A Voyage of Discovery and Research in the Southern and Antarctic Regions/Volume 2/Appendix 1
APPENDIX, No. I.
GEOLOGY OF TASMANIA. BY ROBERT McCORMICK, ESQ. SURGEON OF H. M. S. EREBUS.
The following general description of the geological and physical features of this beautiful portion of Australasia is chiefly derived from observations made in the vicinity of Hobart Town; and during a rapid excursion I made across the island, from the banks of the Derwent to the embouchure of the Tamar in Bass's Strait, including a short visit to Port Arthur and Tasman's Peninsula.
The general aspect of the country is mountainous, the main chain intersects the island from N.E. to S.W. in a somewhat zigzag course, attaining an altitude of about five thousand five hundred feet above the level of the sea, forming ranges or isolated peaks, intersected by the most fertile plains and valleys, which are watered by numerous streams and rivers, and richly clothed with woods and forests.
Five or six large lakes occupy the higher regions, at an elevation of between three and four thousand feet. The prevailing rocks are basalt and greenstone, occasionally passing into amygdaloid and other modifications of the trappean series; and are variously associated with those of the sedimentary class, calcareous, arenaceous, and argillaceous; these are in many places much inclined, disturbed, and dislocated (as evinced by the numerous faults in the coal formation at Port Arthur), by the intrusion of the igneous rocks at different periods of time. A kind of greywacké is widely distributed.
Basalt and greenstone enter very largely into the composition of the mountain range, cresting it in many places, the former rising in magnificent columns at Cape Pillar, Cape Raoul, and Fluted Cape, and capping Mount Wellington. Granite occurs in Ben Lomond; and near Lake St. Clair forms the basis of the Western range, crested by quartz. Serpentine is found in the asbestos hills. Argillaceous slate associated with micaceous and siliceous schist, appears near George Town, on the banks of the Tamar. Limestone and sandstone are very generally diffused over the island. Coal is abundant, occupying two coal-fields; that of the valley of the South Esk, and the Jerusalem formation, including Richmond and Tasman's Peninsula. It has been found south of Ben Lomond; and traces of it have been met with across the island.
The total thickness of the coal deposits amounts to about one hundred and fifty feet; and that of the whole of the stratified rocks together, to upwards of two thousand, more than half of this being occupied by the sandstone alone. The coal bed at Jerusalem is more than eight hundred feet above the level of the sea, having the sandstone superincumbent.
At Richmond, abundance of rich iron-ore occurs. Copper, lead, zinc, and manganese are also found. The principal fossiliferous deposits are at Mount Wellington, Richmond, Jerusalem, and Tasman's Peninsula. Silicified trunks of trees, often beautifully opaline, appear imbedded in a vertical position in vesicular lava, on Macquarie plains. Raised beaches, containing shells, occur at various altitudes; from which we may infer that there has been a progressive upheaval of the land.
In the Australian limestone caverns, remains of extinct mammalia have been found: the Dasyurus, Hypsiprymnus, Phascolomyx, Thylacinus, and Macropus, apparently types of existing marsupial animals; whilst others, as those extraordinary forms, Diprotodon, and Nototherium, are wholly unknown.
In my geological rambles in the vicinity of Hobart Town, I visited a quarry of yellow travertin-limestone, a tertiary deposit coloured by oxide of iron and much decomposed, dipping W.S.W. at an angle of 50 degrees, and abounding in impressions of leaves of Dicotyledonous plants of an extinct flora. Two kinds occur beautifully preserved; but I could discover no traces of shells, although two extinct species, a helix and bulimus, are sparingly imbedded in a similar deposit which appears on the opposite side of the Derwent, at the head of Lindisfern Creek, about two miles from Hobart Town. The limestone is of the same yellowish colour, but more indurated in texture, and has been quarried to the depth of seventy or eighty feet. The shells are found in the upper layer, and both leaves and stems in the lower portion.
The locality of Mount Wellington, which rises to the height of four thousand one hundred and ninety-five feet, on a basis of sandstone, capped, by basalt, two thousand feet in thickness, abounds in organic remains. The sandstone with which the houses in Hobart Town are built, is very soft when first quarried, but hardens on exposure to the atmosphere. In some places it is micaceous, with dark arborescent markings, in others enclosing hard ferruginous-coloured nodules, of various sizes, usually of flattened spheroidal form.
The following species of fossils are found in this locality. Polyparia (corals), Stenopora Tasmaniensis, S. ovata, Fenestella fossula, F. ampla, F. internata, and Hemitrypa sexangula.
Conchifera (bivalve shells), Pecten squamuliferus, P. Fittoni. Brachiopoda,—Spirifera Tasmaniensis, S. subradiatus, Producta subquadratus, and P. brachythserus. Terebratulæ are also met with, and a species of Cypræa (cowrie) has been found in sandy alluvium. The woods on the slopes of Mount Wellington are ornamented with the elegant and magnificent Tree fern.
A large bivalve shell, Pachydomus globosus, occurs in the argillaceous formation at Risdon.
The coal seam at Richmond crops out on the south bank of the coal river; it is about three feet in thickness, and sixty feet above the level of the Derwent. The accompanying sandstone and shales dip south, abounding in impressions of Ferns, as—
Sphenopteris lobifolia, S. alata, Pecopteris Australis, and P. odontopteroides.
About half a mile from the township of Richmond two small knolls of yellowish limestone crop out from the trappean rocks, dipping slightly to the S.W. and much decomposed, assuming a more indurated texture and brownish hue where in contact with the adjacent igneous rocks.
I visited the spot in search of organic remains, but no traces of any could be discovered. Fragments of fossil wood lay scattered about the surface of the neighbouring hills.
In my journey across the country after quitting Hobart Town, I passed Newtown, and crossed the Derwent at Bridgewater. On the left side of the causeway to the ferry is a limestone quarry, dipping 25° to the S.W., and four miles further, a small one of sandstone.
The river Jordan, a narrow stream, intersects the plains of Brighton and Bagdad. At Constitution Hill the sandstone again crops out, dipping S.W. at an angle of 25°. This hill commands one of the finest views in the island. Mount Wellington, with the village of Newtown in its lap, appears in the horizon at the distance of upwards of twenty miles, bounding a rich intervening landscape. The road from this forms a fine curve through the trappean rocks, round a deep wooded glen, resembling the "Simplon," near Richmond, and then continues over the plains of Green Ponds and Cross Marsh to Lovely Banks, a rich fertile tract, studded with lightly wooded knolls, and skirted by sloping banks of green pasture; forming a soft and charming landscape, to Spring Hill, which I ascended, and found the summit composed of greenstone. A fossiliferous deposit occurs here, in which the following species of Conchifera are found; Orthonota compressa, Pterinea macroptera, and Pachydomus globosus.
Stenopora informis, and other species of corals, are also met with, both here and throughout the Jerusalem formation generally, in which, likewise, Spiriferæ and Producta are abundantly distributed.
The seams of coal alternate with layers of clay and shale, in which Zeugophyllites and the fern Pecopteris Australis are imbedded.
Another species, Pecopteris odontopteroides, occurs in the overlying sandstone, the whole dipping to the south, and appearing again at Richmond and Tasman's Peninsula.
After passing Spring Hill, Jericho appears to the westward, backed by hills, and to the eastward of the road the vale of Jerusalem, a rich verdant-looking flat, dotted over with clumps of wood, and strikingly reminding me of some of the vales in Somersetshire. Over Fourteen-Tree Plain the road is enlivened by vast numbers of parrots, and black and white magpies. Eastward of this, the country presented a somewhat desolate aspect from the number of withered trees, some standing erect, others prostrate on the ground, and the whole interspersed with numerous charred stumps, as if caused by some conflagration in the woods.
From Lemon Springs, Table Mountain, three thousand eight hundred feet in height, is seen to the N.W. at a considerable distance.
Approaching the plain in which the township of Oatlands is situated, the country suddenly changes its appearance: gently undulating slopes of sandstone, covered with rich pastures, on which numerous flocks of sheep were feeding, supersede the wild and rugged, though picturesque, hills and ridges of the trappean rocks. To the eastward of the town is a level tract of mud, several miles in circumference, like a drained lake, having in its centre a hummock or small island crowned with a few trees: the margin next the town is flanked by sandstone.
The country between Oatlands and Tunbridge is slightly undulating, with park-like plains, glowing with the warm golden yellow tint of the black wattle (a mimosa, and the emblem of the island), and the equally bright and deep orange blossom of the gorse or furze, which perfumes the whole atmosphere with its sweetness and fragrance.
Quamby's Bluff, three thousand five hundred feet in height, is seen from Tunbridge, bearing N.W., and to the eastward lay the Salt-pan Plains, from which large quantities of salt are collected.
Blackman's River crosses the road to Ross.
An alluvial plain of reddish gravel and other transported materials, containing agglomerated pebbles, agates, and cornelians, extends between Ross and Campbell Town. Greenstone is the prevailing rock, in which very curious nodules, from the size of a pea to that of a hazel nut, are imbedded. These agate-like minerals, from their concentric laminated structure, present in their markings externally the appearance of an eye, more especially when polished, and they are frequently found aggregated in clusters in their rocky matrix. Ben Lomond, rising to the height of five thousand feet, is seen from this. I saw here two eagle hawks (Aquila fucosa) soaring overhead, the first I met with, as the bird is becoming scarce in the colony; and that beautiful and elegant species, the white hawk (Aster albus), is, I much fear, destined ere long to become extinct in the island. I saw only one, on Tasman's Peninsula, during my stay in the country. Elizabeth River passes by Campbell Town, from which Quamby's Bluff has a very conspicuous appearance.
A well cultivated agricultural country lies between Campbell Town and Epping Forest, in which some large estates are situated, with well trimmed fields and enclosures and rich soil, equal in appearance to any I have seen in England. The road winds through the forest for many miles, flanked on either side by the lofty sombre-looking Eucalyptus and the Acacia, the predominant forms of Australian vegetation, forming narrow vistas, across which flocks of miners (Myzantha garrula), parrakeets, and magpies were frequently flitting. On emerging from the forest, a fine sylvan country extends to Perth; greenstone rocks prevailing, and of which the bridge over the South Esk is built. Launceston, one hundred and twenty-four miles from Hobart Town, with Cocked-hat Hill in front, appears skirting the side of a valley, over which a volume of white mist was suspended, half enveloping the town; a frequent occurrence here, from the peculiar locality of the place, situated at the confluence of the two Esk rivers, amidst marshy land. From Launceston I proceeded in a boat down the Tamar to George Town, a distance of forty-two miles. This river, formed by the junction of the North and South Esks, makes a narrow curve in its first reach from Launceston, afterwards expanding and contracting its stream from two miles to a quarter of a mile in breadth. At Whirlpool Reach it is very much narrower, with a strong eddy setting round a ledge of rocks in the centre of the stream. The water is fresh as far as Fresh-water Point, eight miles below Launceston.
Several small bays and islets occur in its course. The Tamar is altogether a far inferior river to the Derwent. The range of hills on either side, scarcely exceeding an altitude of four or five hundred feet, and clothed with sombre woods to their summits, presenting a most monotonous outline. Mount Royal Signal Station, on the eastern bank, about midway between Launceston and George Town, has a striking resemblance to Mount Direction, near Risdon on the Derwent. On the west bank is an argillaceous schist formation, in nearly a vertical position. It has been quarried, and produces abundance of excellent roofing slate. There is also a limestone deposit here.
The Tamar is three miles wide at its mouth, and on Low Head, its eastern point, is a lighthouse rising to one hundred and forty feet above the sea. Thickets of tea-shrub fringe its banks, and here and there the temporary log-hut of some recent settler, who has cleared away just sufficient elbow-room in the woods for himself and family, breaks upon the solitude of the scene.
The only birds I saw in my course down the river were four black swans (Cygnus atratus), a cormorant, and a few gulls.
On my return I made a detour from Tunbridge to the lakes; ascending the western tier to an altitude of above three thousand feet, passing for the most part over greenstone rocks, with an occasional outcrop of sandstone. Lake Sorell is between five and six miles in length, and at its broadest part, about the same in width; its shores encircled with wood, and indented by small sandy beaches. The southern extremity is of irregular form from the number of wooded promontories jutting out, and on its S.W. side is a small islet.
Ten black swans with a few ducks and divers were all that I saw on its broad surface.
Lake Crescent lies to the southward of it, and is about four miles in length and two in breadth; with a level isthmus about half a mile broad, of swampy ground, covered by long grass, intervening between it and Lake Sorell, through which meanders a narrow rivulet uniting both lakes.
From the lakes to Bothwell, the country is wild, rugged, and hilly, interspersed with swamp and marsh, and covered by the primitive forests through which winds the River Clyde. The town of Bothwell is situated in a circular valley, bounded by distant hills of moderate height. The approach to it is over a green plain, about four miles in extent. Ten miles further on I examined a bed of micaceous shale, which crops out in a remarkably deep gorge in the trappean rocks, and dips south at an angle of fifteen degrees.
The country improves in appearance approaching Hamilton, becoming more open, now and then swelling into picturesque wooded hills.
The township occupies the bottom of a green valley, and the beautiful grounds of Lawrenny appear through a finely wooded vista beyond it.
On Macquarie Plains I visited the remarkable silicified fossil-tree, imbedded in a vertical position in vesicular lava. Its height above the ground is six feet; circumference, seven feet three inches; and diameter, at the top, fifteen inches. The wood is silvery white, and covered with the finest silicified filaments of a white satin-like hue, resembling in tenuity the finest blown glass. The bark remaining near the base has a yellowish brown resin-like appearance, and when broken, rivals in lustre and beauty the finest agates. The locality of the tree forms the extremity of a ridge of rock of the same kind in which it is imbedded seventy feet above the river, which is here twelve feet in breadth, winding through a wooded ravine one hundred yards across.
A little further down the ridge, is another specimen also vertically imbedded in a chimney-like cavity on the steep face of the igneous rock, the lower portion having disappeared, the cast left by it in the rock is a foot in diameter, and seven feet in length. In the soil beneath, I found a fragment of it having an opaline lustre. About two miles from Rose Garland, near a reach of the Derwent, I saw excavations in a low ridge of scoria, from which two other silicified trees had been removed some years ago. These had also been vertically imbedded, and I found small silicified fragments of them scattered about the spot. All these trees appear to have been coniferous. Five miles from Rose Garland, and twenty feet above the bank of the river Derwent, a bed of sandstone crops out from the adjacent basalt, dipping forty degrees to the S.W., and enclosing cylindrical masses of greenstone from eighteen to twenty inches in diameter, and six feet in length; presenting the appearance of so many imbedded cannon, pointing down on the road at a considerable angle. Along many of the cliffs on the banks of the Derwent, I observed dark shades and lines of dust, indicating traces of coal. West of the town of New Norfolk is a limestone quarry.
TESSELLATED PAVEMENT OF EAGLE HAWK NECK.
In my excursion to Tasman's Peninsula, this singular fossiliferous formation, so called by the colonists, particularly attracted my attention. It is a kind of argillaceous deposit, situated a little to the northward of the neck, or low sandy isthmus, connecting Forrester's with Tasman's Peninsula; having an inlet of Norfolk Bay on the west, and Pirate's Bay on the east. The latter is bounded by argillaceous cliffs, somewhat steep, and of moderate height; at the base of which the wash of the sea has formed a sort of platform, on which it breaks at high water. The siliceous clay forming this platform is very remarkably divided by the most symmetrical partings into slabs, varying in length and breadth, having their margins frequently bordered in strong relief. The dimensions of those I measured were from three to nine feet long, and from four or eight inches, to six feet broad, and others were eighteen inches square. These divisional planes had a general bearing of E. by N. with a perfectly geometrical parallelism in relation to each other.
The curious structure displayed here may, probably, be due to some re-arrangement of the particles coming under the influence of electro-magnetic forces whilst passing into a solid state, giving a definite direction to the ordinary partings which argillaceous depositions frequently present when contracting under sudden changes of temperature during the process of consolidation.
Spirifera vespertilio and Spirifera avicula are profusely imbedded in this deposit.
The argillaceous cliffs at Point Puer, Port Arthur, contain the same species of shells, with Pachydomus globosus.
The two coal mines at present worked in Tasman's Peninsula, are situated on the west side of Norfolk Bay. In the older mine I descended a shaft twenty-six fathoms deep, sunk in the vicinity of some columnar trap. The coal-seam, from four to six feet in thickness, appears beneath a bed of overlying sandstone, having a greenish tinge. Low and narrow tunnels have been worked to a distance of three hundred feet from the shaft; with the usual swampy muddy floor I have observed, in the coal mines on a larger scale, in the North of England. Sixty men are employed in this mine, and the average quantity of coal daily raised from the pit, amounts to forty chaldrons. In the mine last opened, situated near the beach, the coal which is of better quality is so near the surface that a straight tunnel has been excavated through it horizontally, to the distance of forty-seven yards. It has not yet been worked to its entire depth. The same kinds of fossil plants and wood occur in the two mines, with the sandstone super-incumbent, as at Richmond, &c.
The accompanying rough pen and ink diagram, may convey a better impression of the appearance of the fossiliferous argillaceous formation at Eagle Hawk Neck, than any written description.
