Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/849

 793 AUSTRALIA geology] vary greatly in composition, in texture, and in general appear- luxuriant vegetation, in place of the present desolate and arid ance ; but individual beds preserve their characteristics over region of Central Australia. The lakes of the interior of Australia, very large areas. In Western New South Wales deposits of which are for the most part simply dead levels of white salt, must excellent opal have been found in marls of Cretaceous age. in Pliocene times have been magnificent sheets of water, set amidst These marls are undoubtedly of marine origin. The opal is rich tropical forests. Herds of giant marsupials, birds as large as found filling in seams in the marls; but reptilean bones, wood, the New Zealand moa, saurians 15 feet in length {Nothosaurus), gasteropod shells and belemnites are also found converted into and gigantic monitors tenanted the country. Amongst the noble opal of first quality. The silica of the opal is probably marsupials, Diprotodon was the most notable, with a head 3 feet derived from diatomaceous and radiolarian tests in the marls. in length. A kangaroo, with a skull as large as that of an ox, The Lower Cretaceous rocks of Queensland are believed to be {Palorchestes azael) was the largest known member of the kangaroo sources of artesian water. The Cretaceous rocks of New South family. Nototherium, another great marsupial, almost rivalled Wales were also supposed to yield artesian water. More re- the diprotodon. The remains of a large extinct wombat, equalling cent investigation has shown that, although the artesian bores a tapir in size, have also been described. Associated with these are in many instances put down through Cretaceous rocks, the was a skull with extraordinary adze-shaped teeth. Their long water-bearing strata belong to the underlying Triassic. The chisel-like form suggested the name Sceparnodon. There was also an lower beds form the country known as the Rolling Downs. They animal {Koalemus) probably the ancestral form of the living ‘ ‘ native consist of shales, Cretaceous sandstones, with no great develop- bear.” The existing bear is an animal about 2 feet long, with a ment of Calcareous rocks or limestones. The presence of am- body made to look more clumsy by the almost complete absence of monites and belemnites indicates a marine origin for these beds. a tail. The bear of to-day weighs about 20 lb. The bear of PlioCretaceous rocks cover probably more than 300,000 square miles. cene and post-Pliocene times was a similar animal, more clumsy and It is estimated that the desert sandstone at one time covered an more unwieldy, if such can be pictured. The fossil form was many area of 500 square miles, but has now been reduced by denudation times as large as the “native bear,” and would turn the scale at to isolated table-lands. The most striking feature in connexion 500 to 600 It). The Sarcophilus, or “Tasmanian devil,” is now with the Queensland Cretaceous rocks is their horizontal bedding, extinct on the mainland, but its remains are very common in and their occurrence in fiat-topped, step-cut hills. The desert Pliocene and post-Pliocene drifts, from Victoria to Queensland. sandstone plains present a most desolate appearance, and form Another animal that lived in the days of the diprotodon, is some of the poorest, the driest, and the most inhospitable territory Thylacoleo, a gigantic ally of the phalangers. The skull measured of the continent. The Cretaceous rocks have been very little dis- nearly a foot in length. Various theories have been advanced to explain the extinction of forms so vast and varied. The drying up turbed by the intrusion of the igneous materials. Marine beds of Eocene, Miocene, and probably Pliocene age of the lacustrine area had no doubt much to do with the change. occupy a large extent of country on the southern shores of Pliocene conditions continued into post-Pliocene times. The absence the continent, extending from Gippsland away round of marine beds of this age over the continent makes our subTertiary, to West Australia. Marine Tertiary must cover at divisions of this portion of the geological record somewhat arbitrary. least 20,000 square miles, and near Melbourne very The great alluvial plains are the work of rivers in post-Pliocene considerable thicknesses of Eocene rocks have been found. times, and much of the drifts and “leads” containing gold and The Croydon boring near Adelaide shows a thickness of fully tin-stone are of the same age. All the evidences of the presence 2300 feet of pre-Pliocene Tertiary strata. They reach inland of man are confined to recent deposits. But although, geologicto the south-western corner of New South Wales, where they are ally considered, man is a recent arrival, yet measured in years his over-lain by alluvial deposits of Pleistocene age. Tertiary marine existence is one of vast antiquity on the Australian continent. Granites may be said to be well represented, and are almost beds are also known on the north coast of Tasmania. Quite a different set of beds occur inland in New South Wales, Victoria, everywhere associated with the older stratified rocks. Over the and Queensland. There are also rocks of Tertiary age, but they whole of the interior, there is a notable absence of pjutonic 'volwere laid down under inland and freshwater conditions. Some of basalt or recent lavas, and even of such intrusive rocks the beds contain in abundance fossil plants strongly suggestive of as diorites. Vast tracts of Cretaceous and Tertiary caajc some of the earlier Tertiary European flora. Sufficient is known country show no igneous rocks for many hundreds of now of the marine Tertiary beds of Australia to separate them square miles. The Kosciusko plateau consists for the most part of gneissic granite, cut through sparingly by igneous dykes, and in into Eocene, Miocene, and Pliocene. Eocene.—Eocene rocks are particularly rich in fossils—the one instance, by what seems to be a volcanic neck. The bulk of Eocene beds of Aldinga and Adelaide being notable for gastero- the Australian granites seem to have been intruded into Silurian pods, lamellibranchs, echinoderms and corals. The Eocene beds at and Devonian, and perhaps even into Carboniferous, but they are Muddy Creek, West Victoria, are even more prolific, 649 species older than the Permo-Carboniferous. The first manifestations of of molluscs having been recorded. The Eocene beds are made up igneous outpourings are noted as having occurred in Devonian of clays, calcareous limestones, sandstones, bands of pebbles and times. In the succeeding Carboniferous and Permo-Carboniferous, grits and shelly limestones. The proportion of living species of volcanic activity was also very manifest, and is shown by many mollusca in Australian Eocene rocks nowhere exceeds 3^ per cent., hundred feet of interstratified volcanic products. In Triassic times there is very little sign of volcanic action. The Cretaceous and the percentage is usually under two. Miocene.—Miocene beds occur overlying Eocene rocks in the period, too, was one of quietude. Phe next outburst of volcanic river Murray cliffs. At Aldinga Bay and Hallet’s Cove, on the east tires occurred in the Tertiary, and this continued down almost to side of St Vincent Gulf, Miocene beds rest directly on the Eocene the advent of the aboriginals. In the western district of Victoria —in both cases the newer rocks lying on the eroded surface of the some remarkably fine craters remain quite fresh and very little older. At Muddy Creek, in Western Victoria, Miocene beds are cut into or denuded by atmospheric agencies. Here quite eight of also found overlying the Eocene, and near the Gippsland Lakes the points of eruption have been counted which poured out the they flank an Eocene escarpment. The Australian Eocene con- floods of basalt known in Victoria as newer volcanic. As regards the distribution of recent volcanic rocks, New tains 15 per cent, of living forms. The rocks consist of blue clays, South Wales shows excellent types of trachytes in the Warrummottled sands, calciferous sandstones, and sandy clays. bungle mountains, and also in the Canoblas, a point of eruption Pliocene.—Bores put down at Dry Creek in South Australia show the existence of strata with a fauna comprising 210 species, of near Orange, about one hundred miles west of Sydney. Along the which 150 are gasteropods, and 60 lamellibranchs. As the beds slopes of the Main Dividing Range, there is much basalt of the typical contain about 27 per cent, of living species, Professor Tate pro- basic character. Many of the “ leads ” that have yielded so much visionally called these beds Older Pliocene, “though in this case alluvial gold are old river-beds buried under this basalt. Elatthe percentage system does not fully indicate the strong modern topped basalt-covered hills are a marked character of the western facies exhibited in this collection of fossils.” The Croydon bore, slopes of the eastern Dividing Range. These are remnants of lava near Adelaide, passed through 406 feet of stratified rocks, some of streams that flowed from long extinct, but geologically recent, which are of Pliocene Age. Some beds occurring in the south-west volcanoes. Basalt in these instances filled the old valleys, and of Victoria, at Limestone Creek, have yielded an abundant fauna— the surrounding country rock being worn away, the basalt now the proportions of recent forms being 80 per cent.—and are con- remains above the general level. Thus the lines of the old rivers, sidered late-Pliocene. These rocks are wholly marine. Towards once the lowest part of the country, now stand out as the highest the centre of the continent, however, a totally different set of ground. Water-worn pebbles, sand, and shingle—in other words, beds were laid down, mostly of lacustrine or alluvial origin. In old river-drifts—are invariably found under these cappings. A Pliocene times much of the present arid interior was a well-watered Icucite basalt of Tertiary age occurs in three different localities in country, supporting a luxuriant vegetation. The fauna of this New South Wales. No nephelin basalts have been discovered period included many animals of astonishing proportions and most anywhere on the continent. Serpentines are well represented, in remarkable structural characteristics. These larger forms were all one case evidently derived from an olivine-bearing rock. Basaltic marsupials, and are now extinct. Species of diprotodon, noto- dykes are extremely common, cutting up through the whole therium, phascolomys, macropus, and protemnodon must have been geological series as high as the Triassic. In formations of a later plentifully distributed. To account for the existence of such huge age, dyke rocks are not so common. Rhyolites and true Andesites herbivores, we must suppose a well-watered country, with a are rare as products of recent volcanic action in Australia. S. I. — ioo