Nature's byepaths : a series of recreative papers in natural history

Front matter

SIR WILLIAM CROOKES, D.Sc, FR S.

By the same Author.

FLOWERS : their Origin, Shapes, Perfumes, and Colours. Illustrated with 32 Coloured Figures by Sowerby, and 161 Woodcuts. Second Edition. Crown 8vo, cloth, 7s. 6d.

Contents

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CHAPTER PAGE

I. Subterranean Mountains . . . i

Chapter I. Subterranean Mountains

OSSIBLY no social crises have been without national importance. The general series of strikes for higher wages, and the reckless manner v/ith which the coalminers refused to work except their arbitrary demands were complied with, coupled with the high price to which coals have risen in the market,haveset men a-thinking about the possibility of adding to the coal supply. The geology of Great Britain is pretty well known, and everywhere mining engineers and landed proprietors have been asking ' B

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whether the existing coal-fields may not be profitably worked under strata of a later age ; and whether, here and there, a continuation of the area of certain coal-fields may not be discovered so as to make coal-mining profitable in new localities. Among other places, Norfolk has been suggested, and even our most eminent geologists — those who know most about such matters, as Messrs. Prestwich and Godwin-Austen — listen favourably to the suggestion. The Directors of the Great Eastern Railway are alive to the theoretical advantages, and some time ago invited eminent geologists to confer with them as to the desirability of some time commencing experimental borings somewhere in Norfolk or Suffolk. In West Lancashire, Leicestershire, and near Lincoln, it has been mooted to start similar experimental borings, so that in a short time we may hope to be in possession of some valuable national information on this most important question. Sussex has been distinguished by the Sub-Wealden enterprise, and the odds are, we shall have one commenced in the Eastern Counties ere long.

Let us briefly endeavour to give the reasons for the welcome conjecture that in Norfolk or Suffolk we may have available coal measures lying at no great depth under us. The localities mentioned in the " shires " all lie more or less in the great area which has generally been regarded as coal-producing, and therefore it does not seem such a matter for surprise that the margins of the already-worked coal-fields

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should be extended on every side. What is far more surprisingand unexpected is that faraway to the south-east and east of England it should be deemed possible that the coal strata lie beneath those of the chalk !

It cannot be denied that the subject is one of considerable difficulty to people who don't pretend to know anything about geology. But that science is now exercising such an important influence upon modern thought that it can no longer be withheld from a liberal education. To fully understand everything about how the coal strata lie in the crust of the earth, we are required to be acquainted with the succession and distribution of rock-masses ; and also with the physical geography of the ancient seas along whose bottoms these rocks were originally formed, as well as with the disturbances which have taken place since then. But even with all these scientific difficulties bristling round the subject, and rendering it anything but readable to such as have never attempted anything more than novels, we do not think it impossible to bring it before the attention of a non-scientific public so that the latter will care to read about it, if only for the sake of its probably influential value. Among the many wonders which geology has placed before the pubhc, none is more striking than the immense periods of time during which our earth has been in existence. Accumulations of ancient vegetation have been covered up with other deposits, and quietly stored away as coal. Creation after

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creation of animals and plants has passed away, and their remains packed up in the rocks of the earth's crust. The dry land of every geological epoch has been mapped with mountain-chains, plains, lakes, and rivers, just as the surface of the earth is now. For ages these various mountains were exposed to similar atmospherical wear and tear, the materials thus stripped off being carried away by rivers, etc., to the sea, to form deposits of a later date. There is no reason to suppose that this wear and tear was much more rapid than we see it going on around us, but it extended over such a long period of time that sedimentary rocks were formed, even before man's appearance on the earth, that had reached a total thickness of over 19 miles ! Volcanic outbursts and upheavals compensated for this wear and tear, and thus the relative conditions of land and water were always maintained, although their areas changed with the different epochs.

It will be seen that the physical geography of any of the past geological periods cannot be guessed at merely from the present surface arrangement — that is to say, the mountains and plains, the seas and rivers, which distinguish our globe now, bear Httle or no reference to those of former periods. Let us suppose that the entire area of Great Britain was slowly sinking (as we know the northern parts of Europe to be at the present time), and that this sinking were to be continued say for millions of years ! What would be the result ? That as the land sank the sea would gain, first on the lowest parts, would extend farther and farther, until when the depression had reached its climax the highest hills would be covered. The marine muds and sands would first be strewn over the plains, and the formations would gradually increase in thickness, until eventually they would bury and cover up the very mountain peaks ! If Snowdon or Ben Nevis or such a range of hills as the Pennine Chain were to be thus gradually entombed beneath a layer of rock to the depth of a thousand feet by such a great mud-sheet, then a huge plain would be the effect of the last great geological operation, and when this was uplifted from the sea-bottom, where it was formed, and inhabited by man, few people would dream of the mountain-chains that were buried beneath it. We are here only boldly sketching what the geologist can prove has repeatedly taken place. But suppose that the ancient rocks of such a buried Snowdon or Pennine Chain were coal strata. Then it would be evident, if we wanted to find coal beneath the newly-formed plain, that our plan would be to hit upon the place where these mountains are only a thousand feet below. For if we went farther away we should come over where the origiital plains lay below, at a depth of thousands of feet more, so that even if they were formed of coal rocks the great depth would utterly preclude coal-mining. It may seem strange to apply this supposition, but it can be

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proved that such a chain of buried mountains does extend beneath the eastern and south-eastern counties of England. This ancient ridge appears to slope off gradually north and south, so that, farther away from a given point, strata overlying it grow thicker. Of the rocks comprising this buried mountain-chain, those containing coal form only a part. We know, further, that these ancient mountains must have long been exposed to meteorological wear and tear, so that thick strata have been denuded and stripped off.

Let us briefly sketch the brilliant generalisation — it can be called by no other name — which led to the belief that a ridge of ancient rocks thus stretched across the eastern and south-eastern counties long before any facts concerning it were known. More than twenty years ago Mr. Godwin-Austen read a paper before the Geological Society of London on the subject, and as he stands in the first rank of those geologists who have studied the physical geography of the ancient seas, and, therefore, of the distribution of their sedimentary deposits or rocks, his views were listened to with the respect they deserved. In that paper Mr. Austen showed that the coal-fields of Bristol and South Wales, on the west, and of Northern France, in the east, were merely " outcrops " (or places where the coal strata came to the surface), of one continued area. Both these widely separated coal-fields have the same " strike " or run of their rocks. It is evident, therefore, that the Somersetshire range of hills, and that of the Ardennes, in Belgium, are continuations ; and the deduction is that this continuityis oitly broken as far as surface appearance is concerned, and that, beneath subsequently formed strata which have covered these hills up in the manner we have asked our readers to suppose it is still maintained. In fact, a great depression has bellied down an extensive part of this ancient ridge, and it is in the hollow that the later rocks rest.

A few years after Mr. Godwin-Austen's theory was put forth it obtained verification in a remarkable manner. A deep well had to be bored in Kentish Town, in London, over where, if the theory were true, the ancient ridge lay below. The borers passed through the Tertiary strata, as well as through the various subdivisions of the chalk, and then, at a depth of about 1300 feet, they came upon some queer-looking primary or ancient rocks. In fact, they had hit upon a part of the mountain-chain buried below ! No fossils were obtained, and this was an unfortunate circumstance, as geologists are as well able to tell the age and position of a rock by its fossils, as a gardener is to tell a tree by its fruit. But, as far as appearance and mineralogical structure went, the best authorities were of the belief that the ancient rocks thus reached belonged to the geological formation known as the Devonian. Now, this formation lies below the true coal-bearing rocks, so that in this particular spot the latter had either been stripped off

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or had never been deposited there. The former idea is believed to be the correct one. Professor Prestwich had been of the same opinion as Mr. Austen respecting the extent of the ancient and buried-up ridge, and both these gentlemen gave their full reasons for holding it before the Royal Coal Commission, where our readers who are curious may find their complete evidence printed in 1871.

Not long after the boring at Kentish Town had taken place, the Harwich Corporation commenced their waterworks, and several wells were bored to furnish the necessary water supply. The deepest went to 1070 feet, and here, at a depth of nearly 200 feet less than in London, the old ridge was again reached. This time, however, the Carboniferous or "coal-bearing" rocks were touched, and fossils obtained which are peculiar to that formation, so that the age and relative position of the ridge were at once known. But the part of the coal rocks thus reached was the lowest — that immediately resting on the sloping Devonian rocks, which we have seen underlie the metropolis. The part of the coal formation where the coal seams occur is the upper, consequently, if the latter rocks incline on the Harwich beds as those do on the metropoHtan, we should have to go a little farther north to find them, and this would bring us to the borders of Norfolk and Suffolk,

To prove that the ancient coal vegetation extended over the Norfolk and Suffolk area, we have only to notice the fact that coal is worked much farther to the south, near Calais, in France, where it is profitably wrought at a depth of 1500 feet. There the true coal-bearing rocks lie just underneath the chalk, the secondary strata being absent as they are under Harwich and London. The great axis of rocks, chiefly Silurian, Devonian, and Carboniferous, which constitute the Ardennes chain, in Belgium, dips westwards under the chalk of Northern France, and has been traced underground nearly as far as Calais. Nearer Boulogne these rocks are close to the surface, and they crop up in the Boulonnais from beneath the chalk. There can be no doubt that the ancient mountain-chain of which the Mendips still stand above at the western end, and the Ardennes at the eastern, like all mountain-chains, did not run in a straight direction, but was a curving line. Hence there is a difficulty to know how the line runs beneath us. Only repeated experiments can give us the necessary information, and a rich nation like England ought to conduct some borings in the interest of coming generations. Another thing which has to be considered in enquiring whether coal actually lies under the Eastern Counties is, whether the original rocks in which the coal-seams are imbedded have been denuded away before the later formed rocks were deposited. There is reason for supposing they have in some places, but not in all. Supposing the old ridge to extend at an available depth, and seeing

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the probability of the upper coal, measures succeeding the lower as the latter must succeed the Devonian, then, at no great distance north of Harwich, they will most probably be met with. For many technical reasons Hunstanton has been suggested as a likely place for a trial boring — chiefly because the lowest part of the secondary rocks crop up there, and Professor Hull has shown that these thin out so rapidly to the south that the Oolite and New Red Sandstone formations are not likely to occur beneath London, Subsequent deep borings in and near the metropolis have proved the sagacity of Professor Hull's hypothesis. From the pebbles enclosed in the strata about Hunstanton, geologists know that when they were deposited some part of the ancient ridge of rocks stood above water, for the latter were actually broken up, and these fragments have entered into the composition of those now occupying the surface. The composition of the Lower Greensand in Cambridgeshire, Bedfordshire, Buckinghamshire, and elsewhere, shows, by its large angular grairis of quartzite, that this deposit must have been formed in shoal water ; and the abundant pebbles of old rocks, grains of quartzite, and even of coal, plainly indicate the neighbourhood of land composed of Palaeozoic rocks. The number of Oolitic and other extraneous fossils, mixed up in the Cambridgeshire and Bedfordshire Greensand beds, with the fossils belonging to that formation, also indicates that the older secondary

SUBTERRANEAN MOUNTAINS. ii

formation must have been very much denuded and otherwise broken up (as well as the Primary rocks), or we should not have such a strange medley of organic remains as we find in the Greensand phosphatic nodules.

On the other hand, it would appear certain that south of the ridge of underground mountains, the Oolitic and associated rocks are actually thicker than elsewhere in England. This was proved by the famous Sub-Wealden boring — an enterprise which was at once a glory and a disgrace to the British nation. It was a splendid scientific experiment to originate and carry on ; it is a deeply lamented fact that in a wealthy country like England it had to come to an end just when every additional foot sunk deeper in the boring was getting of greater importance.

Whilst the boring was still in progress we had the opportunity of visiting the locality in Sussex where the " Sub-Wealden Exploration," as it was called, was made. St. Leonards was reached by rail, and we proceeded thence, by way of the famous town of Battle, to Netherfield. For miles after leaving Hastings, the road runs over hill and dale in nearly a straight line, thus indicating its possible Roman origin. The Norman invaders must have proceeded by this very road. Reaching Battle, we noticed how the physical character of the scenery suddenly changes close to the town, and we are told that this is due to an enormous "fault" or dislocation of the solid rocks beneath, which has brought up the lower beds to the surface. It is certain that this has operated upon English history, for it was chiefly by taking advantage of the position thus brought about that the Conqueror defeated the Saxons under Harold, and established a Norman dynasty in England. As we passed along these straight roads, with their high hedge-banks, we could not but think of that important event which occurred hereabouts eight centuries ago, and wonder what the England of the nineteenth century would have been, had the Norman invasion never taken place ! The many bright flowers of an English summer were competing for the mastery of the luxuriant hedge-banks. Gnarled old oaks, chiefly dwarfed by the sea-breezes, lined the road-side, and occupied the same places they possibly held when the Norman troops marched past. But our eyes caught glimpses of a period infinitely older than that of the Normans, for the flag-stones which pave the sides of the road are each distinctly impressed with ripple-marks ! They come from the formation known as the " Hastings Sands," one of the upper beds of the great Wealden deposit which underlies the whole of this part of Sussex. The Wealden was a secondary formation elaborated at the mouth of an enormous river as a delta, where the materials brought down accumulated until they reached a thickness of two thousand feet. Since then, these sedimentary beds have been converted into rock, but, as in the case of these ripple-marked sandstone flags, they still retain many traces of the physical and vital conditions that were in force at that distant epoch. In these deposits you obtain the remains of the great fossil reptiles, Igtianodon, Megalosaurus, etc., huge monsters thirty feet or more in length. We get fossil relics of palmtrees and tree-ferns which then clothed the dry land, and, as we approached Netherfield, we came to limestone beds composed entirely of the remains of freshwater shells. A wonderful country is this, and one cannot wonder it should be so fertile, seeing that its subsoils are the remains of an old river delta, just as the fertile land of Egypt owes its riches to a similar physical cause.

We passed through the quiet old town of Battle, which seems as if the onward tide of progress had left it high and dry. From Battle we passed Normanhurst, and other villages whose termination of hurst shows that they were founded in an ancient forest. The scenery is most magnificent. The country is one grand undulating, forest-clad area, and as such has very little altered its character since the days of the Saxons. Indeed, one cannot doubt that the surprised and affrighted remnants of Harold's army must have sought shelter amid these very coverts from the dreaded pursuit of their Norman foes ! In our rapidly changed country, it is quite refreshing to come across spots so rich in incident, and so thoroughly unchanged since those incidents took place. At

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length we came to a denser part of the wood, away from the main road, and found our way down a tolerably steep declivity, covered with brushwood and loose fragments of limestone. The brook, which is still cutting its way through these beds, may be heard and seen below ; and there we get a good section of the beds, and as many of such fossil freshwater shells as we like, for these limestone layers are literally composed of nothing else. Just before we reached the stream, a little wooden shanty and the three wooden legs of a crane met our eyes. A portable engine was puffing out balls of steam and smoke, and the otherwise unbroken silence of this old English forest scene was disturbed. It would not have been difficult for us to imagine we were in Australia, and that this was some exploring party in search of gold. Here, however, was being carried on an underground exploration, suggested by the profoundest scientific sagacity, to which the keen attention of every geologist in Great Britain was directed.

We all know the result of this boring before it was stopped for want of funds. The upper Oolitic beds were found to be of extraordinary thickness, and practically all the money raised for carrying on the exploration was spent in boring through this one formation. But even the unsuccessful result of the Sub-Wealden Exploration has not been without its scientific value in tracking out the strike of the subterranean mountain-chain. We now know that the experiment was carried on too far to the south of London. And this conclusion is strengthened by more recent discoveries, the result of several deep-well borings in and near the Metropolis which, had they been known at an earlier period, would probably have had the effect of commencing the experiment elsewhere than in Sussex, and very probably in the north of Essex or the south of Suffolk.

Let us glance briefly at the nature of these metropolitan well-borings. We have already seen the effect which the deep borings at Kentish Town and Harwich had upon the idea that a ridge of Palaeozoic rocks extended beneath the south-eastern counties. The well — and other borings at Burford (in Oxfordshire), Ware in Hertfordshire, Turnford, and at Meux's Brewery in Tottenham Court Road, London,- have furnished the most complete confirmatory evidence of the trend of the subterranean mountain-range, and have also furnished us with specimens of its rocks and their fossils. At Burford, coal was actually reached beneath the overlapping of Oolitic rocks, and the nature of the fossil plants, etc., indicates that this is an underground extension of the Bristol and Forest of Dean coal-field. Burford is thirty-five miles east of either, and thus we have actually proved the Somersetshire coal-field to be running in an easterly direction, whilst similar borings beneath the secondary rocks of Northern France have proved that the Ardennes coal strata extend underground as far i6

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as Calais. The trial-boring for these buried-up strata will have to be made, in the future, somewhere along the line connecting Burford with Calais, which line will be more or less mapped out for speculative geologists by the recent well-borings in and about London.

Between Ware and Burford is a distance of about eighty miles. It would be a most valuable experiment to institute a trial-boring somewhere between these two points. The depth to which the shaft was sunk at Ware before the buried-up Palaeozoic rocks were reached was only eight hundred feet. Here the Upper Silurian strata were struck, and the boring tool brought up cores which showed the very angle at which the rocks were dipping, although, owing to the tool having to be turned round, it was not possible to tell the direction of the dip. Mr. Etheridge, of the Geological Survey, has published a list of thirty-three species of well-known Silurian fossils which were imbedded in the cores. At Turnford, at the depth of nine hundred and eighty feet, Devonian rocks were reached. At Loughton, palaeozoic strata were struck at one thousand and ninety-two feet from the surface ; and at Meux's deep well-boring, at a depth of eleven hundred and forty-eight feet, Devonian rocks were also reached, in which certain characteristic Devonian fossils were imbedded ; whilst the mineralogical character of the strata is said to be wonderfully similar to the Devonian rocks of the Ardennes, and of other places in Northern France. The Lower Greensand was found to be resting on the Devonian rocks at Meux's, but so altered from its general character as to indicate it was here a shore deposit, formed on and around the ancient Palaeozoic chain of hills. Here is evidence, therefore, of the last disappearance beneath the sea of what may have been for ages a range of Silurian and Devonian and even Carboniferous rocks, continuing as an elevated track from North Wales and the Mendip Hills right away into Germany.

Notice has already been made of the striking resemblances, in many physical characters, between the Somersetshire coal-field in the west, and the French and Belgian coal-fields in the east. The Carboniferous strata occur in small basins, owing to the rocks having been squeezed and folded. In France the strata are so much folded that in places the formations have been reversed ; and recently, in the full belief that the foldings had thus flexed the rocks until they had been locally reversed, borings were carried on in the Boulonnais district through the Devonian and Carboniferous limestone rock, and the coal strata were found beneath ! All along the trend of the subterranean range of hills there can be no doubt the strata are highly inclined and considerably folded, so that the coal-fields must lie in comparatively small basins.

We have said enough, however, to show how modern science seizes isolated and seemingly discon-

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nected facts, and, by properly connecting them, arrives at the most nationally important generalisations. Should the reasoning hold good, and the experiments we have mentioned, past and future, end in the discovery of new coal-fields, we shall have another illustration of the genuine seership of science.

Chapter II. Over An Old-land Surface

T was recently our good for- 2 tune to explore and personally examine one of the most interesting old-land surfaces in Europe.

In the south-easterly parts of France the geological formation termed the Oolite extends over an enormous area.

Large rivers, such as the Lot, Aveyron, and Garonne, have excavated wide valleys out of the rolling and uneven table-land of Oolitic limestone.

Nowhere in sunny France is more beautiful scenery to be found than in the bosoms of these valleys. Life flows along in a quiet, unbroken fashion. The land of the vales is wonderfully rich, for the inundations of the rivers cover the cultivated areas from time to time with newly fertilising soils.

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Maize, tobacco, lucerne, and rye carpet the low-lying lands with sheets of splendid greenery. The slopes of the somewhat precipitous hills are clad with little vineyards, or packed with forests of young trees intended ultimately to serve for fuel in the absence of coal. No railways have as yet invaded this pleasant region, and the aspect of things has not much changed during the last two hundred years. There are no large landed proprietors, and no capital therefore has to be expended in the newest scientific agricultural improvements. Ploughing is still performed with a rude wooden implement drawn by a couple of oxen or cows. The whole country is mapped out into neat little enclosures. Every peasant is a landowner, and has a "stocking" in which his hard-earned savings are stored away. The people labour from early morn to dewy eve, for each is working for himself.

A new industry has recently arisen in this part of France. A few years ago a retired army surgeon, who had picked up a smattering of geology, found some queer-looking stones lying on the surface of the rubbly ground in the department of the Aveyron. He was curious enough to have them analysed, when they were discovered to be phosphate of lime, now called " phosphorite." Few minerals have been more largely sought after than this since Liebig suggested the application of mineral manures to soils, and when the phosphate of lime deposits in the south of France were heard of in England, companies and private firms commenced exploring them. Plots of land belonging to peasant proprietors were purchased, or the phosphates known to underlie them were worked bypaying a royalty of so much per ton.

These phosphate deposits have brought to light the remains not only of an old-land surface but of the animals which swarmed over it during the period known as the later Eocene or the early Miocene. The places where the phosphate mines are most extensively worked are Montauban, Caylux, St. Antonin, Quercy, and Bach. All these are upon the widestretching plateau of Oolitic limestone. The phosphate occurs, as a rule, only on the highest ground, or on the summits of the rounded hills. There it partly fills up old caverns or gullies which were once ravines or gorges. During the process of extraction these ancient modifications of the surface are in a great measure restored to their original conditions — the caverns are once more hollow and empty, and the gorges are opened out just as they naturally existed during the early Miocene period. All around the summits of the hills the limestone must have been sorely weathered and denuded since that distant epoch, so that these caverns and gorges are only the relics of a dry land surface which was once as chequered with them as is now the mountain limestone of Yorkshire and Lancashire with "swallow-holes " and caves, and that of Derbyshire with such gorges as "Cave Dale" and the "Winnats." If we

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could restore all the carbonate of lime which the rivers have carried away to unknown seas in the interim, it would go far towards levelling up the valleys and hollows to the height of the hill-tops. There are no traces of " drift " deposits hereabouts, and therefore we have every reason to suppose the country was dry land at the time when Great Britain was wrapped in a vast Greenland-like ice-sheet.

In these ancient French gorges are a few traces of another formation which overlay the limestone in places, but has since been removed by denuding agencies. Some of the larger of them are filled up with a kind of coarse, well-bedded sandstone, resembling many of those formed during the middle Tertiary epoch. Evidently this once extended over part of the country, but the wear and tear of atmospherical action has removed it elsewhere, having planed down the rocks of the neighbourhood and left nothing of the sandstone formation but what was preserved in these sheltered positions. The sides of these ancient gorges and caverns are covered with layers of stalagmite, which has evidently oozed out of the limestone rocks, just as we may see it now in the well-known and much visited caves of Matlock and the Peak district. It is caused by water in which lime is dissolved finding its way out of the rock along the walls of caves, etc. Part of the water is absorbed by the air, and thus the lime which it previously contained is precipitated as a fine film. In time it upholsters the rugged rock