AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.
New South Wales.
Sandstone formation.
Embedded pseudo-fragments of shale.
Stratification.
Current-cleavage.
Great valleys.
Van Diemen's Land.
Palaeozoic formation.
Newer formation with volcanic rocks.
Travertin with leaves of extinct plants.
Elevation of the land.
New Zealand.
King George's Sound.
Superficial ferruginous beds.
Superficial calcareous deposits, with casts of branches.
Their origin from drifted particles of shells and corals.
Their extent.
Cape of Good Hope.
Junction of the granite and clay-slate.
Sandstone formation.
The "Beagle," in her homeward voyage, touched at New Zealand, Australia, Van Diemen's Land, and the Cape of Good Hope.In order to confine the Third Part of these Geological Observations to South America, I will here briefly describe all that I observed at these places worthy of the attention of geologists.
NEW SOUTH WALES.
My opportunities of observation consisted of a ride of ninety geographical miles to Bathurst, in a W.N.W.direction from Sydney.The first thirty miles from the coast passes over a sandstone country, broken up in many places by trap-rocks, and separated by a bold escarpment overhanging the river Nepean, from the great sandstone platform of the Blue Mountains.This upper platform is 1,000 feet high at the edge of the escarpment, and rises in a distance of twenty-five miles to between three and four thousand feet above the level of the sea.At this distance the road descends to a country rather less elevated, and composed in chief part of primary rocks.There is much granite, in one part passing into a red porphyry with octagonal crystals of quartz, and intersected in some places by trap-dikes.Near the Downs of Bathurst I passed over much pale-brown, glossy clay-slate, with the shattered laminae running north and south; I mention this fact, because Captain King informs me that, in the country a hundred miles southward, near Lake George, the mica-slate ranges so invariably north and south that the inhabitants take advantage of it in finding their way through the forests.
The sandstone of the Blue Mountains is at least 1,200 feet thick, and in some parts is apparently of greater thickness; it consists of small grains of quartz, cemented by white earthy matter, and it abounds with ferruginous veins.The lower beds sometimes alternate with shales and coal: at Wolgan Ifound in carbonaceous shale leaves of the Glossopteris Brownii, a fern which so frequently accompanies the coal of Australia.The sandstone contains pebbles of quartz; and these generally increase in number and size (seldom, however, exceeding an inch or two in diameter) in the upper beds:
I observed a similar circumstance in the grand sandstone formation at the Cape of Good Hope.On the South American coast, where tertiary and supra-tertiary beds have been extensively elevated, I repeatedly noticed that the uppermost beds were formed of coarser materials than the lower: this appears to indicate that, as the sea became shallower, the force of the waves or currents increased.On the lower platform, however, between the Blue Mountains and the coast, I observed that the upper beds of the sandstone frequently passed into argillaceous shale,--the effect, probably, of this lower space having been protected from strong currents during its elevation.The sandstone of the Blue Mountains evidently having been of mechanical origin, and not having suffered any metamorphic action, I was surprised at observing that, in some specimens, nearly all the grains of quartz were so perfectly crystallised with brilliant facets that they evidently had not in their PRESENT form been aggregated in any previously existing rock.(I have lately seen, in a paper by Smith (the father of English geologists), in the "Magazine of Natural History," that the grains of quartz in the millstone grit of England are often crystallised.Sir David Brewster, in a paper read before the British Association, 1840, states, that in old decomposed glass, the silex and metals separate into concentric rings, and that the silex regains its crystalline structure, as is shown by its action on light.) It is difficult to imagine how these crystals could have been formed; one can hardly believe that they were separately precipitated in their present crystallised state.Is it possible that rounded grains of quartz may have been acted on by a fluid corroding their surfaces, and depositing on them fresh silica? I may remark that, in the sandstone formation of the Cape of Good Hope, it is evident that silica has been profusely deposited from aqueous solution.
