A sketch of the physical structure of Australia, so far as it is at present known

Front matter

A SKETCH

of the

PHYSICAL STRUCTURE

Preface

It is now four years since the notes were written on which this little book is founded. I read an abstract of it at the meeting of the British Association, at Southampton, in 1846; another abstract was read at the Geological Society, in November, 1847. In consequence, however, of its length, and of its being not entirely composed of original observations, it could not be published in extenso in the proceeding's or transactions of either of those bodies. I have since frequently intended to publish it myself, in order that the labour I expended on its compilation might not be entirely thrown away; but have been too much occupied with the duties of the Geological Survey of Great Britain, to which I am now attached, to spare the necessary time for its completion. Buried for the most part in the heart of the Welsh mountains, I have been unable to do more now than take advantage of some rainy days this spring for revising my original rough notes. I mention these facts in some measure to excuse the incompleteness of many parts, and the hastily-written style of all, and trust the reader will pardon me accordingly should his eye or his ear be offended.

Introduction

I fear it will appear to most persons that an attempt to give any tolerable account of the geological structure of so large a country as Australia, from the few observations that have been as yet made and published concerning it, must be a very rash one. This must be especially the case with geologists, accustomed only to the full, varied, and complex structure of western Europe. It is quite clear that any such account must be very incomplete and imperfect. Still I think that sufficient materials have been collected to give a connected outline, a sketch of the structure of that country, which shall exhibit its principal features in a rude and approximate way. My own reasons for attempting such a sketch are, that having visited some portion or portions of all the coasts of Australia, and having made geological notes upon them, I was enabled to understand and connect the observations of others, and that by studying and comparing all the observations I could find in any published accounts, I conceived a general but distinct notion of its structure in my own mind. This notion I shall endeavour to convey to the reader. If it should eventually turn out that my speculations and generalizations are incorrect, no one will be more rejoiced than myself at the acquisition of the more accurate and fuller knowledge that shall overthrow them.

In the meantime I would remind the reader that detached observations have a far greater relative value in such a country as Australia than they would have in a complicated country like Europe. When mountain chains and sheets of rock stretch for hundreds of miles without an interruption, or a change in their general character, cursory observations at great distances one from the other suffice to give us an outline of the country. In Russia, in N. and S. America, as in Australia, observations made across one line of country would thus be applicable to great tracts, while in western Europe a series of observations along" any one line might give no information as to the rocks or formations to be met with, even a few miles on each side of it. Australia especially seems the very land of uniformity and monotony, the same dull and sombre vegetation, the same marsupial type of animals, spread over the whole, land from the gloomy capes of the south coast of Tasmania, and the stormy Leeuwin, to the cloudless and burning skies of Torres Straits and Port Essington. The reader will, I think, see reason in the following pages to believe that the rock formations of Australia are equally uniform and extensive.

In the colouring of the two maps that accompany this volume, I must warn the reader not to expect any accuracy of outline. They are not geological maps in the strict sense of the word, the geological colours being only dabbed on roughly about the place where the rock indicated by it was observed, as nearly as I could guess its size and locality. The colours are only intended to come in aid of the description, and to guide the eye and help the memory of the reader.

PRINCIPAL PHYSICAL FEATURES.

The great physical features of Australia, as far as they are at present known, may be very briefly described.

1. A long, but not lofty, mountain chain runs along the whole eastern coast, crossing Bass's Straits into Tasmania, and running under Torres Straits to the shores of New Guinea.

2. On the landward, or western side of this chain are great plains, declining gradually to the west, but at first often broken by detached hills or groups of mountains. These plains are traversed by the rivers Murray and Darling and their tributaries, on the south, disemboguing into Lake Alexandrina,—by Victoria River (and perhaps some others) in the centre, which drain into Sturt's central desert, and in seasons of flood probably disembogue by Lake Torrens, and by the small rivers which, on the north, run into the south-eastern portion of the Gulf of Carpentaria.

3. West of the tracts thus described appear immense desert plains, which seem to extend to the sea coast round the Gulf of Carpentaria on the north,—to that of the Great Australian Bight on the south, and to stretch along the N.W. coast, from Northwest Cape to Collier Bay.

As minor, but sufficiently important features, may be mentioned,—

4. The mountain chain of South Australia running north from Cape Jervis to the singular horse-shoe shaped depression of Lake Torrens.

5. The high land of Western Australia, running north from Point D'Entrécasteaux and King George's Sound to the neighbourhood of Shark Bay.

6. The high land which forms the coast from Collier's Bay to Wickham's Victoria River, and seems to stretch in an east and west direction across the interior of Arnhem Land, south of Port Essington to the western shores of the Gulf of Carpentaria.

I shall endeavour to describe the structure of each of the districts above named somewhat in the above order.

I.—The Eastern Coast.

The mountain chain of the eastern coast is, apparently, a rather irregularly formed and complex one, but, it seems to preserve the same or very similar features throughout its extent. It is no where, so far as is known, a single ridge of mountain, but is made up of many masses of various character,—sometimes peaked and serrated ridge—sometimes detached hills rising from slightly elevated ground, —sometimes great table lands, often ending towards the sea in nearly perpendicular escarpments,—sometimes having on one side or other gently sloping plains furrowed by innumerable precipitous gulleys and ravines. Large lateral spurs often diverge on either hand from the central portion of the chain, and, especially on its seaward side, appear like separate and independent mountain masses.

Tasmania may be said to be entirely occupied by this chain, as it is a complete net-work of ridges (called there "tiers") enclosing a multitude of small plains and valleys. In at least two places the summits of these ridges attain a height of upwards of 5,000 feet, while several reach that of 4,000. From the N.E. corner of Tasmania the chain may be traced across Bass's Straits by the curved line of lofty and rugged islands that lead up to Wilson's Promontory. From this point the chain strikes into the interior of New South Wales, and shortly attains its greatest elevation in the "Australian Alps." The height of the loftiest peak of this group, named by Count Strzelecki Mount Kosciusko, is stated by that observer to be 6,500 feet. These are the only mountains in all Australia that are covered by perpetual snow; from which it follows (as a necessary consequence in that climate*) that the only perpetually flowing river of the country, the River Murray, rises at their feet. The chain runs thence through the whole of New South Wales nearly parallel to the coast, or about N.N.E., its water-shed preserving a mean distance from the sea of about eighty or one hundred miles. It sends out, however, many lateral spurs, such as those which on the land side divide the head-waters of the rivers Murray, Murrumbidgee, Lachlan and Macquarrie, the Namoy, the Gwydir, and the Karaula, and on the sea-side separate the respective basins of the districts of Gipps' Land and Twofold Bay, the Shoalhaven River, the Hawksbury and Hunter Rivers, the Port Macquarrie basin, that of the Clarence River, and that of the district of Moreton Bay. Throughout this part of its course the elevation of the highest points of the main chain varies from about 2,000 to about 4,000 feet above the sea.f About Sandy Cape, or in S. lat. 25° the coast line changes its direction from N.N.E. to N.N.W., which latter bearing it maintains to its termination at Cape York in S. lat. 10° 43'. Through this northern portion of its course the constitution of the chain is only known from observations made upon the coast, and those of Dr. Leichhardt in his overland journey from Moreton Bay to Port Essington. Numerous ranges of considerable magnitude, having a north and south course, strike out upon the coast with many bays and indentations behind their headlands. There are, also, lofty islands stretching off the coast to a distance sometimes of thirty or forty miles. In the interior, Leichhardt makes mention of many mountain masses, whose waters, between the parallels of 25° and 18°, all seem to flow down to the eastern coast. Mount Pluto, described by Sir T. Mitchell in his last expedition, in lat. 25°, and long. 147° 30', or nearly 200 miles from the coast, seems to be the most distant source of some of these waters, as the M'Kenzie River of Leichhardt, and the River Beylando of Mitchell, or Cape of Leichhardt; and from it also proceeds Mitchell's Victoria River, running into the interior. North of these, however, the water-shed seems again to approach the coast line, as the head waters of the Burdekin and Separation creek, about 18° and 19°, are not at a greater distance from the sea than 100 miles, and the river Lynd which Leichhardt struck on near lat. 18° flows to the N.W. into the Gulf of Carpentaria. In this neighbourhood also, or between Cape Upstart and Cape Melville, the loftiest and most massive mountain ranges strike out upon the very coast as if the heart or central axis of the chain was here actually washed by the sea. Thus far several of the highest points still attain the height of 4,000 feet above the level of the sea, and its mean elevation cannot be much less than in New South Wales. North of Cape Melville the chain rapidly diminishes in height and importance, few parts being higher than 1,000 feet, and at Cape York we have only insignificant hills of 300 or 400 feet high. As in Bass's Straits, however, so also in Torres' Straits, the submarine continuation of the chain may still be traced in a line of steep, rugged, and often peaked islands, several of which are 7 and 800 feet high, which run in a north and south direction up to Mount Cornwallis, which is close to the coast of New Guinea. The distance in a straight line between the S. Cape of Tasmania in 43 35', and Mount Cornwallis in 9° 45', is 43° 35' of latitude, or about 2050 geographical, 2370 statute miles; following the slightly curved line of the chain itself, its extreme length will be about 2,400 geographical, or 2,780 statute miles.

We have now to describe the geological structure of this chain. It cannot be said to have any one well-defined axis of granitic or other rocks, or if it has, that axis is often deep-seated, and is not apparent at the surface. Large granitic masses, however, shew themselves at different portions of its course, sometimes forming whole mountain groups, and there is probably no very considerable portion of the chain without the exhibition of granitic rocks, either in its central ridges or on some of its flanking or lateral ranges. Gneiss and mica slate do not seem to be any where abundantly exhibited, although they not unfrequently occur, together with considerable masses of clay—slate and other rocks more or less metamorphic. Great masses of porphyry and greenstone are often associated with these, especially in the lateral ranges.

Upon these rocks lies a large palæozoic formation, which is principally composed of very thick masses of sandstone interstratified occasionally with beds or groups of shale and some beds of limestone. In this palæozoic formation, perhaps only in a particular part of it, beds of good coal are found. Masses of porphyry, greenstone, basalt, and other igneous rocks, are associated with the palæozoic formation, often cutting through, dislocating, and altering various parts of it, but sometimes seemingly supporting it as if existing before its production.

Tertiary rocks again are found reposing at various places on all the above-mentioned rocks, up to a certain height above the sea, and these are likewise associated with still more recent igneous rocks, which sometimes retain all the characters of veritable subaerial lavas.

A. Tasmania.—Granitic and metamorphic rocks are described by Count Strzelecki as existing in large mass in the N.E. corner of Tasmania, and in still larger force over its S. W. portion. They shew themselves also at three other smaller districts in the centre and on the northern coast. The rest of the island seems to be generally formed of great intersecting ridges of greenstone, enclosing plains or valleys composed of the palæozoic formation. Some of the more massive greenstones are evidently anterior to the palæozoic formation, as sections may be seen in which horizontal beds of sandstone abut against cliffs of greenstone, or even repose upon steps and ledges of that rock. In other cases, however, sandstones of the palæozoic formation appear to be capped by immense masses of greenstone, and they are certainly often broken through by basaltic and other igneous rocks. The following details on the geology of Tasmania are extracted from a paper of mine in Journal of Geological Society for 1847.

The Valley of the Derwent River.

Along the S.W. side of the valley of the Derwent runs a bold range of flat-topped hills, of which one of the principal promontories is Mount Wellington, rising immediately behind Hobarton to a height of 4,200 feet above the sea. The upper portion of this range is composed of massive greenstone, often forming rude columns of great size, frequently as much as ten feet in diameter. The lower slope of this range, and much of the country forming the opposite side of the valley, is composed of the palæozoic rocks. These lie generally in a nearly horizontal position, and I believe abut horizontally against the greenstones; but as I never found a clear section near the junction of the two, I cannot positively say that they do not pass under them,—that the greenstones of the hill-tops are not a thick capping resting on the palæozoic formation. In ascending Mount Wellington from Hobarton we first pass over a great thickness of white and yellow sandstones nearly horizontal; above these are shales and thin beds of limestone, likewise horizontal; over which again other sandstones are found. These rocks occur to a height of 2,500 feet above the sea, and apparently form a solid mass of that thickness at least. Above this point greenstone alone is to be seen, forming a mass 1,700 feet thick at least. Its total thickness depends of course on the undecided question, of whether it be a capping to the palæozoic rocks, or what I believe is more probable, a solid mass with the sedimentary beds resting against its sides.

Both the sandstones and limestones are quarried at several points. At Mr. Hull's limestone quarries at Tolosa, about four miles from Hobarton, I found dark grey limestone, sometimes compact, sometimes finely laminated, with fragments of shells and corals. The beds of limestone were about two feet thick, and in one place were some beds of soft brown sandstone interstratified with thin beds of limestone. These sandstones were scarcely consolidated, and fell to pieces on being taken from the quarry. They often contained fossil shells, both Spiriferæ and Productæ, quite perfect in appearance, but so much decomposed as not to bear extraction, falling into white powdery fibrous carbonate of lime. I procured from other parts of these quarries the following fossils :—

Fossils from Mr. Hull's Quarries.

Corals.

Stenopora Tasmaniensis. —— informis. Fenestella ampla. | Fenestella internata. —— fossula? Caryophyllæa

Mollusks.

Producta rugata. —— brachythærus. Spirifer subradiatus. —— Darwinii. —— Taamaniensis. | Spirifer Stokesii. —— Vespertilio. —— avicula. Pecten squamuliferus. —— Limæformis.

A few miles above New Norfolk, the banks of the Derwent shewed cliffs consisting of alternations of sandstone with black and brown shales, producing a precise resemblance to parts of the English coal-measures. Much fossil wood, apparently parts of large trees, lay in these rocks.

Similar rocks to these were frequently observed in the cuttings of the road-side as far as Oatlands in the centre of the island, and they almost invariably lay in positions so nearly approaching horizontality, that their dip was not appreciable to the eye. Still their continuity did not appear to extend unbroken over any large district, as not only were dykes and other masses of intrusive trap rocks frequent, but solid ridges of crystalline greenstone often intervened, and evidently cut off one portion of the palæozoic rocks from the other.

In the immediate vicinity of Hobarton there were places, as near Stoke, and at the mouth of the valley of Risdon, where the palæozoic rocks had evidently been tilted up and altered by masses of trap rock, which could be traced to have a perfect passage from compact tabular or amorphous basalt into hills of solid crystalline greenstone.

In other places quarries were opened in sandstones of the palæozoic age, forming small patches either embosomed in greenstone, or resting upon it. About a mile from a place called Ralph's Bay Neck, on the S.E. side of North Bay, I found a cliff where the sandstones were shewn clearly to be posterior to the igneous rock. In this case a dark, rudely columnar trap rock ended in a succession of small cliffs and terraces in one direction, upon which terraces and against which little cliffs rested the sandstone perfectly undisturbed, and evidently in the position in which it had been originally deposited.

A parallel instance was observed in the cliffs a little to the eastward of the entrance of Port Arthur.

It appears, then, that there are masses of greenstone both of more ancient and more modern date than the palæozoic rocks.

At Macquarrie Plains, about ten miles above New Norfolk, there is a large exhibition of igneous rock, which from its cellular character seems certainly to have flowed as lava in the open air. It forms a mass of considerable thickness, as shewn in the brooks and ravines, and appears to have been gradually accumulated by successive accessions of melted matter. I infer this from the fact of its including fossil trees, apparently in the position of growth, which seem to have been enveloped, while living, in the lava.

There are two small patches of tertiary travertinous limestones: one mentioned by Mr. Darwin, and found in the outskirts of Hobarton, where it appears to have been tilted by the intrusion of an adjacent mass of trap; another in a little cove called James's Bay, about three miles above Hobarton, on the opposite side of the Derwent. It rests here nearly horizontally, and is but little elevated above the level of the sea. A Helix and a Bulimus, and the leaves and portions of the stems of several plants, have been found in each locality.

Fossils from James's Bay.

Plants, unnamed : one figured by Morris. Helix. Bulimus.

There are very thick masses of gravel, consisting of pebbles as large as the fist, accumulated on the sides of the Derwent River at some places, and Count Strzelecki mentions great accumulations of loose sand, from beneath which he procured a large Cypræa. This was at Newton, a short distance from Hobarton.

B. Norfolk Bay and Tasman's Peninsula.

The principal mass of Tasman's Peninsula appears to be columnar greenstone, forming the highest and most rugged of its hills, and the gigantic perpendicular cliffs of Cape Raoul and the intermediate shores round the entrance to Port Arthur. Just to the eastward of the mouth of that harbour, a mass of the sandstone of the palæozoic formation, a quarter of a mile across and 200 feet high, may be seen resting against these perpendicular cliffs of columnar greenstone with its beds quite horizontal and apparently unaltered.

Point Puer, one of the projections inside the port is composed of a white compact, rather argillaceous sandstone, which among others contains the following fossils :

Fossils from Point Puer.

Producta rugata. Spirifer subradiatus. —— crassicostatus, MSS., sp. n. —— Stokesii. —— Vespertilio | Pterinea macroptera. Orthonota compressa. Allorisma, n.s. Pachydomus carinatus. Pecten squamuliferus.

Eagle Hawk Neck, the connecting link of Tasman's and Forrester's Peninsulas, is one of the celebrities of Tasmania, on account of the peculiar jointed structure of its rocks, forming what is called "the tessellated pavement." The rock is a very hard, brittle, fine-grained and compact grey sandstone or gritstone, lying in a horizontal position. It occasionally contains pebbles of granite, porphyry, or quartz rock.

The rocks abound in fossils, especially at the south point of Pirates' Bay. Among others I collected fine specimens of the following:—

Fossils from Eagle Hawk Neck.

Fenestella internata. Producta rugata. Spirifer crebristriatus. —— Darwinii. —— avicula. | Spirifer subradiatus. —— Vespertilio. Platychisma Oculus? Pachydomus carinatus.

On the opposite side of Norfolk Bay is a small peninsula about three miles across, in which is a large convict-station called The Mines. The mass of this piece of land consists of sandstone with some trap, but immediately at the back of the station is a small colliery. A bed of coal, of slight thickness and extent, is here worked. The following was the shaft-section as given me by the overseer:—

|Yards.

"Ironstone" (a fine-grained trap rock) | |20

Sandstone | |20

Sandstone and shale | |10

Coal | |1½

This coal, which in the deepest part is about seven or eight feet thick, rises pretty rapidly in every direction from that point, and as it rises it thins out to about two feet. It thus forms a small basin, not half a mile across, and its outcrop is everywhere covered by beds of loose sand. A little beyond its outcrop on the seashore was the following section:—

|Yards.

Trap (in small prismatic pieces) | 7

Sandstone, formed of grains of some trap rock | 18

Sandstone, soft and rather shaly | 6

Shale and bind | 2

Coal | 0½

Near this spot they had bored to a farther depth of nearly 100 yards and passed through one twenty-inch coal; but the rest of the mass was almost entirely sandstone. I got from these coal-measures fossil plants, among which were Pecopteris Australis, a Sphenopteris and a Zeugophyllites.

There are other places in Tasmania where coal is worked, but they are chiefly detached and isolated spots separated by greenstone ridges one from the other. I was not able to visit any other of these localities, but I should fear that the beds of coal in Tasmania are comparatively insignificant in an economic point of view, that the true coal-measures of the country have no great thickness, and that the seams of coal contained in them are but partial, thickening and thinning out perhaps along the same horizontal lines, and thus forming limited cakes rather than regular and persistent beds.

C. East Coast of Tasmania.

Rocks of the palæozoic formation, chiefly sandstones, are found at various point of the eastern coast, but greatly broken and obscured by the usual greenstone ranges and local exhibitions of other trap rocks. In Maria Island are limestone quarries which I did not visit, but from which I procured fossils, among which were some of the large Pachydomi, of precisely the same species as those from Wollongong in New South Wales.

At Spring Vale, about ten miles above Great Swan Port, is a patch of palaeozoic rocks, not more than a mile or two in extent, forming a low gently undulating ground surrounded by hills of igneous rock. No section is exhibited, but blocks of the rock protrude through the soil. It is a fine compact quartz rock, charged with the usual fossils of the formation in great abundance. The rock reminded me strongly of the quartz rock of the Lickey Hill. The fossils of this locality were

Fossils from Spring Vale.

Fenestella ampla. Producta rugata? Spirifer radiatus. ——Darwinii. ——Tasmaniensis. | Spirifer Stokesii. —— crassicostatus, sp. nov. —— three others. Stem of a Crinoidal animal.

B.—The islands of Bass's Straits appear to be chiefly granitic.

C.—New South Wales.

Wilson's promontory is all granite, and granite and metamorphic rocks appear from Count Strzelecki's description to extend thence to the Australian Alps, which are almost or entirely composed of them. To the east of this part of the chain occurs the low land of Gipps's Land, which appears to be chiefly tertiary. On the west of the chain palæozoic rocks, with coal, occur around Western Port, and are found in highly inclined positions at some points around Port Phillip where horizontal tertiary strata rest upon them. About Goulbourn and Bredalbane Plains the palæozoic rocks appear to stretch across the whole chain, but farther north granite and metamorphic rocks again appear at the surface, forming a wide district round the head waters of the river Macquarrie, and run thence in a narrower band to Liverpool plains, whence they again expand to the north, and occupy a wide band of country as, far north as lat. 28°. On the western or inland flank of the chain, limestone and other rocks of the palæozoic formation are mentioned by Sir T. Mitchell as occurring about Yass plains near the sources of the Murrumbidgee, between the heads of the Lachlan and Macquarrie rivers, and about the head of the Namoy or Peel river. On the eastern flank of the chain, the rocks on the coast of Twofold Bay appeared to me to be palæozoic, in which case they probably form part of a large tract of such rocks spreading over that district. North of this a granitic spur from the main chain with porphyry and other igneous rocks reaches the coast about the Shoalhaven river. North of that again is the largest and best known district of palæozoic rocks in New South Wales, stretching from Illawarra to the flanks of the Liverpool range, and traversed by the rivers Nepean and Hawksbury on the south, and the Hunter river on the north, with their various tributaries.

From the descriptions of my friend the Rev. W. B. Clarke, of Sydney, and from some excursions made either alone or in his company, as well as from the published accounts of Count Strzelecki, Mr. Darwin, and others, I am enabled to give a more definite account of this district of New South Wales than of any of the others. Immediately north of Illawarra, and on a line running thence nearly due north, about forty miles, to Campbeltown, we get the following section.

In the descending order.

1. Black and brown shales (named, I think, by Mr. Clarke, Wyanamatta shales,) 300 ft. and upwards.

2. The Sydney sandstone (or Hawksbury sandstone), thick sandstones, with a few thin beds of shale in its upper and lower parts; about 700 or 800 ft.

3. Alternations of sandstone and shale; about 400 ft.

4. Alternations of sandstone and shale, with much fossil wood (often drifted) and some beds of coal; 200 or 300 feet.

5. Wollongong sandstones, with calcareous concretions, containing many fossil shells and corals, and some fossil wood; 300 or 400 feet.

Total, 1,900 to 2,200 ft.

The beds here enumerated are probably but a portion of the palæozoic formation of Australia, as there may be found in other places beds much above No. 1, and there are almost certainly beds lower than No. 5.

In these four upper divisions, No. 1 to 4, the fossils are chiefly vegetable, containing among others:

Glossopteris Browniana. Pecopteris australis. | Vertebraria indica. Phyllotheca australis.

There are also, I believe, in No. 1, fossil fish to be found, which have not yet been examined so as to determine their genus and species.

In No. 5, the Wollongong sandstone, which is frequently calcareous, and has large and small calcareous nodular concretions, I collected the following fossils:

Stenopora crinita. Producta rugata. Spirifer subradiatus. —— Stokesii. —— avicula. | Pachydomus carinatus. ———— ovalis. Orthonota, sp. nov. Pleurotomana Strzeleckiana. Bellerophon contractus, sp. nov.

From other localities, whose precise geological or geographical place I do not know, the Rev. W. B. Clarke had, in addition to the above, collected the following fossils:

Favosites gothlandica. One species of Crinoidea, apparently related to Platycrinus. A form belonging to the radiata, and resembling an Echinoderma. A small trilobite. Two new sp. of Spirifer. | Two species of leptæna. A terebratula. An eurydesina. An inoceramus. A pleurotomaria. A conularia.

Some persons have been struck with the oolitic aspect of the fossil plants collected in New South Wales (as also of those of India), and have been led to imagine, in consequence, that they did not belong to the same formation as that in which the productæ, spiriferæ, &c. above named, are found. All the physical characters and relations of the rocks, however, both in New South Wales and Tasmania, led me to look upon the whole series as one great continuous formation, and Mr. Clarke has since distinctly informed me, that he has obtained the same spiriferæ, productæ, &c. from beds above those which contain the fossil plants as are found in the beds below.