, 300, 306, 309, 315, 318, 325, 330, 333, 345, 377, 379, 386, 403, 421; ii. 45, 57, 96, 125, 137, 184, 236, 424 Anglesey, gneisses and schists of, i. 126; volcanic rocks of, 189, 219 Anhydrite deposits, ii. 54 Annandale, Permian volcanic rocks of, ii. 56, 58, 60, 61, 66 Antrim, Old Red Sandstone volcanic rocks of, i. 314; Tertiary volcanic rocks of, 47, 52; ii. 109, 110, 113, 139, 140, 199; basalts of, 192, 193, 199, 202, 206; clays and iron-ore of, 204; rhyolites of, 185, 364, 370, 371, 426, 445; deceptive agglomerate of, 188; rhyolitic conglomerate of, 195, 206; plateau of, 199; tuffs of, 202, 204; vents of, 271, 277; sills of, 298; central subsidence of basalt-plateau of, 448 Apatite, ii. 135 Apjohn, J., ii. 42 Applecross, volcanic vents in, ii. 292 Arans, the, i. 175, 176, 179, 184, 186, 207 Archæan period, i. 110, 111; volcanic rocks of, 120 Ardnamurchan, dykes and veins of, ii. 154, 320; basalt-plateau of, 208; vents of, 287; sills of, 318; gabbro of, 355 Arenig group, i. 175; lower limit of, 177, 185; top of, 178, 228, 246 ---- volcano of, i. 42, 175, 176, 179, 186, 207 ---- rocks in Scottish Highlands, i. 123, 126; in Merionethshire, 176, 179; of Shropshire, 189; of Ayrshire, 196; of Scottish Highlands, 201; of Anglesey, 221; of Lake district, 229; of Ireland, 239 Argyll, Duke of, ii. 113, 114, 198 Argyllshire, dykes of, ii. 127, 128, 138, 142, 146, 171, 172; vents of, 278 Arizona, explosion crater in, i. 58; laccolites in, 86 Arran, Old Red Sandstone volcanic rocks of, i. 298, 311; Carboniferous volcanic rocks of, 386, 392; possible Permian volcanic rocks of, ii. 58; granite of, i. 93; ii. 366, 367, 418; pitchstone of, i. 19; ii. 445; dykes of, 123, 139, 140, 142, 146, 154, 161 Arthur Seat, i. 364, 373, 378, 385, 386; ii. 67 "Arvonian," i. 145, 156 Asbestos in volcanic breccia, ii. 51 Ascension Island, cellular lava of, i. 15 Ashes, volcanic (see Tuffs) Ashprington volcanic series, i. 262 Asphalt, ii. 79 Atherstone, i. 170 Augite, loose crystals of, in volcanic vents, i. 62, 178, 181; ii. 58, 79; lumps of, in volcanic vents, i. 352 Augite-aphanites, i. 178 Auvergne, old volcanoes of, i. 29, 32, 66, 70, 100; ii. 373 Aveline, Mr. W. T., i. 227, 230; ii. 32 Ayrshire, example of volcanic neck in, i. 56; Silurian volcanic rocks of, 192; Old Red Sandstone volcanic rocks of, 275, 282, 283, 285, 291, 331; Carboniferous volcanic plateau of, 102, 368, 388, 393, 398, 410; Carboniferous Puys of, 415, 416, 434, 440, 474; Permian volcanic rocks of, ii. 55, 58, 62 Azoic period, i. 109

Bäckström, Mr., ii. 266 Baily, W. H., i. 251, 252; ii. 198, 449 Bala group, i. 175, 190, 196, 201, 206, 207, 223, 242; limestone of, 47, 175, 229, 245, 251; volcanic rocks of, 186, 190, 207, 213, 221, 241, 248 Balbriggan, igneous rocks of, i. 244 Ballagan beds (Lower Carboniferous), i. 384, 387, 392, 393, 412, 447 Ballantrae, volcanic rocks at, i. 192, 199 Ballypallidy, tuffs and leaf-beds of, ii. 204, 429 Bamborough, Whin Sill at, ii. 2, 3, 5 Banding of igneous rocks, i. 84, 207; ii. 189, 294, 329, 354, 357, 476 ---- of gneiss, i. 116 Bangor group, i. 166 Banks, Sir Joseph, ii. 109 Barnavave, eruptive rocks of, ii. 421 Barrow, Mr. G., i. 201, 226, 272, 279, 380; ii. 147, 148 Basalt, columnar structure of, i. 24, 25; relation to gabbro, 78; altered by carbonaceous strata, 95; shells supposed to occur in, ii. 110; banded, 189; thickness of sheets of, 192; meaning of red layer between sheets of, 197, 203, 206, 254; metamorphism of, 272, 276, 337, 339, 340, 347, 355, 356, 357, 358, 362, 378, 383, 386, 397, 399, 400, 404, 413

---- pre-Cambrian, i. 119, 131; Silurian, 206, 207, 230, 245; Carboniferous, 378, 403, 407, 417; ii. 11, 45, 46; Permian, 57, 96; Tertiary, 125, 136, 183, 199, 208, 291

Basalt-conglomerate, ii. 195 Basic volcanic rocks, silica-percentage of, i. 14; devitrification of, 20; flow-structure of, 21; occur in thinner sheets than the acid, 24; metamorphic action of, 94; erupted at low levels, 98; scenery of, 102; converted into schists by deformation, 75, 114, 118, 119, 124, 129; alternation with acid, 28, 61, 131, 157, 165, 207, 213, 233, 284, 318; ii. 236, 266, 278 Bass Rock, i. 372, 373, 403 Bassenthwaite Lake, i. 335 Bathgate, puy eruptions of, i. 440, 442, 445, 456, 461 Bauer, Dr. M., i. 62 Bauxite, ii. 197, 204 Bayley, Mr. W. S., ii. 330 Bedding in lavas, i. 24 Bell, Sir I. Lowthian, ii. 1, 113, 137, 165 Bemrose, Mr. H. A., ii. 10, 11, 13, 16, 17, 18, 20, 21 Ben Cruachan, alteration of granite at, i. 343 ---- Hiant, basic sills of, ii. 318 Benaun More, felsite of, i. 347 Berger, J. F., ii. 22, 95, 110, 113, 139, 140, 141, 145, 199, 364, 426 Bertrand, Prof. M., i. 28 Berwickshire, i. 272, 290, 338, 375, 385, 401, 413 Berwyn Hills, i. 176, 186, 208, 218 Biggar, volcanic area, i. 287, 325 Binney, E., ii. 56 Binny Craig type of basalt, i. 419, 421 (444) Biotite (see Mica) Bitumen in intrusive rocks, i. 421 Blackstone (Derbyshire), ii. 18, 21 Blair-Atholl Limestone, i. 122 Blake, Rev. J. F., i. 126, 130, 144, 160, 161, 162, 163, 165, 166, 168, 220, 221, 222 Blocks, ejected, i. 36, 423, 438; ii. 197, 221 Bole between lavas, i. 442; ii. 197, 203, 206, 254 Bombay, volcanic plateau of, ii. 180 Bombs, volcanic, i. 60; ii. 39 Bonney, Prof., i. 95, 126, 130, 136, 144, 160, 162, 163, 164, 165, 166, 167, 168, 192, 210, 227 Borrowdale Volcanic Series, i. 227 Bosses, volcanic, i. 56, 78, 88; petrography of, 89; differentiation in, 90; ii. 476; granitic, i. 93; metamorphism around, 94, 95; conditions of their intrusion, 97, 98; weathering of, 102 ---- Silurian, i. 215, 235; Old Red Sandstone, 277, 288; Carboniferous, 403, 458 ---- Tertiary, ii. 271, 284, 327, 366, 378, 395, 403; boundaries of, 382; relation to older eruptive vents, 280, 384, 399; relation to plateau basalts, 386, 396, 402, 404; relation to gabbro intrusions, 391, 402, 404; relation to the basic dykes, 395 Bostonite, ii. 47 Boué, Ami, i. 268, 363; ii. 112, 372 Boule, M., i. 27, 29, 44, 45, 46, 61; ii. 375 Boutan, M., i. 62 Bowden Hill, type of doleritic basalt, i. 418, 421 Braid Hills, great vent of, i. 289, 293, 311, 318, 323 Branco, Prof. W., i. 46, 417 Breccias, volcanic, i. 31, 32, 120, 131, 135, 147, 165, 189, 190, 197, 213, 224, 225, 233, 234, 246, 252, 255, 289, 347; ii. 39, 41, 49, 195 ---- of non-volcanic materials, ii. 196, 423 Brecciated structure, i. 162, 211 Breidden Hills, i. 176, 190, 208 Brent Tor, ii. 33, 35, 36 Bréon, M. R., ii. 191 Britain, advantageous position of, for the study of ancient volcanic action, i. 6; completeness of the Geological Record in, 6; direction of folds and fractures in, 11; chief lavas found in, 31; Vesuvian cones of, 42; volcanic plateaux of, 43; puys of, 46; lacustrine volcanoes of, 49; fissure eruptions of, 52; scenery of volcanic rocks of, 100, 101; pre-Cambrian rocks of, 111; in Cambrian time, 141; in Silurian time, 173; in Devonian time, 258; in Old Red Sandstone time, 263; in Carboniferous time, 355; in Permian time, ii. 53; in older Tertiary time, 108 Brögger, Prof., i. 28, 88, 90, 91, 92 Bryce, J., i. 314, 369 Buch, L. von, i. 27; ii. 381 Buckland, W., ii. 95, 110, 113 Buddle, J., ii. 113 Builth, i. 176, 203 Burdiehouse Limestone, i. 361, 374, 388, 415, 463 Burnt Country of Asia Minor, i. 2 Burntisland, Binn of, i. 428, 429, 433, 435, 457, 459 Burntisland Sill type of dolerite, i. 418, 421 Busz, Mr. K., i. 261 Bute, Isle of, i. 369, 378, 407

Cadell, Mr. H. M., i. 114, 423; ii. 334 Cader Idris, volcanic rocks of, i. 42, 175, 176, 177, 178, 179, 180, 181, 182, 188, 207 Caer Caradoc, i. 131, 132, 170 Caerfai group (Cambrian), i. 155 Caernarvonshire, volcanic rocks of, i. 159, 207 Caithness Flags, i. 343, 352 ---- volcanic vents in, i. 352 Calciferous Sandstones, i. 361, 366, 415 Calcite as a matrix of tuffs, ii. 27, 39, 41 Caldecote volcanic rocks, i. 170 Callaway, Dr. C., i. 126, 130, 132, 134, 220, 221 Calton Hill, lavas and tuffs of, i. 373, 378, 385, 386, 389 Cambrian system, i. 112, 123, 133, 139, 143, 144; volcanoes of, 145, 159 Campbeltown, volcanic rocks of, i. 312, 386 Campsie Fells, i. 102, 368, 369, 384, 386, 389, 393, 397, 398, 400, 403, 410, 412, 447 Canary Islands, i. 27 Canna, basalts of, ii. 184, 187, 190, 215, 216; vent in, 288 Cantyre, volcanic rocks of, i. 311, 369, 370, 386 Caradoc group, i. 175, 196 Carbonaceous rocks, influence of, on igneous masses, i. 95, 426, 449, 456; ii. 65, 87, 104, 165 Carboniferous Limestone, origin of, i. 357 ---- system, subdivisions of, in Britain, i. 358, 360, 366; ancient geography of, 355, 361, 362, 432, 462; flora and fauna of, 356 ---- volcanic plateaux, distribution of, i. 364, 367; nature of materials constituting, 377; structure of, 383; bedded lavas and tuffs of, 383; vents of, 54, 394, 399 ---- Puys, i. 46, 47, 308, 364; of Scotland, 414; nature of the materials erupted by, 416; necks of, 424; bedded lavas and tuffs of, 417, 436, 440; sills of, 446, 472; bosses of, 458, 465; dykes of, 460; of Derbyshire, ii. 8; Isle of Man, 22; of Somerset, 32; of Devonshire, 32; of King's County, 37; of Limerick, 41 Carlingford, igneous rocks of, i. 96; ii. 175, 371, 420 Carnedd Dafydd, i. 209 Carnmony Hill, ii. 272 Carrock Fell, differentiation in rocks of, i. 91; metamorphism at, 94, 96; as a volcanic boss, 235, 236 Cement-stone group, i. 362, 366, 387, 418, 462 Cellular structure of volcanic rocks, i. 15, 33 Chalk, metamorphism of, by a dyke, ii. 164 Champernowne, A., i. 260, 262 Charnwood Forest, i. 134; ii. 53 Cherts associated with volcanic rocks, i. 123, 167, 169, 173, 174, 184, 196, 197, 240, 254; ii. 25, 36 Cheviot Hills, i. 102, 271, 272, 274, 275, 277, 278, 290, 293, 336 Chilled margin in intrusive rocks, i. 81, 83; ii. 126, 158, 160, 172, 299, 303, 310, 317, 321, 392, 402 Christiania, eruptive rocks of, i. 28 Chronology, volcanic, how determined, i. 46 Clark, Mr. G. T., ii. 180 Claystone, i. 277, 279, 318, 324, 327; ii. 403 Cleavage, effects of, on igneous rocks, i. 162, 165, 224, 231, 232, 234, 237, 260, 261; ii. 36 Clee Hills, ii. 101, 102 Cleveland Dyke, ii. 1, 122, 139, 140, 142, 144, 146, 147, 150, 153, 167, 168, 169 Clough, Mr. C. T., i. 114, 201, 236, 274, 290, 337; ii. 123, 124, 127, 128, 132, 137, 138, 142, 145, 146, 152, 162, 171, 172, 316, 384, 437 Clyde, Carboniferous volcanic plateau of, i. 368, 384, 385, 393, 400, 407, 411 Coal interbedded among volcanic rocks, i. 392, 423; ii. 198, 213, 251, 287 ---- alteration of, at volcanic vents, i. 72; ii. 64 ---- alteration of intrusive rocks by, i. 95, 451 ---- alteration of by sills, dykes, etc., ii. 67, 164, 166 Coal-measures, i. 358, 360, 366 Coalbrookdale Coal-field, ii. 103 Cole, Prof. G. A., i. 176, 177, 178, 179, 180, 181, 184, 187, 188, 210, 211; ii. 134, 205, 212, 245, 370, 371, 378, 426 Colorado, Grand Cañon of, i. 30; laccolites of, 86 Columnar structure, i. 25, 27, 343, 378, 385, 459; ii. 164, 186, 206, 301 Comley Sandstone, i. 144 Cones, volcanic, connection of, with necks, i. 70; contemporaneous denudation of, 73; ii. 202, 218, 230; entombment of, i. 433, 463; ii. 66 Conglomerates, volcanic, i. 31, 37, 183, 190, 286, 300, 307, 309, 310, 314, 315, 330, 341; ii. 195, 198, 218, 284 Coniston Limestone, i. 229, 231 Contemporaneity in Geology, 201 Continents, origin of, i. 11 Contraction, effects of terrestrial, i. 12, 97, 98 Conybeare, J. J., ii. 95 ---- W., i. 171; ii. 9, 95, 110, 113, 199 Cooling, effects of, in inducing varieties of texture in igneous rocks, i. 78, 79, 81; ii. 274, 275, 299, 303, 310, 317, 392, 402 Coon Butte, Arizona, i. 58 Cork, County, volcanic breccias of, ii. 49 Corndon, sill of, i. 176, 189, 190 Corston Hill, i. 373, 386, 387 Craiglockhart type of dolerite and basalt, i. 418 Crater, consolidation of tuff within a, i. 429 Crater-lakes, i. 58; ii. 266, 275 Cretaceous period, geography of the, ii. 108, 182 Cross Fell, i. 228, 229, 238 Cross, Mr. Whitman, i. 86 Crush-conglomerates or breccias, i. 32, 220, 223, 225, 244; ii. 281, 347, 352 Crushing, mechanical effects of, i. 315 (see Schist) Crust, contraction of the terrestrial, i. 12, 97, 98; oldest rocks of, 110; deformation of, 117, 121, 264, 295, 297 Cryptocrystalline type of basalt, i. 419 Crystallites of volcanic rocks, i. 18 Crystals, different periods of formation of, in volcanic rocks, i. 19, 20, 21, 421; ii. 128, 131, 134, 135; ejected by volcanic vents, i. 62, 178, 180, 181, 195, 213, 234, 245; ii. 27, 49, 58, 79 Cuillin Hills, scenery of, i. 106; gabbro of, ii. 329, 361; acid rocks of, 391 Culots, i. 78, 88 Culm-measures, ii. 33 Cumbrae Islands, i. 368, 369, 378, 407 Cumming, J. G., ii. 22 Cycles, volcanic, i. 27, 92; ii. 116

Dacite, i. 230; ii. 185 Dakyns, Mr. J. R., i. 90, 229, 272; ii. 10 Dalmellington, volcanic rocks at, i. 333; ii. 62 Dalmeny type of dolerite and basalt, i. 418, 420 Dalradian rocks, probable crushed necks of, i. 75, 125; lavas and sills of, 121; green schists of, 124 Dalry, Ayrshire, buried volcanoes of, i. 434 Dana, J. D., ii. 189 Darwin, C., i. 27 Daubrée, A., i. 72, 404 Davies, J., i. 156, 157 Dechen, H. von, i. 46; ii. 112, 280, 333, 340, 367, 372, 381 Deformation, effects of, on volcanic rocks, i. 75, 115, 117, 119, 121, 127, 129, 162 De la Beche, H., i. 142, 143, 170, 175, 204, 205, 207, 259; ii. 9, 10, 19, 33, 95, 96, 97 Delessite, ii. 79 Denudation, influence of, on volcanoes, i. 3, 4, 8, 40, 43, 45, 46, 54, 58, 71, 73, 75, 79, 87, 100-107, 370, 433, 434, 436, 476; ii. 55, 61, 62, 179, 181, 182, 241, 245, 248, 249, 255, 257, 282, 283, 292, 316, 317, 363, 373, 407, 455 Derbyshire, toadstones of, i. 359; ii. 8 Desmarest, ii. 373 Devitrification of volcanic rocks, i. 18, 19, 78; ii. 437, 446 Devonian system, i. 257; volcanoes of, 259 Devonshire, volcanic scenery of, i. 103; Devonian volcanic rocks of, 259; Carboniferous volcanic rocks of, ii. 32; Permian volcanic rocks of, 94 Diabase, i. 151, 153 (analyses), 156, 192, 194, 204, 206, 214, 217, 235, 240, 247, 249, 273 (analyses), 278, 292, 318, 320, 330, 335, 344, 345, 351, 403; ii. 5, 136, 415 Diabase-porphyrite, i. 192, 204 Diamond found in volcanic vents, i. 62 Dick, Mr. A., jun., i. 380 Dickson, Mr. E., ii. 23 Differentiation in igneous rocks, i. 22, 27, 84, 85, 90, 91, 449; ii. 300, 475 "Dimetian," i. 145 Dingle-beds, i. 346 Dingle, Upper Silurian nodular lavas of, i. 20, 254 Diorite, i. 78, 247, 249, 277, 278, 288; ii. 36 Dirrington Law, i. 290 Dittmar, Prof., ii. 137 Dolerite, i. 119, 134, 178, 190, 206, 230, 247, 261, 378, 403, 407, 417, 448; ii. 5, 11, 35, 49, 102, 103, 104, 125, 136, 157, 183, 271, 299, 303, 307, 319, 328 Dolgelli, i. 169, 178, 188 Donegal, Dalradian rocks of, i. 122; dykes in, ii. 124 Drogheda, volcanic rocks near, i. 244 Duffin, W. le S., ii. 426 Dumbarton, rocks near, i. 402, 404 Dumfoyn, a volcanic neck, i. 395, 398, 400 Dumgoyn, a volcanic neck, i. 395, 397, 398, 400 Dundee, sills and bosses near, i. 292, 306 Duneaton Water, volcanic rocks of, i. 329 Dunite, ii. 309 Du Noyer, G. V., i. 245, 250, 254; ii. 272, 426 Durham, Mr., i. 275 Durness Limestone, i. 112, 121, 123, 141 Dust, volcanic, i. 13 Dutton, Capt. C. E., i. 68; ii. 267 Dykes, vitreous margins of, i. 18; formation of, 54, 98; in necks, 66; filled with agglomerate, 70; grouping of, among intrusive rocks, 77; character of, 79; extent of, in Britain, 80; age of, 81; compound, 81; ii. 59; expulsion of lava from, i. 82; ii. 128; connected with the surface, i. 82; pre-Cambrian, 118; flow-structure in, 161; Cambrian, 156; Silurian, 187, 216, 235, 237, 248, 249; Old Red Sandstone, 277, 291, 338, 345; Carboniferous, 406, 429, 460; ii. 1, 30; Permian, 83; amygdaloidal structure of, 85 ---- Tertiary, ii. 114; arguments for their geological age, 118, 125, 171; geographical distribution, 121; two types of protrusion of, 122; nature of component rocks of, 125; external character of, 126; classification of basic, 129; enclosed fragments in, 129, 131, 144; porphyritic and amygdaloidal structures of, 128, 129, 130; veins in, 130; joints in, 132, 166; microscopic characters of, 134; chemical characters of, 137, 139; hade of, 139; breadth of, 139; interruptions of, 142; length of, 142; persistence of mineral characters of, 144; direction of, 145, 159; upward termination of, 147; known vertical extension of, 150; evidence of movement of molten rock of, 151; branches and veins from, 152; connection with sills, 155; intersecting, 158; compound or of more than one infilling, 159; double, treble, and multiple, 160, 318, 417, 439; compound, with basic and acid bands, 161, 435; contact metamorphism of, 163; relation of, to geological structure, 166; origin and history of, 175; Icelandic example of, 261; example communicating with cinder cone in Utah, 268; connection of, with surface, 179, 269, 280; latest protrusions of, 381, 408, 416; of granophyre, 435, 436, 437, 439

Earth, condition of the interior of the, i. 10; fractures in crust of the, 11 Earthquakes, influence of, on early man, i. 1; transient effects of, 3, 8 East Lothian, trachyte lavas of, i. 24; Carboniferous volcanic plateau of, 370, 389, 403, 409 Edinburgh, volcanic rocks near, i. 24, 102, 104, 269, 273, 276, 279, 281, 285, 287, 289, 291, 293, 311, 317, 318, 323, 364, 370, 373, 385, 386, 387, 389, 410, 420, 436, 449 Egan, Mr. F. W., i. 242; ii. 201, 423 Eifel, i. 4, 46, 58, 100 Eigg, Isle of, ii. 115; pitchstone of, 185, 217, 242, 445; brecciated basalt in, 189, 192; basalt plateau of, 215, 234; Scuir of, 217, 234, 447; sills of, 318; acid bosses of, 403; acid sills of, 431; proofs of subsidence at, 447; enormous denudation of, 239, 447, 458 Eildon Hills, i. 375 Electric Peak, i. 79, 82, 84 Elvans, i. 249, 281 Engulphment craters, i. 58 Ennerdale, granite of, i. 236 Enniscorthy, volcanic rocks near, i. 245 Environment, influence of, on early man, i. 1 Eozoic period, i. 110 Epidiorite, i. 118, 124, 129, 184, 247, 249 Erosion, laws of, i. 101 Eruptions, transient effects of, i. 3; old submarine, how ascertained, 48; lacustrine, 49; fluviatile, 49; terrestrial, 50; evidence of intervals between, 283, 287, 300, 442; ii. 42, 59, 203, 205, 221, 251, 254, 287 Erzeroum, old volcanoes near, i. 32 Eskdale Dyke, ii. 127, 133, 136, 137, 140, 143, 145, 146, 153 ---- (Lake District), granite of, i. 236 Etheridge, Mr. R, jun., ii. 24 Etna, i. 2, 4, 10, 55; ii. 261 Eurite, i. 188 Europe, basalt plateaux of north-western, i. 51, 52; ii. 181; pre-Cambrian disturbances of north-western, i. 117 Explosion-craters, i. 58; ii. 266 Explosions, volcanic, i. 246; ii. 196, 266, 425, 472 Extrusive rocks, defined, i. 14; textures of, 78

Fair Head, sills of, ii. 301 Farey, J., ii. 9 Farne Islands, ii. 2 Faroe Isles, basalt plateaux of, i. 52, 102; ii. 191, 192, 194, 256; vents in, i. 63; ii. 293; dykes of, 122, 133; tuffs and lignites of, 258; sills of, 322; absence of gabbro bosses in, 355; subsidence of, 447; dip of basalts in, 448; proofs of denudation in, 458 Faujas St. Fond, ii. 109, 112 Faults, connexion of volcanic vents with, i. 69; ii. 65; boundary, i. 294, 303, 305, 369; ii. 169; effects of, i. 446; ii. 200; connection with, dykes, 168 ---- of Tertiary basalt-plateau, ii. 452 Felsite (Felstone), Torridonian, i. 120; Uriconian, 130, 133; of Malverns, 134; Cambrian, 151, 160, 161, 164, 165, 167, 168; Silurian, 184, 199, 205, 206, 207, 210, 212, 218, 231, 232, 246, 247, 252, 255; Old Red Sandstone, 276, 277, 291, 293, 321, 327, 335, 346; Carboniferous, ii. 36, 49; Permian, 85; Tertiary, 174, 369, 424, 446 Felsitic breccia, ii. 195 ---- type of devitrification, i. 19 Felspar, ejected crystals of, i. 181; ii. 58, 79; large porphyritic crystals of, in dykes, 129, 135 Fife, Old Red Sandstone volcanic rocks of, i. 307; Carboniferous volcanic rocks of, 428, 429, 430, 433, 437, 448; Permian volcanic rocks of, ii. 56, 69 Fingal's Cave, i. 25; ii. 210 Fisher, Rev. O., i. 98 Fishguard, volcanic rocks at, i. 205 Fissure type of volcanoes, i. 42, 52; ii. 108, 115, 267 Fissures, volcanic, i. 42, 52, 53, 54, 425; ii. 141, 145, 159, 176; filled with agglomerate, i. 70; filled with dykes, 81, 118; compound, 82, ii. 159; pre-Cambrian, i. 118, 119; Carboniferous, 425; Tertiary, ii. 141, 159, 176, 425; modern of Iceland, 262; cause of, 177 Fleming, John, i. 268 Flow-structure, i. 21, 157, 160, 161, 162, 184, 210, 232, 246, 248, 255, 315, 321, 327, 346; ii. 129, 152, 190, 191, 332, 369, 392, 402, 424, 437, 441 Foot, F. J., i. 316 Forbes, D., ii. 370 ---- Edward, ii. 66, 113, 114, 198 ---- J. D., ii. 112, 333, 372, 381 Forellenstein, ii. 332 Forest of Wyre coal-field, ii. 102 Forfarshire, volcanic rocks of, i. 285, 299; flagstones of, 299 Forster, M., ii. 113 Forth-basin, Carboniferous system of, i. 361; Carboniferous plateaux of, 370; Carboniferous puys of, 416, 427, 429, 430, 432, 434, 437, 440, 446, 462; Permian volcanoes of, ii. 55, 67 Foster, Mr. C. le Neve, ii. 10 Fouqué, Prof., i. 18, 21; ii. 134 Fox, Mr. Howard, ii. 36 Fox Strangways, Mr. C., i, 135 Fragmental volcanic rocks, i. 14; only arise from explosions which reach the surface, 57 (see Agglomerates, Conglomerates, Tuffs) France, Tertiary volcanoes of Central, i. 4, 10, 29, 41, 45, 49, 58, 60, 70; ii. 31, 271, 281, 373; Carboniferous volcanic action in, i. 357 Frankland, Prof. E., i. 273, 278 Fundamental complex of oldest gneiss, i. 114, 115 ---- gneiss, i. 115

Gabbro, granulitic, ii. 329; banded structure of, i. 116; ii. 329, 354, 357, 476; coarse-grained massive, 330; pale varieties in veins, 330; gneiss-like aspect of, 342, 254, 358 ---- of Carrock Fell, i. 91; Silurian, 195, 206, 247; Devonian, 262; Tertiary, 84, 90, 116; ii. 307, 308, 309, 319, 327, 334, 349, 355, 358, 391, 406, 407 Gairloch, peculiar pre-Cambrian rocks of, i. 115, 117 Galapagos Islands, i. 27 Gallaston type of dolerite and basalt, i. 418 Galloway, granites of, i. 93, 95, 272, 277, 290, 331 Garabol Hill, differentiation at, i. 90 Gardiner, Miss, i. 95 Gardiner, Mr. C. J., i. 256 Gardner, Mr. Starkie, ii. 196, 198, 212 Garlton Hills, i. 102, 370, 377, 378, 379, 380, 386, 390, 405, 412 Garnet found in volcanic vents, i. 62 Garth Grit, i. 177, 185, 208 Gases dissolved in the volcanic magma, i. 13, 15, 72, 97, 99 Geikie, Prof. J., i. 277, 306, 308, 331, 336, 339, 340, 369, 375, 426; ii. 57, 191, 259, 322 Genèvre, Mont, i. 194 Geological action, supposed former greater intensity of, i, 139 ---- contrasts, i. 103 ---- history, i. 109, 113 ---- Survey of Great Britain, i. 113, 115, 118, 119, 121, 122, 123, 124, 125, 126, 129, 130, 133, 135, 142, 143, 144, 145, 159, 160, 166, 170, 171, 175, 176, 179, 181, 182, 183, 186, 187, 188, 190, 196, 198, 201, 204, 205, 207, 208, 212, 214, 215, 216, 217, 218, 219, 220, 221, 225, 227, 228, 232, 233, 238, 239, 240, 242, 243, 244, 245, 250, 251, 254, 259, 270, 275, 278, 294, 299, 306, 307, 308, 314, 315, 317, 318, 325, 329, 331, 336, 339, 340, 344, 346, 349, 350, 352, 364, 369, 372, 373, 375, 397, 403, 404, 406, 407, 411, 423, 425, 434, 449, 462, 475, 476; ii. 3, 4, 9, 10, 12, 13, 16, 17, 20, 23, 33, 36, 37, 42, 43, 46, 48, 49, 56, 58, 65, 66, 68, 94, 95, 102, 103, 118, 121, 125, 127, 144, 145, 148, 162, 164, 170, 174, 175, 190, 192, 199, 201, 203, 253, 272, 277, 292, 347, 384, 391, 420, 422, 423, 426, 428, 433, 435, 446, 449 Giant's Causeway, ii. 80, 109, 186, 188, 192, 206 Gilbert, Mr. G. K., i. 87; ii. 362, 363 Girvan, i. 192, 200 Glaciation, absence of, in Devonshire, i. 261 Glass in volcanic rocks, i. 18, 33, 60, 78, 180, 211, 216, 230, 232, 235, 316; ii. 85, 120, 126, 133, 135, 137, 184, 204, 247, 272, 285, 316, 317 Globulites, ii. 135 Gloucestershire, Silurian volcanoes of, i. 238 Gneiss, analogies of, with igneous rocks, i. 93; ii. 476; oldest, i. 110, 115 Godwin-Austen, A. C., i. 259, 262 Goodchild, Mr. J. G., i. 229, 449; ii. 150 Grainger, Rev. Dr., ii. 198 Grand Sarcoui, ii. 373, 374, 381 Granite, bosses of, i. 88, 90, 93; plutonic and volcanic, 89; metamorphism by, 95; altered by dykes, ii. 164; pre-Cambrian, i. 119; post-Arenig in Highlands, 126, 310; in Cambrian rocks, 155; in Silurian rocks, 200, 229, 236, 238, 249; of probably Old Red Sandstone age, 272, 277, 290, 331, 337; Tertiary, ii. 366, 418, 420 Granitite, i. 188, 277, 290, 337; ii. 367 Granophyre, alteration of rocks by, i. 95, 96; scenery of, 105; solvent action of, 82, 84, 85, 96, 99; ii. 163, 392, 415, 422, 433; brecciated, 382; spherulitic, 381; bedded structure of, 381, 403, 404; apt to be intruded at the base of a volcanic series, 403; shattering of rocks invaded by, 405, 411, 413, 416, 439; veins of, 409, 410, 432, 437; Silurian, i. 214, 215 ---- Tertiary, i. 339; ii. 368, 395, 408, 430; boundaries of, 382, 409; relation to older vents, 280, 384, 399; relation to plateau-basalts, 386, 396, 402, 404; relation to gabbro, 391, 402, 404, 410; relation to basic dykes, 395; proof of liquidity of, 413; sills of, 430, 436, 437; dykes of, 435 Granophyric structure, i. 20; ii. 366 Graphite in Tertiary volcanic series, ii. 198 Graptolites, i. 174, 196, 197 Grauwacke or Devonian rocks, De la Beche on, i. 259, ii. 33 Graves, Lieut. T., ii. 451 Great Glen of Scotland, i. 121 Greeks, influence of volcanoes on, i. 1 Green, A. H., i. 133, 134, 163; ii. 10, 12 Greenland, Tertiary basalts of, ii. 182 Greenly, Mr. E., i. 129, 214 Greenock, Lord, i. 363 Green schists of the Scottish Highlands, i. 122; of Anglesey, 129 Greenstone, i. 183, 187, 206, 217, 219, 249, 259, 261; ii. 34, 35, 37, 103, 104, 355 Greenstone-ash, i. 219 Griffith, Sir R., ii. 299, 422 Gunn, Mr. W., i. 114, 298, 311, 336, 369, 407, 410; ii. 58, 172, 420 Gypsum deposits, ii. 54

Hade of dykes, ii. 139 Hæmatitic iron-ore, ii. 197 Hall, Sir James, i. 72, 363 Hälleflinta, i. 131, 167 Hardman, E. T., ii. 365, 449 Harker, Mr. A., i. 90, 91, 93, 95, 96, 99, 188, 209, 210, 211, 212, 213, 214, 217, 218, 222, 227, 228, 230, 231, 232, 235, 236, 237, 238, 290; ii. 124, 125, 126, 129, 130, 139, 144, 146, 160, 162, 163, 164, 174, 185, 189, 190, 223, 224, 247, 269, 281, 284, 285, 309, 310, 318, 320, 334, 339, 347, 348, 368, 382, 384, 385, 387, 389, 392, 407, 408, 409, 413, 415, 433, 434, 437, 441, 446 Harkness, R., i. 228; ii. 56 Harlech anticline, i. 159, 179, 187; group, 176 Hatch, Dr. F. H., i. 183, 184, 187, 188, 229, 230, 246, 247, 248, 249, 261, 277, 278, 306, 377, 380, 381, 417, 419, 420; ii. 57, 96, 184, 274, 276, 299, 319, 332, 367, 368, 369, 370, 388, 398 Haughton, Prof. S., i. 346; ii. 422 Hawaii, lava-fountains of, i. 12; differentiation in lavas of, 27; lava-cauldron of, 58 Haworth, Mr. E., ii. 96 Hay Cunningham, R. I., i. 269, 317, 363, 372, 373, 449, 451; ii. 237, 238, 244 Heaphy, Mr. C., i. 432 Hebrides, basalt-sheets of, i. 24, 47, 52, 102; acid rocks of, 95, 102; gabbros of, 84, 90, 102; scenery of, 105; pre-Cambrian rocks of, 112, 114, 117, 121; Cambrian land of, 141; early observations on the Tertiary volcanic rocks of, ii. 109, 110, 111; dykes of, 118, 146, 158, 174; basalts of, 181, 186, 215; pitchstone lava of, 238, 246; plateau-scenery of, 249; Tertiary rivers and lakes of, 217, 228, 231, 234, 252; vents of, 274; basic sills of, 304; gabbro intrusions of, 327; acid intrusions of, 364, 379, 430, 437; dislocations of, 452; denudation of, 455 Heddle, Dr., i. 274, 302; ii. 78, 79, 246, 307, 406 Helland, Prof. A., ii. 191, 261, 263, 264 Henderson, Mr. J., i. 449 Henry Mountains, laccolites of, i. 86; ii. 362 Henslow, J. S., ii. 22, 224 Heterogeneity in volcanic magmas, i. 85, 90; ii, 190, 334 Hett Dyke, ii. 1, 7, 147 Hibbert, S., i. 46 Hicks, Dr. H., i. 126, 145, 154, 158, 159, 166, 206 Hill, Mr. J. B., ii. 140 Hill, Rev. E., i. 135 Hinde, Dr. G. J., i. 198; ii. 35 Hinxman, Mr. L., i. 114, 344; ii. 121 Hobson, Mr. B., i. 260; ii. 23, 27, 96, 99 Holden, Mr. J. S., ii. 204 Holl, H. B., i. 133, 134, 170 Holland, Mr. P., i. 177, 178, 179; ii. 23 Hollybush Sandstone, i. 133, 170 Holocrystalline structure, i. 78; ii. 136, 184 Hopkins, W., ii. 177, 179, 268 Hornblende, ejected crystals of, i. 178, 181; ii. 49, 51, 58, 79 Hornblende-schists formed from basic igneous rocks, i. 75, 114, 118, 119, 124, 129 Horne, Mr. John, i. 114, 196, 199, 200, 344, 345, 375; ii. 23, 144, 292 Hornito of a lava-stream, i. 55; ii. 264 Hornstone, i. 131, 136, 277, 278 (analyses), 324 Houston Marls, i. 423, 436, 440, 444, 466 Howard, Mr. H. T., i. 207 Howell, Mr. H. H., i. 294, 307, 364 Hoy, Island of, i. 350 Hughes, Prof. T. M'K., i. 126, 144, 160, 161, 166, 168, 220, 222, 223, 227 Hull, Mr. E., ii. 42, 95, 103, 272, 421, 426, 449 Hurlet Limestone, i. 360, 366, 394, 410, 415, 444, 452, 456, 467, 470, 474 Huronian rocks, i. 111 Hutchings, Mr. W. M., i. 227, 230, 233 Hutton, James, i. 363; ii. 9, 110 Hutton, W., ii. 3 Hyperite, i. 279 Hysgeir, pitchstone of, ii. 246

Iceland, Tertiary basalts of, ii. 182, 260; Tertiary gabbros and liparites or granophyres of, 261; continuity of volcanic phenomenon of, 261; lava-fields of, i. 24, 42, 53, 100; ii. 260; lava-domes of, i. 10; ii. 265; fissures of, i. 70; ii. 262, 271, 454; dykes of, 122, 261; cinder cones of, 264, 271; subsidence of, 447 Idaho, lava-fields of, ii. 267 Iddings, Prof., i. 28, 29, 30, 78, 79, 82, 84, 90; ii. 128, 178 Idiomorphic crystals, i. 21, 417, 420; ii. 40 Index Limestone of the Scottish coal-fields, i. 360, 444, 452 India, fissure-eruptions of, i. 10; volcanic plateau of, ii. 180 Intermediate volcanic rocks, silica-percentage of, i. 14 Intersertal structure, i. 417; ii. 136 Intrusive rooks, defined, i. 14; occasional cellular character of, 16; flow-structure in, 22, 161; varieties of, 77; textures of, 78, 449; ii. 274, 360, 392; in sheets, sills, and laccolites, i. 83; melting of rocks by, 82, 84; ii. 129, 163, 392; consolidation of, i. 84; banding of, 84, 450; ii. 329, 342; heterogeneity of, i. 85; ii. 344; metamorphism by, i. 94, 451; influence of surrounding rocks on, 95; conditions of their intrusion, 97; columnar structure in, ii. 187, 291, 301 ---- Pre-Cambrian, i. 116; Cambrian, 156; Silurian, 187, 195, 206, 216, 235, 237, 248, 249; Devonian, 261; Old Red Sandstone, 277, 291, 321, 335, 338, 343, 345; Carboniferous, 406, 408, 420, 446; ii. 1, 21, 30, 33, 40, 48; Permian, 58, 64; Tertiary, 270, 298 Ireland, submarine eruptions of, i. 48; Dalradian rocks of, 122, 123, 126; Arenig rocks in, 123; Silurian volcanic rocks of, 239, 251; granites of, 290; Old Red Sandstone volcanic rocks of, 346, 348; Carboniferous volcanic rocks of, 359; ii. 37; early observers among the Tertiary volcanic rooks of, 109; Tertiary basalt plateau of, 364, 370, 371; gabbros of, 359; acid rocks of, 420 Iron-ore, pisolitic (Arenig), i. 181, 208; Tertiary, of Antrim, ii. 204 Irvine, Mr. D. R., i. 294, 299 Irving, Rev. A., ii. 95 Isogeotherms, shifting of, i. 98 Italy, old volcanoes of, i. 4; ii. 474, 477

Jack, Mr. R. L., i. 294, 308, 369, 375, 396, 404; ii. 57, 145 Jameson, Robert, i. 268, 269, 317, 363; ii. 109, 161, 244, 333, 355, 364 Jan Mayen, ii. 182 Jedburgh type of dolerite and basalt, i. 418 Jennings, Mr. C. V., i. 176, 177, 179, 180, 181, 184, 185, 186, 187, 188 Johnston-Lavis, Dr., ii. 261 Joints in dykes, ii. 132 Judd, Prof. J. W., i. 157, 275; ii. 115, 116, 134, 137, 162, 185, 209, 211, 245, 247, 267, 274, 278, 280, 303, 307, 309, 315, 316, 319, 322, 328, 329, 332, 333, 349, 356, 360, 372, 388, 410, 439 Jukes, J. B., i. 143, 171, 175, 208, 218, 219, 245, 246, 250, 254, 316; ii. 10, 20, 42, 47, 49, 101, 103, 105 Jurassic period, physical conditions of the, ii. 108, 182

Kelly, J., i. 314 Kenmare, Old Red Sandstone volcanic rocks of, i. 350 Keratophyre, i. 247 Kerrera, Isle of, i. 342 Kersantite, i. 261 Keswick, i. 229 Kildare, Chair of, Bala volcanic rocks at, i. 245, 256 Killarney, nodular lavas of, i. 20, 272, 346 Kilpatrick Hills, i. 385, 388, 403, 410 Kilroe, Mr. J. R., i. 251, 253, 315 Kilsyth type of dolerite and basalt, i. 418 Kinahan, Mr. G. H., i. 349; ii. 45, 49, 426 Kincardineshire, volcanic necks of, i. 281, 286, 293, 299; Old Red Sandstone of, 301 King, Mr. Clarence, i. 27 King's County, volcanic necks of, ii. 37 Kippie Law type of basalt, i. 418 Kirkby, Mr. J., ii. 106 Kirwan, R., ii. 110 Knockfeerina, Old Red Sandstone volcanic rocks of, i. 349 Knocklayd, ii. 200 Kynaston, Mr. H., i. 343

Labyrinthodonts, i. 356 Laccolites, i. 77, 83, 86, 88, 98, 99, 190; ii. 363 Lacroix, Prof., i. 96 Lacustrine volcanic eruptions, i. 49 Lagorio, Dr. A., ii. 137 Lake, Mr. P., i. 177, 179 Lake-District, i. 227, 290; Vesuvian cone of, i. 42, 45 "Lake Caledonia," i. 272, 294, 296 "Lake of Lorne," i. 341 "Lake Orcadie," i. 266, 271, 343, 350 Lakes, eruptions in, i. 49; crater, 58; of Old Red Sandstone, 264 Lambay Island, conglomerates of, i. 244 Lammermuir, granites of, i. 290, 340 Lamplugh, Mr. G. W., i. 32, 220; ii. 23, 28 Lamprophyre, i. 291, 293 Lanarkshire, i. 291, 368, 416 Land, sculpture of the, i. 101, 102 Landslips, ii. 200, 287 Lankester, Prof. E. Ray, i. 310 Lapilli, volcanic, i. 33, 34, 61, 151 Lapworth, Prof. C., i. 130, 132, 137, 171, 172, 189, 190, 196 Largs, volcanic vent near, i. 56, 396, 397, 401 Lasaulx, Prof. von, ii. 365, 371, 426 Laurentian gneiss, i. 110 Lavas, classification of, i. 14; flow-structure of, 16, 21; vesicular structure of, 17; glass in, 18; devitrification of, 19; bedding of, 24; effect of water on molten, 25, 334; sack-like or pillow-structure of, 26, 184, 193, 201, 240, 244, 252; seldom occur in solitary sheets, 26; variations in structure in, 27; sequence of, in eruptions, 28, 92, 377, 386; crusts of, disrupted in volcanic explosions, 58, 59, 60; ii. 189; alternations of acid and basic, i. 28, 61, 152, 157, 165, 207, 213, 284, 318; ii. 236, 266; contrasted with intrusive rocks, i. 78; sandstone veins in, 283, 300, 303, 320, 327, 333, 337; ii. 59, 98; shattered or agglomerate structure of, 99; metamorphism of, i. 231, 240, 338; ii. 272, 276, 337, 339, 340, 347, 355, 379, 386, 397, 399, 400, 404, 413 ---- Cambrian, i. 152, 168 ---- Silurian of Merionethshire, i. 183; Scotland, 191; Builth, 203; Pembrokeshire, 205; Caernarvonshire, 207; Berwyn Hills, 218; Anglesey, 219; Lake District, 227; Gloucestershire, 238; Ireland, 239, 254 ---- Lower Old Red Sandstone, i. 273, 281, 294, 317 ---- Carboniferous, i. 377, 384, 417, 436, 440, 443; ii. 8, 18, 34, 45 ---- Permian, ii. 68, 96 ---- Tertiary, ii. 183; types of, 186; banding of, 189; thickness of, 192; lenticular character of, 193; of Antrim, 199; irregular bedding of the vitreous, 243 ---- modern Icelandic eruptions of, ii. 261 Lava-domes, ii. 265 Lava-plug of volcanic funnels, permanence of, i. 40, 41, 55, 73, 76, 430 Lawson, Prof. A. C., i. 82 Leaf-beds, ii. 198 Lebour, Prof., i. 336; ii. 2, 3, 5, 7 Leckstone, i. 419, 442, 443 Lecoq, H., i. 45; ii. 373 Leinster granite, ii. 245, 249, 290 Lewisian Gneiss, i. 81, 110, 111, 113, 118 Liddesdale, Carboniferous volcanic vents of, i. 55, 416, 425, 440, 475 Life, earliest traces of, i. 140 Lignite in Tertiary volcanic series, ii. 198 Limburgite, i. 377, 408, 417, 420, 448; ii. 40, 46 ---- type, i. 418 Limerick, Carboniferous volcanic rocks of, i. 421, 430; ii. 41; Old Red Sandstone volcanic rocks of, i. 348 Limestone, metamorphism of, i. 72, 451; ii. 14, 22, 164, 280, 383 Lindley and Hutton on Eigg conifer, ii. 238 Lingula Flags, i. 144, 177 Linlithgowshire (see West Lothian) Lintrathen, porphyry of, i. 277, 292, 311 Lion's haunch type of dolerite and basalt, i. 418 Lithomarge, ii. 197, 204 Lizard, rocks at the, i. 194 Llanberis, Pass of, i. 159, 163 ---- group, i. 144 Llandeilo group, i. 175, 196, 242; volcanic rocks of, 186, 202, 221, 227, 241 Llandeiniolen, i. 160 Llandovery group, i. 175, 196; possible volcanic rocks of, 238 Llangadock, i. 205 Llangefni, i. 220 Llanllyfni, i. 161 Llanwrtyd, i. 204 Lleyn Peninsula, i. 208, 209, 213, 215 Lloyd Morgan, Prof., i. 145, 147, 154 Llyn Padarn, i. 157, 159, 160 Loch Carron, pre-Cambrian rocks of, i. 115, 117 Loch Lomond, dykes at, ii. 180 Loch Tay Limestone, i. 122, 124, 125 Lomas, Mr. J., ii. 191, 322 Longulites, ii. 135 Lonsdale, W., i. 257 Longmyndian rocks, i. 111, 129, 132 Lorne, volcanic rocks of, i. 102, 271, 281, 341 Lough Mask, Silurian volcanic rocks of, i. 251 ---- Nafooey, Silurian volcanic rocks of, i. 251 ---- Neagh, subsidence of site of, ii. 201, 205; history of, 448 Ludlow group, i. 175 Lycopods, fossil, i. 174

Maare, volcanic, i. 58; ii. 287, 288, 296 Macconochie, Mr. A., ii. 58 Macculloch, John, i. 95, 269, 270; ii. 22, 111, 113, 123, 140, 154, 156, 159, 172, 174, 175, 213, 217, 231, 237, 244, 251, 280, 293, 304-307, 310, 314, 315, 327, 349, 364, 371, 403, 406, 408, 409, 418 Macknight, Dr., i. 268, 317 Maclaren, Charles, i. 269, 317, 325, 363, 372, 373, 451, 462; ii. 67 M'Henry, Mr. A., i. 32, 240, 242, 244, 314, 347; ii. 201, 204, 272, 293, 426, 427, 428, 429 M'Mahon, General, i. 260; ii. 35, 36 Magma, volcanic, explosive energy of, i. 13; gases and vapours dissolved in, 13, 72, 97, 99; differentiation of, 22, 84, 91; solvent action of, 82, 84, 85, 99; ii. 392, 415, 422, 433; heterogeneity of, i. 85, 90, 91; ii. 344, 360, 476; metamorphic action of, i. 94; alteration of, by incorporation of foreign material, 96; ii. 386, 390, 392; conditions for the injection of, i. 97, 99 Magnesian Limestone, ii. 54 Malvern, pre-Cambrian volcanic rocks of, i. 133; Cambrian volcanic rocks of, 169 Man, Isle of, Carboniferous volcanic rocks of, ii. 22 Manod, i. 184 Marl Slate, ii. 54 Marl, volcanic, i. 423, 436, 440, 444, 466 Marr, Mr. J. E., i. 227, 228, 230, 231, 232, 236, 237, 238, 290; ii. 189 Matlock Bath, ii. 13, 22 Mediterranean, earthquakes and volcanoes of, i. 1 Melaphyre, i. 131 Mello, Mr. J. M., ii. 22 Melrose, rocks near, i. 397, 398, 400, 425 Melting of rocks by igneous intrusions, i. 82, 84, 85, 96, 99; ii. 163, 392, 415, 422, 433 Menai Strait, i. 159 Menevian group, i. 114 Merse, volcanic plateau of the, i. 375 Metamorphism of tuffs, i. 157; of lavas (see under Lavas) ---- by lavas, i. 27; by sills and bosses, 87, 94, 95, 216, 236, 331, 338, 431; ii. 7, 22, 36, 131, 148, 163, 299, 300, 310, 337, 339, 340, 347, 355, 356, 357, 358, 362, 378, 383, 386, 397, 399, 400, 404, 413 ---- in vents, i. 67, 71, 82, 93, 399, 404; ii. 39, 78, 292 ---- around vents, i. 72, 349, 350, 352, 399, 404, 432; ii. 76, 272, 273, 276, 280, 292 ---- regional, i. 121, 123; ii. 35 Mica, ejected crystals of, in volcanic breccias, ii. 49, 58, 79, 80 Mica-porphyrite, i. 277, 338 Michel Lévy, M., i. 18, 21, 46, 88; ii. 373, 374 Microgranite, i. 131, 215, 235, 249; ii. 367, 437 Microlites of igneous rocks, i. 18, 21, 33; ii. 135, 275 Micropegmatitic structure, i. 20, 418, 449; ii. 5, 368, 437 Microscopic examination of rocks, i. 18, 21 Midlands, eruptive rocks of English, ii. 100 Midlothian, Carboniferous volcanic plateau of, i. 373, 385, 387; sills of, 446 Miller, Hugh, ii. 237 ---- Mr. H., i. 336 Mills, Abraham, ii. 109 Millstone grit, i. 358, 360, 366 Minette, i. 277, 278, 291, 293 Minto Crags, i. 375, 397 Mitchell, Rev. Hugh, i. 301 Moel Siabod, i. 175 ---- Wyn, i. 175, 176, 184, 185 Monckton, Mr. H. W., i. 449; ii. 224 Montana, lava-fields of, ii. 115, 267 Montrose, volcanoes of, i. 299 Moray Firth, basin of, i. 271, 343 Morton, Mr. G. H., i. 189 Morven, basalt-plateau of, ii. 208 Mountain-chains, origin of, i. 11, 12, 98 Mourne mountains, granite of, i. 93; ii. 124, 366, 367, 420 Muck, Isle of, ii. 215 Mud-lava, ii. 85 Mudstone, volcanic, i. 423, 436, 440, 444, 466; ii. 86, 222, 258 Mull, branching amygdales of, i. 17; perlitic glass from, 19; pale lavas of, ii. 184, 213; basalt of, 188, 192, 193; breccias of, 196; plateau of, i. 24, ii. 208; non-volcanic breccias in, 196, 211; flint gravel in, 211; leaf-beds of, 212; vents in, 274, 278; gabbro of, 355; acid bosses of, i. 20; ii. 395; acid sills of, 430; acid dykes and veins of, 443; enormous denudation of, 457, 461 Murchison, R. I., i. 113, 121, 129, 142, 173, 175, 189, 204, 205, 207, 238, 257; ii. 56, 95 Mynydd-mawr, i. 209, 211, 216 Mythology, influence of earthquakes and volcanoes on, i. 2

Nant Francon, i. 161 Naples, puys of, i. 100, 429 Napoleonite, i. 22 Necker, L. A., ii. 112, 123, 139, 140, 146 Necks, volcanic, i. 56; of fragmentary materials, 56; of non-volcanic detritus, 57, 289, 343, 426; of agglomerate, 58; ii. 276; internal stratification in, i. 63; ii. 80, 294; with central lava-plug, i. 64, 430; with dykes and veins, 66, 430; ii, 291; of lava-form material, i. 67, 430; ii. 271; parasitic, i. 69; connection of, with cones, 70, 435; ii, 70, 89, 277, 281, 290; metamorphism of, i. 67, 71, 82, 93, 399; ii. 39, 78; metamorphism of rocks around, i. 72, 349, 350, 352, 399, 404, 432; ii. 76, 272, 273, 274, 276, 280, 292; inward dip of rocks towards, i. 73, 352; ii. 80; connection of, with bosses, i. 93; ii. 276, 284; entombment and exposure of, i. 434; connection of with valleys, 272, 366, 375; ii. 61, 65; relation between their size and the character of the agglomerate, 76; connection of, with sheets of tuff or lava, 70, 89, 277, 284 ---- Silurian, i. 215, 235; Old Red Sandstone, 277, 288, 293, 311, 318, 323, 328; Carboniferous, 394, 399, 400, 404, 406, 424, 465; ii. 13, 28, 47; Permian, ii. 62, 67; Tertiary, ii. 202, 270, 276 Neptunist and Plutonist controversy, i. 363; ii. 67, 95, 110, 112 New Mexico, necks in, i. 68 Newry granite, i. 290 Nicholson, Prof. Alleyne, i. 228, 229 Nicol, James, i. 311, 369 ---- W., ii. 238 Nigrine, ii. 79 Nithsdale, Permian volcanic rocks of, ii. 58, 60, 62, 65 Nodular structure of lavas, i. 20, 162, 204, 206, 207, 211, 232, 247, 255, 274, 346 Nolan, Mr. J., i. 240, 251, 315; ii. 423, 424, 425 Non-volcanic debris, among volcanic rocks, i. 31, 57, 289, 313, 345, 381, 399, 402, 422, 426, 437; ii. 18, 27, 28, 58, 64, 76, 78, 99, 195, 196, 281, 423; indicates comparatively feeble eruptions, i. 57, 289, 345, 426, 438; ii. 293; points to earliest eruptions of a vent, ii. 76 Nordenskjöld, Mr. O., i. 120 North Berwick Law, i. 371, 373, 403 North, Mr. Barker, ii. 244 Norway, eruptive rocks of, i. 28 Nuneaton, i. 171

Obsidian, i. 18, 19; ii. 370 Ochil Hills, i. 274, 276, 277, 279, 281, 286, 287, 288, 293, 303, 308, 311 Oil-shales of the Lothians, i. 361, 362, 462 O'Kelly, J., i. 349; ii. 49 Oldham, T., ii. 299 Old Red Sandstone, lines of vents in, i. 69; granite protrusions of, 236, 272, 277, 290, 337; of County Waterford, 251; distribution in Britain, 257; an exceptional stratigraphical type, 258; conditions of its deposit, 259, 263, 297; original scenery of, 265; vegetation of, 265; isolation of the water-basins of, shown by fossil evidence, 265; classification of, 260; history of the investigation of, 268; volcanic centres in, 271; nature of volcanic products in, 273; structure of lavas and tuffs of, 281; volcanoes of, 259, 263, 294, 303, 323, 325, 337, 341, 343, 346, 348, 352; subdivisions of, 297; thickest conglomerates of, 301; composition of conglomerates of, 302, 315, 341; unconformabilities in, 267, 328, 333; Upper division of, 348, 375, 383; ii. 42 Olenellus-zone, i. 112, 130, 132, 140, 144 Olenus-zone, i. 144 Olivine, i. 154, 418, 420; ii. 58, 135 Omagh, i. 315 Ophitic structure, i. 21, 417; ii. 136, 184, 274 ---- type of dolerite, i. 418, 421 Oregon, crater lake in, i. 58 Orkney Isles, i. 271, 344, 350; ii. 121 Orthoclase, ejected crystals of, ii. 79 Orthophyre, i. 273, 276, 277, 308 Osann, A., ii. 191 Oyenhausen, C. von, ii. 112, 280, 333, 340, 367, 372, 381

Palæopicrite, i. 261 Palæozoic systems, i. 139; volcanic rocks resemble modern, i. 30 Palagonite, i. 33, 61, 151, 180, 246, 422, 423; ii. 44, 46, 57, 223 Paradoxides-zone, i. 144 Paramorphism, i. 249 Peach, Mr. B. N., i. 114, 128, 147, 168, 191, 192, 195, 196, 197, 198, 199, 200, 216, 240, 277-294, 307, 308, 329, 331, 344, 345, 369, 375, 425, 426, 476; ii. 133, 145 Pebidian, i. 145 Pegmatite, i. 20, 119, 127, 418, 449; ii. 5, 368, 437 Pembrokeshire, volcanic rocks of, i. 145, 159, 205 Penmaen-mawr, i. 209, 215 Pennant, T., ii. 109 Pennine chain, ii. 8 Pentland Hills, volcanic series of the, i. 102, 269, 273, 276, 279, 281, 285, 287, 289, 291, 311, 317 Perlite, i. 130 Perlitic structure, i. 19, 196, 199, 206, 211, 216, 232, 274 Permian system, geographical conditions accompanying the deposition of, ii. 53, 97; subdivisions of, in S. W. England, 94; volcanic phenomena of, i. 46; ii. 55; lavas and tuffs of, 57, 58; vents of, 62, 67, 70, 96; sills of, 91, 100 Permo-carboniferous strata, ii. 54 Petersen, Dr., i. 275, 336 Phillips, J., i. 133, 170, 205, 238; ii. 3 ---- J. A., i. 260, 261 ---- W., i. 171; ii. 95 Phonolite, i. 380 (analysis); ii. 375 Phyllite, i. 162, 222 Picrite, i. 377, 417, 420, 448, 450; ii. 57 ---- type, i. 418 Pillow-structure in lavas, i. 26, 184, 193, 201, 240, 244, 252; ii. 189, 259 Pitchstone, i. 18, 19, 130; ii. 134, 174, 204, 238, 242, 246, 370, 437, 444 Plagioclase, ejected crystals of, ii. 79 Plants fossil, in tuffs, i. 392; ii. 113, 198, 212, 222 Platania, G., i. 26 Plateau-type of volcanoes, i. 42, 100, 308, 341, 364 Plateaux, Carboniferous, of Scotland, i. 364; distribution of, 367; composition of, 377; structure of, 383; lavas and tuffs of, 383; vents of, 394; dykes and sills of, 406; close of eruption of, 410; Tertiary, ii. 181; formation of modern Icelandic, 265 Player, Mr. J. H., analyses by, i. 377, 381; ii. 138, 330 Playfair, John, i. 363; ii. 110 Plinthite, ii. 197 Pliocene (supposed) of Lough Neagh, ii. 449 Plutonic operations of volcanoes, i. 77; granite, 88 Plutonists and Neptunists, i. 363; ii. 67, 95, 110, 112 Pomeroy, volcanic series near, i. 315 Porphyrite, i. 190, 193, 207, 229, 240, 252, 273, 274, 377, 379 Porphyritic structure, i. 19, 274; ii. 128 Portlock, J. E., ii. 110, 111, 113, 199, 201, 299, 364 Portraine, conglomerates at, i. 244 Portrush, shells in supposed basalt at, ii. 110, 299 Potstone, i. 125 Pre-Cambrian rocks, i. 111, 121, 126 Pressure, experimental proof of effects of, i. 24 Prestwich, Sir J., ii. 103 Propylites, ii. 185, 388 Proterobase, i. 247 Pterygotus, i. 265 Pumice in tuffs, i. 244, 422; ii. 17, 27, 28, 32, 44, 46, 286, 288; in volcanic necks, i. 60, 180; ii. 17, 39, 195 Pumiceous structure, i. 15, 33, 34, 60 Puys, as a type of volcano, i. 10, 44, 100, 414; probable subærial nature of some, 432; Carboniferous, 308, 364, 414, 424, 463; ii. 13, 28, 34, 47; Permian, ii. 62; Tertiary, 271, 276 Puy de Chopine, i. 32; ii. 374 ---- Dôme, ii. 373 ---- Montchar, i. 32 ---- Pariou, i. 66, 70; ii. 31, 281 Pyroclastic detritus, i. 31, 58, 61 Pyromeride, i. 211 Pyrope, ii. 58, 79 Pyroxene, ii. 135

Quartz-porphyry, i. 19, 156, 159, 160, 161, 165, 277, 291, 314; ii. 96, 369, 420, 423, 430, 431 Quartz-trachyte, ii. 371 Quartzite, i. 112, 170

Raasay, basalt of, ii. 192; neck-like breccias in, 293; acid sill of, 430 Raddling or red-staining of rocks, i. 250, 261 Radiolarian cherts, i. 123, 167, 169, 173, 174, 184, 196, 201, 244 Rain-pittings in strata, i. 342 Raisin, Miss, i. 161, 163, 164, 165, 210 Ramsay, A. C., i. 126, 142, 143, 144, 145, 158, 159, 168, 175, 176, 177, 178, 179, 180, 182, 183, 204, 205, 208, 210, 212, 214, 223, 237, 364 Ratho type of dolerite, i. 418, 421 Red Head, section at, i. 300 Red Hills, Skye, scenery of, i. 105; ii. 379 Reed, Mr. Cowper, i. 205 Reid, Mr. Clement, ii. 449, 450 Renard, Prof. A., i. 148, 149 Renfrewshire, Carboniferous volcanic rocks of, i. 368, 385, 397, 400, 404, 408, 416, 430, 447 Reyer, Prof. E., ii. 474 Reynolds, Mr. S. H., i. 177, 179, 256 Rhobel Fawr, i. 177, 178, 186 Rhyolite, i. 19, 22, 24, 131, 161, 165, 167, 168, 178, 204, 210, 231, 232, 255, 276, 278; ii. 185, 205, 371, 424, 437 Rhyolitic conglomerate, ii. 195, 206, 429 Richardson, Rev. W., ii. 110 Richthofen, F. von, i. 28; ii. 115, 116 Rivers made to shift their channels by volcanic eruptions, i. 42, 49; of the Tertiary volcanic period, ii. 217, 228, 231, 234, 456 Rocks, oldest known, i. 110 Roscommon, volcanic rocks of, i. 316 Rosenbusch, Prof. H., ii. 136, 137 Ross, Mr. Alexander, ii. 406, 409 Rothliegende, ii. 95 Roxburghshire, Carboniferous vents of, i. 55, 403, 404 Rubers Law, i. 375, 380, 404 Rum, i. 112; basalt-plateau of, ii. 215; gabbros of, 332, 349; acid bosses of, 403; acid sills of, 431; pitchstone of, 445 Rutley, Mr. F., i. 131, 133, 170, 207, 210, 227, 231, 232, 238, 260; ii. 23, 35

Sahlite found in volcanic vents, i. 62 Saline Hill, volcanic vents of, i. 433, 435, 440 Sanday, basalts of, ii. 215; conglomerates of, 226 Sandstone altered into quartzite, i. 72, 349, 350, 404, 432, 451; ii. 76, 164; veinings of, in lava, i. 283, 300, 303, 320, 327, 333, 337; ii. 59, 98 Sandwich Islands, lava-cones of, i. 10 Sanidine ejected from volcanic vents, ii. 58, 79 Sanquhar, Silurian volcanic rocks at, i. 192, 195, 199; Permian volcanic rocks at, ii. 62 Santorin, ii. 134 Saponite, ii. 79 Scenery, origin of, i. 8, 100 Schalstein, i. 262; ii. 36 Schists, primeval, i. 110, 114, 118, 119; produced by deformation of igneous rocks, 75, 114, 118, 119, 121, 162, 240, 249, 252, 261 Schmidt, Dr. C. W., ii. 266 Scoriaceous structure, i. 15, 16, 282, 327, 339 Scorpions, fossil, i. 174, 356, 466 Scotland, lines of fault in, i. 11; Vesuvian cones of, 42; plateaux of, 43; puys of, 46; submarine lavas of, 48; Carboniferous vents of, 55; volcanic scenery of, 104; pre-Cambrian rocks of, 111; Cambrian rocks of, 112; Lewisian gneiss of, 114; Dalradian rocks of, 121; Arenig rocks of, 123, 191; Old Red Sandstone of, 266, 273, 281, 291; Carboniferous geography of, 356; Carboniferous volcanoes of, 359; Carboniferous plateaux of, 367; Carboniferous puys of, 414; Permian volcanoes of, ii. 55; Tertiary dykes of, 122; Tertiary basalt-plateaux of, 208, 274, 304; Tertiary gabbros of, 327; Tertiary acid rocks of, 379 Scrope, G. P., i. 27, 82, 45, 116; ii. 373, 374, 381 Sedgwick, A., i. 142, 166, 175, 218, 227, 257; ii. 1, 2, 3, 5, 113, 139, 153, 157 Segregation (see Differentiation) Segregation-veins, i. 84, 92; ii. 66, 130, 300, 303 Selwyn, Mr. A. C. R., i. 143, 175, 208, 221 Semi-opal, ii. 79 Sepulchre Mountain, i. 79 Serpentine, i. 195, 293 Shale, alteration of, i. 72, 451; ii. 164 Shap, granite of, i. 236, 238, 271, 290 Sheets, intrusive (see Sills) Shelve, i. 176, 190 Shetland, i. 271, 289, 292, 293, 345 Shiant Isles, ii. 307 Shineton Shales, i. 144 Shore-lines, traces of ancient, i. 295, 305 Shropshire, ancient volcanic rocks of, i. 129, 189; latest eruptive rocks of, ii. 101 Sicily, i. 26 Sidlaw Hills, i. 286, 294, 303 Sills, vitreous margins of, i. 18; tectonic relations of, 77, 83, 451; origin of name, 83; differentiation (segregation) in, 81, 450; ii. 476; ordinary stratigraphical position of, i. 85; considered as parts of incompleted volcanoes, 86; metamorphism by, 87, 94, 451; ii. 299, 303, 310; conditions for injection of, i. 97, 98, 99, 458; columnar structure of, ii. 187, 291, 301, 306, 308, 319; amygdaloidal structure in, 299, 312; banding of, 309; split by later sills, 310, 316; extreme subdivision of, 311; slaggy surface in some, 312; give off veins, 313; double and multiple, 318, 434; connection with vents, 322 ---- pre-Cambrian, i. 118, 124; Cambrian, 155, 170, 171; Silurian, 187, 195, 206, 216, 237, 249; Devonian, 261; Old Red Sandstone, 277, 291, 321, 335, 343, 345; Carboniferous, 408, 446, 472; ii. 2, 21, 30, 48; Permian, 64, 66; of Midlands, 102, 103; Tertiary, (1) Basic, 298; (2) Acid, 366, 430 Silurian system, i. 173; vegetation of, 174; geography of, 263; volcanoes of, 175; classification of, 175; two volcanic series of, 177 ---- volcanoes in Shropshire, i. 189; in Scotland, 191; at Builth, 203; in Pembrokeshire, 205; in Caernarvonshire, 207; in the Berwyn Hills, 218; in Anglesey, 219; in the Lake District, 227; in Gloucestershire, 238; in Ireland, 239, 254 Skae, H. M., i. 294, 299, 306, 375; ii. 57 Skiddaw, i. 228; granite of, 236 ---- Slate, i. 229 Skomer Island, i. 207 Skye, spherulitic dykes and sills of, i. 20; ophitic structure from, 20; basalt-terraces of, 24; metamorphism by granophyre of, 95, 96; volcanic scenery of, 103, 105; dykes of, ii. 123, 124, 129, 139, 140, 146, 150, 152, 154, 160, 162, 164, 165, 173, 269; bedded basalts of, 192, 249, 269; tuffs of, 251; connection of dykes and superficial lavas in, 269; vents in, 280; sills of, 304; gabbro bosses of, i. 116; ii. 334; acid bosses of, 379; acid sills of, 431; acid dykes of, 437; pitchstone veins of, 445; subsidence of, 447 Slaggy structure, i. 16, 33, 59, 282, 327, 339; ii. 98, 187 Slane, volcanic rocks near, i. 244 Slate-tuffs, i. 180, 213, 234 Slemish a volcanic neck, ii. 271 Slieve Foye, ii. 421 ---- Gallion, ii. 200 ---- Gullion, ii. 422 Small Isles, basalt-plateau of, ii. 215; vents of, 288; sills of, 318; acid bosses of, 403; acid sills of, 431 Small, Mr. E. W., i. 207 Smaragdite found in volcanic vents, i. 62 Snowdon, volcanic rocks of, i. 20, 42, 47, 102, 175, 208, 209, 210, 211, 212, 213, 218, 226 Soda-felsites, i. 183, 196, 247 Solfataric action, i. 71; ii. 185, 205, 388 Sollas, Prof., i. 96; ii. 175, 293, 415, 421, 422 Solway, Carboniferous volcanic plateau of, i. 375, 385, 413 Somerset, volcanic rocks of, ii. 32 Somma, denudation of, i. 3, 100 Spheroidal structure of dolerite, i. 456 Spherulitic structure, i. 19, 20, 95, 120, 130, 155, 162, 184, 211, 232, 235, 346; ii. 369, 381, 392, 432, 435, 437, 441, 446 Spilosite, i. 262 Springs, mineral, connected with volcanic action, i. 390, 445 St. Abb's Head, i. 338 Staffa, i. 25; first notice of, ii. 109; columnar basalts of, 186, 188, 210; basalt conglomerate of, 195 Staffordshire, latest eruptive rocks of, ii. 101, 103 St. Andrews, old volcanoes near, ii. 71, 73, 87 St. David's, Cambrian volcanic rocks of, i. 145 ---- Head, i. 205 Steam in volcanic action, i. 13, 15, 16, 71 Stecher, Dr., i. 421, 451; ii. 165 St. Kilda, dykes of, ii. 173, 416; gabbro of, 358; general account of geology of, 405; granophyre of, 408 Stocks, or bosses, i. 78, 88 Strahan, Mr. A., i. 171; ii. 10, 12, 23, 28, 32 Strathaird, ii. 123, 140, 164, 269 Strathbogie, i. 344; ii. 121 Strathmore, i. 304 Stromboli, i. 4 Sublimations, traces of ancient, i. 445 Submarine eruptions, i. 48 Sub-ophitic structure, i. 417 Subsidence and volcanic action, i. 295, 297, 444, 463; ii. 42, 205, 447, 470 Subterranean igneous injections, i. 77 (see Bosses, Dykes, Sills) Suess, Prof. E., ii. 474 Sun-cracks, i. 342 Sweden, Archæan volcanic rocks of, i. 120 Syenite, ii. 366 Symes, Mr. R. G., i. 311, 343, 369; ii. 201, 428

Tate, G., ii. 3, 113 ---- R., ii. 204 Tatlock, Mr. R. R., i. 273, 278 Tawney, E. B., i. 157 Teall, Mr. J. J. H., i. 90, 114, 116, 117, 118, 119, 120, 156, 192, 194, 200, 207, 210, 275, 277, 290, 311, 336, 338, 346, 407, 449; ii. 2, 3, 5, 7, 11, 32, 44, 113, 131, 134, 135, 137, 138, 140, 144, 149, 292, 293, 329, 367, 368, 369 Teesdale, i. 228 Termier, M. P., ii. 375 Terrestrial volcanic eruptions, i. 50 Tertiary Volcanic Series, ii. 181 Subaerial character of eruptions, ii. 103, 198 Scenery of, ii. 255, 256, 349, 391, 405, 408 The Plateaux, ii. 183, 249; lavas of, 183, 218, 236, 256; thickness of individual sheets, 192, 206, 254, 257; lenticular character of lavas of, 193, 257; greatest depth of, 210, 211, 213, 260; tuffs and clays of, 194, 202, 204, 211, 222, 225, 251, 258, 277, 284, 287; non-volcanic fragments in, 196, 211, 213, 219; lignites of, 198, 203, 208, 213, 251; gravels and conglomerates of, 198, 212, 238, 256; coal of, 213, 251, 256, 287; leaf-beds of, 204, 212, 222, 225, 288; carbonaceous nature of the upper parts of intercalated sediments in, 223, 226, 227, 229, 232, 251, 288; evidence for intervals between the eruptions in, 203, 205, 208, 221, 228, 240, 245, 251, 254, 288; no evidence of great central vents in, 208, 214, 255, 258, 260, 267; faulted condition of, 200, 208, 209, 452; subsidences of, 205, 208, 209, 214, 447; ancient river channels of, 217, 228, 231, 234, 456; volcanic cones of, 202, 218, 230, 277, 281, 285; paralleled by the modern Icelandic eruptions, 260; vents of, 202, 218, 230, 270, 276 The Basic sills, ii. 298, 304 The Gabbro bosses, ii. 327, 349, 355, 358; history of the gabbro intrusions, 359 The Acid rocks, ii. 364; petrography of, 366; history of their investigation, 371; analogies with trachytes of Central France, 373; intruded at base of the gabbros or of the bedded basalts, 337, 353, 357, 431, 432, 444; bosses of Skye, 378; of Mull, 395; of Small Isles, 405; of St. Kilda, 405; of Arran, 418; of Carlingford, 420; of Slieve Foye, and Barnavave, 421; of Slieve Gullion, 422; of Antrim, 426; acid sills, 430; acid dykes and veins, 437 Metamorphism of the basalts, ii. 272, 276, 337, 339, 340, 347, 355, 356, 357, 358, 362, 378, 383, 386, 397, 399, 400, 404, 413 Texture, varieties of, in igneous rocks, i. 78, 449; ii. 5, 299, 360 Tholeiites, ii. 137, 158 Tholeiite type of basalt, i. 419, 421 Thornhill, volcanic rocks of, ii. 60 Thoroddsen, Th., ii. 261, 262, 263, 264, 265, 266, 278 Thrust-planes, i. 229 Time in geological history, ii. 107, 461, 465 Timmins, J. H., i. 133 Tinto, i. 278, 288, 329 Titterstone Clee Hill, ii. 101 Toadstones of Derbyshire, i. 359; ii. 8 Topley, W., i. 147; ii. 3, 5, 7 Torridonian rocks, i. 111, 112, 113, 120; ii. 350 Tortworth, volcanic rocks at, i. 238 Tourmakeady, volcanic rocks of Bala age at, i. 251 Townson, R., i. 363 Trachyte, i. 183, 230, 246, 273, 276 (analysis), 379 (analysis), 386, 403, 407, 421; ii. 36, 47, 96, 138, 152, 184, 236 Traill, Mr. W., ii. 175, 421, 422 Traprain Law, i. 372, 380, 403, 405 Traquair, Dr. R. H., i. 266 Tremadoc group, i. 144, 177 Trevelyan, W. C., ii. 3 Triassic eruptive rocks, i. 29; geography, ii. 108 Trichites, i. 19; ii. 136 Troctolite, ii. 332 Tuffs, i. 31; association of, 33; composition of, 34; alternations of, 34, 61; blending of, with non-volcanic sediment, 35, 437; fossiliferous, 36; without lava, 36; necks of, 58; relation of, to lavas, 61; metamorphism of, ii. 224 ---- pre-Cambrian, i, 125, 135; Cambrian, 147, 151, 155, 163, 165, 167; Silurian, 178, 189, 190, 195, 205, 209, 212, 213, 222, 224, 229, 232, 241, 245, 246, 254, 255; Devonian, 262; Old Red Sandstone, 279, 281, 289, 337, 339, 351; Carboniferous, 381, 384, 387, 399, 422, 427, 429, 432, 436, 466; ii. 11, 18, 24, 36; Permian, 57, 58; Tertiary, 194, 197, 202, 204, 211, 222 Tyrol, Triassic eruptive rocks of, i. 29 Tyrone, Old Red Sandstone of, i. 314

Ulster, Old Red Sandstone volcanic rocks of, i. 314 Ultra-basic rocks, i. 14, 118, 377, 417 Unconformability, deceptive case of, i. 163 Urgneiss, i. 110 Uriconian volcanic rocks, i. 129 Ussher, Mr. W. A. E., i. 260, 262; ii. 35, 95 Utah, laccolites of, i. 86; volcanic regions of, ii. 115, 267

Valleys, tendency of vents to appear in, i. 272, 368, 376; ii. 61, 65, 96 Vapours, action of volcanic, i. 13, 15, 16, 17, 31, 57, 72, 78, 97, 99, 180, 289, 426 Variolitic structure, i. 21, 206, 235 Veins, intrusive, i. 66, 77, 79, 98, 426, 429; ii. 311, 313, 400, 410, 432, 437 Velay, volcanic rocks of, i. 26, 27, 29, 60; ii. 271, 373, 375 Vents, volcanic, i. 53; ground-plans of, 54; size of, 55; filled with non-volcanic detritus, 57; ejected crystals found in, 62; agglomerates of, 62; ii. 13, 28, 47, 61, 69, 276, 280, 284, 288, 289; stratification in, i. 63; ii. 80; metamorphism in, i. 67, 71; metamorphism of rocks around, 72, 349, 350, 352, 399, 404, 432; ii. 76, 272, 280; connection of, with geological structure-lines, i. 68; occurrence of, in lines and in groups, 69; double and multiple, 69; possible indications of length of activity of, 72; inward dip of strata around, 73, 352; ii. 76, 295; stages in history of, i. 74; tendency of, to rise in lines of valley, 272, 368, 376; ii. 61, 65, 96, 468; criteria for the relative ages of, 270; connection of, with later eruptive bosses, 280, 384, 399, 400 ---- Silurian, i. 209, 214, 234; Old Red Sandstone, 272, 287, 298, 305, 323, 328, 337; Carboniferous, 394, 399, 400, 404, 406, 424, 465; ii. 13, 28, 47; Permian, 61, 69; Tertiary, 202, 270, 294, 400 Vesicular structure of lavas, i. 15; ii. 187 Vesuvius, denudation of, i. 3; as an active volcano, 4; as a type of volcano, 10, 39, 53, 100; ii. 108, 115, 261, 266 Vicary, Mr. W., ii. 95 Vogesite, i. 277, 293 Volcanello Island, i. 70 Volcanic action, permanent traces of, i. 4; of present time elucidates that of the past, 5; submarine, 5; transient effects of, 8; chief factors in, 10; explosive energy of, 13, 99; uniformity of, in geological time, 13, ii. 470; metamorphism by, i. 67, 71; underground phases of, 77; proofs of gradual quiescence of, 155, 157, 166; connected with subsidence, 295, 297; ii. 205, 444, 463, 470; repetition of, in the same region, i. 368, 375, 377; ii. 42, 69, 94, 467; developed along continental borders, 466; persistence of, in Britain, i. 7; ii. 466; connection of, with lines of geological structure, 468; connection of, with terrestrial disturbance, 469; gradual decline of, during Palæozoic time, 471; quiescence of, during Mesozoic time, 472 Volcanic cycles, i. 27, 92 ---- products, general characters of, i. 14; persistent uniformity of, 30, 46; thickest mass of, in Britain, 229 (see Agglomerate, Lava, Tuff) Volcano, Island of, i. 4, 24 Volcanoes, their influence on mythology, i. 1, 2; denudation of, 3; number of extinct, 4; ancient, of Britain, 6; influence on scenery, 8, 100, 102; defined, 10; types of, 10, 39; ii. 471; determination of relative dates of, i. 46; their geographical condition in old times, how ascertained, 48; parasitic, 69; contemporaneous denudation of, 73, 100; connected with granite, 89; incompleted, 86, 93, 99 Vom Rath, G., ii. 474

Wacke, i. 157 Walcott, Mr., i. 30 Wales, pre-Cambrian rocks of, i. 126, 142; early geological work in, 142; Cambrian volcanoes of, 145, 159; volcanic scenery of, 176; Silurian volcanoes of, 176, 202, 205, 207, 218, 219; Old Red Sandstone of, 257, 259 Waller, Mr. T. H., i. 171, 278 Ward, J. C., i. 227, 228, 229, 230, 231, 233, 234, 235, 236, 237; ii. 23 Warwickshire, Cambrian rocks of, i. 137, 171 Waterford, volcanic region of, i. 247 Watts, Mr. W. W., i. 131, 132, 135, 137, 189, 190, 191, 243, 276, 278, 336, 347, 417, 421, 423; ii. 40, 42, 43, 45, 57, 96, 184, 204, 224, 272, 424, 425 Weaver, T., i. 238 Wenlock group, i. 175; volcanic rocks of, 552 Wernerian School, ii. 109 West Lothian, volcanic rocks of, i. 47, 55, 415, 433, 437 Whin Sill of England, i. 83, 85, 97, 449; ii. 2 Whitehurst, J., ii. 9, 109 White trap, i. 96, 426, 449, 456; ii. 65, 87, 103, 165, 252 Williams, Mr. G. J., i. 179, 185, 186, 188 Williamson, W. C., i. 392 Wilson, Mr. J. S. Grant, i. 148, 149, 153, 276, 344, 375, 379, 380; ii. 137, 164 Wilson, Mr. A., ii. 49 Winch, N. T., ii. 113, 147 Witham, H. T. M., ii. 113, 238 Wood, N., ii. 113 Woods, Mr. H., i. 204 Woodward, Dr. Henry, ii. 449 Woodward, Mr. H. B., ii. 32, 435, 453 Worcestershire, latest eruptive rocks of, ii. 101 Worth, Mr. R. N., ii. 99 Wrekin, i. 130 Wright, J. R., ii. 102 Wunsch, E., i. 369, 392 Würtemberg, puys of, i. 46 Wyoming, lava-fields of, ii. 115

Yates, J., i. 171 Yellowstone Park, volcanic phenomena of, i. 29, 31 Y-foel-frâs, i. 209, 214 Y Glyder-Fach, i. 209 Yoredale group, ii. 9, 13, 17 Young, Mr. John, i. 369, 392 Young, Prof. John, i. 294, 308

Zircon, found in volcanic vents, i. 62 Zirkel, Prof., ii. 327, 329, 334, 356, 364, 370, 372, 379, 430

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