Trap-rocks are displayed in a great variety of situations. They often occur connected with limestone in valleys, sometimes const.i.tute lofty ranges as on the north or left bank of the Hunter, and along the seash.o.r.e at the Illawarra; they likewise cap the summit of isolated hills, but no particular place can be a.s.signed to them with reference to the position of any other rocks. Trap forms a good soil on decomposition as is shown in the rich districts of the Illawarra, Cowpastures, Valley of the Hunter, Liverpool Plains, Wellington Valley, and Buree.
Vesicular lava and amygdaloid are the chief ingredients of some of the best parts of Australia Felix. In that region volcanic phenomena are more apparent than in other parts of Eastern Australia, especially where the Grampians, consisting of a ma.s.s of sandstone 4000 feet thick, seem a portion of the great formation covering the districts of the north. The strata in these mountains are inclined to the north-west, as if in obedience to the upheavings of Murroa or Mount Napier, an extinct volcano in the very line of their outcrop.
Observation. We found in the interior, hills of sandstone only, but at this extremity of the great Coast range granite is extensively exposed in ridges, between which, in one extensive district, are round heights of mammeloid form, consisting of pure lava, and in another, tabular ma.s.ses of trap reposing on granite occupy one side of a valley.
GRAVEL.
Beds of gravel are not common in these parts of Australia; but occur partially in the basins of the larger streams on the interior side of the Coast range where the pebbles in general consist of quartz.
SANDSTONE.
The prevailing geological feature in all Eastern Australia is the great abundance of a ferruginous sandstone in proportion to any other rocks.
The sterility of the country where it occurs has been frequently noticed in these volumes. It is found on the coast at Port Jackson and it was the furthest rock seen by me in the interior beyond the Darling.
A deposit upwards of 1200 feet thick forms the Blue Mountains west of Sydney, ranging thence, with the intersection of no other rock of importance, to the Hawkesbury; and although declining towards the sea at the rate of only 100 feet per mile, or 1 in 52, or at an angle of about 1 degree with the horizon; yet it is traversed by ravines which increase in depth in proportion as the sandstone attains a greater elevation, and present perpendicular crags and cliffs of a very remarkable character.
A region consisting of a sandstone deposit of so great thickness and so slightly inclined necessarily presents a monotonous aspect in all directions; and when it is compared with European countries composed of many formations and presenting great diversity of scenery it proves how much geological structure influences pictorial and physical outlines.
(See Plate 10 Volume 1, also Plate 38 above.)
In the eastern part of Australia the geologist will certainly find sections in abundance but they are nearly all of sandstone for, with few exceptions no other rocks have been denuded in situations similarly exposed.
GEOLOGICAL STRUCTURE AND PHYSICAL OUTLINE. VALLEYS OF EXCAVATION. EXTENT OF THAT OF THE c.o.x.
The ravines which discharge their waters into the little river c.o.x occupy an area of 1,212 square miles, or one-half of the county of Westmoreland on the right or south side of that river, and one-fourth of the county of Cook on the other. Of this area 796 square miles, equal to one-half of the county of Westmoreland, are on the right or south side of that river, and 416, or one-fourth of the county of Cook, on the left. The whole extent comprises the basin of this mountain stream, and is bounded by heights rising very gradually from about 1000 feet at the gorge or outlet of the c.o.x, to 3,400 feet on the north side at Blackheath, and on the south to Murruin and Werong, summits of still greater elevation; the lowest part of the ridge bounding this basin on the west or interior side being nearly 3000 feet above the level of the sea. c.o.x"s river flows over a bed of water-worn rocks which, in the upper part of the valley, is 2,150 feet above the sea, and on the road to Bathurst this bed consists of trap and granite. The river falls rapidly on leaving the granite of the vale of Clwyd to a level not much above that of the sea, and it escapes near its junction with the Warragamba from this s.p.a.cious basin through a gorge about 2,200 yards wide and flanked on each side by points about 800 feet high.
QUANt.i.tY OF ROCK REMOVED.
Supposing but two-thirds of the enclosed area of sandstone to have been excavated to the depth of 880 feet only, which I allow as the mean thickness of the stratum thus broken into, and considering the inclination of the c.o.x and other valleys, then 134 CUBIC MILES of stone must have been removed from this basin of the c.o.x alone.
VALLEY OF THE GROSE.
The valley of the Grose, whose basin is contiguous to that of the c.o.x on the north, is of less extent but enclosed by cliffs of greater perpendicular height. That river has been already described in the journal, and the general character of the valley through which it flows is represented in Plate 10 Volume 1.* We now perceive but slight indications of the action by which the great area of stone in the valley of the c.o.x, the Grose etc. has been removed. There are no acc.u.mulations of sand but huge blocks of rock, scarcely worn by attrition, occur in great abundance in the bed of the stream; neither do we find in the larger channels of the rivers below any sand deposits, but on the contrary the very rich alluvium which distinguishes the banks of the Hawkesbury.
(*Footnote. This book is already almost too full of plates and I beg to refer the geological reader to my three-sheet map of the Colony for the superficial forms and extent of these valleys.)
WELLINGTON VALLEY.
In the year 1830, after I had traced out the new line of descent from the Blue Mountains to the interior country by the pa.s.s which I then named Mount Victoria, I extended my survey to the heights beyond Wellington Valley. This includes a rich alluvial tract watered by the river Bell, one of the princ.i.p.al tributaries of the Macquarie, and is about 170 miles to the westward of Newcastle. It is bounded on each side by a compact calcareous rock resembling the mountain limestone of England and rising on the east side to about 100 feet above the Bell.
On the west side of this valley hills of greater elevation, consisting of a red sandstone and conglomerate, extend parallel to the limestone; and on the east side of it is another range composed of trap-rocks. The basis of a tract still further eastward, dividing the watershed of the interior from that which sends its streams to the sea is, as has been already observed, of granite.
The limestone presents a naked and rugged surface composed of pointed, weather-worn blocks between which are small crevices leading to caves and fissures. From these crevices a warm air ascends, accompanied by a smell peculiar to the caves. The worn aspect of the external rock, resembling half-dissolved ice, is very remarkable, particularly near the largest caverns.
An account of the survey of these caves was communicated to the Geological Society in a paper read on the 13th of April 1831, of which an abstract was published in its Proceedings, but the particulars respecting the animal remains found by me have derived great additional importance from the discoveries made by Professor Owen since my return to England. I may be excused therefore for again calling attention to the situation of those curious caves respecting which the following details are now published with the consent of the Council of the Society.
LIMESTONE CAVERNS.
The entrance to the caves of Wellington Valley is in the side of a low hill and 65 feet above the adjacent alluvial flat. It consists of two crevices between large blocks of limestone in one side of a hollow about 12 feet deep; and which has evidently been widened by water. (Plate 41.)
DESCRIPTION AND VIEW OF THE LARGEST.
We first descended the fissure at the mouth of the large cave, and then clambered over great rocks until, at 125 feet from the entrance, we found these inequalities to be covered by a deep bed of dry, reddish dust, forming an even floor. This red earth lay also in heaps under lateral crevices, through which it seemed to have been washed down from above. On digging to a considerable depth at this point, we found a few fragments of bone, apparently of the kangaroo. At 180 feet from the mouth is the largest part of the cavern, the breadth being 25 feet and the height about 50 feet. The floor consisted of the same reddish earth, but a thick stalagmitic crust extended for a short distance from a gigantic stalact.i.te at the further end of the cavern. On again digging several feet deep into the red earth here we met with no lower layer of stalagmite nor any animal remains.
On a corner of the floor behind the stalact.i.te and nearly under a vertical fissure we found a heap of dry white dust into which one of the party sunk to the waist.* (G. Plate 44.)
(*Footnote. The dust when chemically examined by Dr. Turner was found to consist princ.i.p.ally of carbonate of lime with some phosphate of lime and animal matter. Proceedings of the Geological Society for 1831.)
Pa.s.sing through an opening to the left of the stalact.i.te we came upon an abrupt descent into a lower cavern. Having reached the latter with some difficulty, we found that its floor was about 20 feet below that of the cavern above. It was equally level and covered to a great but unascertained depth with the same dry red earth which had been worn down about five feet in a hollow or rut.
A considerable portion of the farthest part of the floor (at H) was occupied with white dust or ashes similar to that found in the corner of the upper floor (at G).
This lower cavern terminated in a nearly vertical fissure which not only ascended towards the external surface but descended to an unascertained depth beneath the floor. At about 30 feet below the lowest part of the cavern it was found to contain water, the surface of which I ascertained was nearly on a level with that of the river Bell. Having descended by a rope I found that the water was very transparent but unfit to drink, having a disagreeable, brackish flavour.
This lower cavern is much contracted by stalact.i.tes and stalagmites.
After having broken through some hollow-sounding portions (at O and N) we entered two small lateral caverns and in one of these, after cutting through (at I) about eight inches of stalagmitic floor, we discovered the same reddish earth. We dug into this deposit also, but discovered no pebbles or organic fragments; but at the depth of two and a half feet met with another stalagmitic layer which was not penetrated. This fine red earth or dust seems to be a sediment that was deposited from water which stood in the caves about 40 feet below the exterior surface; for the earth is found exactly at that height both towards the entrance of the first cavern and in the lateral caverns. (See Plate 44.)
That this cave had been enlarged by a partial sinking of the floor is not improbable, as broken stalagmitic columns, and pillars like broken shafts, once probably in contact with the roof, are still apparent. (See the view of the largest cavern Plate 43.)
OF THAT CONTAINING OSSEOUS BRECCIA.
Eighty feet to the westward of this cave is the mouth of another of a different description. Here the surface consists of a breccia full of fragments of bones; and a similar compound, confusedly mixed with large blocks of limestone, forms the sides of the cavity. This cave presents in all its features a striking contrast to that already described. Its entrance is a sort of pit, having a wide orifice nearly vertical, and its recesses are accessible only by means of ladders and ropes. Instead of walls and a roof of solid limestone rock we found shattered ma.s.ses apparently held together by breccia, also of a reddish colour and full of fragments of bones. (Plate 45.) The opening in the surface appears to have been formed by the subsidence of these rocks at the time when they were hurled down, mixed with breccia, into the position which they still retain. Bones were but slightly attached to the surface of this cement, as if it had never been in a very soft state, and this we have reason to infer also from its being the only substance supporting several large rocks and at the same time keeping them asunder. On the other hand we find portions of even very small bones, and also small fragments of the limestone, dispersed through this cementing substance or breccia.
FIRST DISCOVERY OF BONES.
The pit had been first entered only a short time before I examined it by Mr. Rankin, to whose a.s.sistance in these researches I am much indebted.
He went down by means of a rope to one landing-place and then, fixing the rope to what seemed a projecting portion of rock, he let himself down to another stage where he discovered, on the fragment giving way, that the rope had been fastened to a very large bone, and thus these fossils were discovered. The large bone projected from the upper part of the breccia, the only substance which supported as well as separated several large blocks, as shown in the accompanying view of the cave (Plate 45) and it was covered with a rough tuffaceous encrustation resembling mortar. No other bone of so great dimensions has since been discovered within the breccia. (See Figures 12 and 13, Plate 51.)
From the second landing-place we descended through a narrow pa.s.sage between the solid rock on one side and huge fragments chiefly supported by breccia on the other, the roof being also formed of the latter and the floor of loose earth and stones.
SMALL CAVITY AND STALAGMITIC CRUST.
We then reached a small cavern ending in several fissures choked up with the breccia. One of these crevices (K. Plate 44) terminated in an oven-shaped opening in the solid rock (Plate 50) and was completely filled in the lower part with soft red earth which formed also the floor in front of it and resembled that in the large cavern already described.
Osseous breccia filled the upper part of this small recess and portions of it adhered to the sides and roof adjoining, as if this substance had formerly filled the whole cavity. At about three feet from the floor of this cavity (Plate 50) the breccia was separated from the loose earth below by three layers of stalagmitic concretion, each about two inches thick and three apart; and they appeared to be only the remains of layers once of greater extension, as fragments of stalagmite adhered to the sides of the cavity as shown in Plate 50. The s.p.a.ces between what remained of these layers were filled with red ochreous matter and bones embedded partially in the stalagmite. Those in the lower sides of the layers were most thickly encrusted with tuffaceous matter; those in the upper surfaces on the contrary were very white and free from the red ferruginous ochre which filled the cavities of those in the breccia, although they contained minute transparent crystals of carbonate of lime.
TEETH FOUND IN THE FLOOR.
On digging (at K) into the soft red earth forming the floor of this recess, some fragments of bone, apparently heavier than those in the breccia, were found, and one portion seemed to have been gnawed by a small animal. We obtained also in this earth the last phalange of the greatest toe of a kangaroo, and a small water-worn pebble of quartz. By creeping about 15 feet under a ma.s.s of solid rock which left an opening less than a foot and a half above the floor, we reached a recess about 15 feet high and 12 feet wide (L). The floor consisted of dry red earth and, on digging some feet down, we found fragments of bones, a very large kangaroo tooth (Figure 6 Plate 47) a large tooth of an unknown animal (Figures 4 and 5 Plate 51) and one resembling some fragments of teeth found in the breccia. (See Figures 6, 7, 8, and 9, Plate 51.)
A THIRD CAVERN.
We next examined a third cave about 100 yards to the westward of the last described. The entrance, like that of the first, was tolerably easy, but the descent over the limestone rocks was steeper and very moist and slimy. Our progress downwards was terminated by water which probably communicated with the river Bell, as its level was much lower when the cave was first visited during a dry season. I found very pure iron ochre in some of the fissures of this cavern but not a fragment of bone.
BRECCIA ON THE SURFACE.
Perceiving that the breccia, where it occurred below, extended to the surface, I directed a pit to be dug on the exterior about 20 feet from the mouth of the cave and at a part where no rocks projected. (N, Plate 44.) we found that the hill there consisted of breccia only; which was harder and more compact than that in the cave and abounded likewise in organic remains.
Finally I found on the summit of the same hill some weathered blocks of breccia from which bones protruded, as shown in the accompanying drawing of a large and remarkable specimen. (Plate 46.)
SIMILAR CAVERNS IN OTHER PARTS OF THE COUNTRY.