May 2, 1895] 



NATURE 



gypsum are of common occurrence. On the northern coast the 

 •submergence was greater, as the sandstones and shales have a 

 thickness of more than a thousand feet. 



The Cretaceous Deposits. 

 One characteristic of the later part of the Cretaceous deposits 

 is that in the lower pari they consist chiefly of white, blue, anil 

 pale red shales, which reailily disintegrate, while the ujiper 

 portion consists of variegated sandstones of a harder character, 

 with a comparatively thin covering of ferruginous concretionary 

 pebbles or nodules, often with a nucleus of organic origin. (Jn 

 the west co;ist (latitude 29'), on Moresby's Klat-topped Range. | 

 these features are well developed, and in the upper part a ^ 

 bed of limestone, containing .Vunnonites and other mollusca of 

 the Cretaceous series. And it was from this locality that the first 

 proofs of the existence of the Cretaceous formation in Australia ] 

 ■were furnished to Prof. M'Coy. Closely associated with these 

 limestones are ferruginous sandstones, containing casts of large 

 accumulations of fragments of wood and vegetable ilebris, such as 

 may be found after floods on the margins of rivers, indicating an 

 «stHarine system, where fresh and salt water alternated. 



AUSTR.M.IA A.N ISLA.NL). ' 



The Mesozoic period closetl with .\ustralia reducetl to the area 

 <A a large island on the east coast and some small islar.Js on the 

 south-west and north-west of the present continent, and then 

 the connection with I'apua was severed 



A New ELEV.vnoN. 

 Karly in the Tertiary period a new elevation of the land 

 commenced, but the rise was not attended by any great dis- 

 turbance of the strata, as in almost every instance where the 

 Up|)er Cretaceous rocks remain they are remarkable for their 

 horizontal position. The elevation of the continent on this 

 <x:casion was nearly equal in all parts ; the ultimate altitude was 

 at least 500 feet greater than at ]iresent, and the geographical 

 ^fifect was that .Vvistralia assumed nearly its present limits. 



Features ok the Continent. 

 The features of the continent at this time appear as high ranges 

 on the east coast and a nearly level tableland extending to the 

 west coast, but the whole of the interior with a general incline 

 towartls Spencer's ( iulf. Slu)rt watercourses flow ed direct to 

 the sea, but far the greater area was drained by much longer 

 streams towards S|iencer's (uilf, while a secondary series 

 occupied the basin of the Murray and Darling Rivers. The 

 climate evidently diftered greatly from that now existent, as the 

 denudations of the tableland removed tracts of country many 

 hundreds of s(|uare miles, each forming immense valleys bounded 

 by flat-topped hills and ranges representing the marginal 

 remnants of the original surface. Enormous quantities of the 

 finer-grained portions of the degr.aded shales must have been 

 sivept into the ocean by the rivers, but the coarser sands have 

 beeii left in what is now the desert interior, where the wind 

 <lrifts it into long steep ridges of bright red sand, having a 

 northerly direction near the south coast, but spreading out like 

 I iAW to the east and west in the northern interior. 



\AI.LEVS AND RiVKR SYSTEMS. 

 The interior rivers formed a grand feature of the covmtry so 

 long .as the rainfall continued sufficiently copious to maintain their 

 flow, but in the arid climate which now obtains it does not even 

 <ompensate for the evaporation. The river channels have been 

 nearly obliterated, and some parts of the wider valleys changed 

 to salt marshes or lakes, such .as Lakes .\madeus and Torrens, 

 while the entrance to .Spencer's C.ulf is choked with sand. It 

 was during this period when the great valleys of the river 

 systems were being excavated that a great proportion of the 

 outbursts of volcanic rock in the form of basalt occurred. The 

 ige of these ba.salts is established by their superposition on 

 cretaceous rocks. 'I'hus, at Roma, the CIrafton Range is a mass 

 of bas.dt, resting on the cretaceous sandstones and shales. 

 Mount liindango is a similar instance. On the Upper Warrego 

 there is a deep ravine through cretaceous rocks partly under- 

 mining a basaltic cone. On the \ictoria Kiver a large basin 

 has l)een eroded in the cretaceous rocks and then several hundred 

 ■~quare miles flooded by an eruption of basalt, through which 

 watercourses have cut instructive sections, showing the subordin- 

 ate sandstones baked and fused by contact and the cracks fdled 

 'V the covering basalt. 

 It does not appear that the eruption of basalt has materially 



NO. I 33 I. VOL. 52] 



affected the geographical outline of the coast, but there were 

 considerable variations (jf level and important tracts of fertile 

 country formeil by the ba.s;iltic detritus, such as Peak Downs and 

 Darling Downs in (Queensland, and to the west of Melbourne in 

 the .south. 



Larc;e .\ni.\i,\l Period. 

 It was not till after the convulsions which attended this out- 

 flow of ba,salt, and lakes, marshes, and rivers had been formed, 

 and produced a luxuriant growth of vegetation, that the gigantic 

 marsupials gave any ilecisive evidence of their aflvcnt, as their 

 fossil remains are found in the drifts of watercourses mixed with 

 basaltic pebbles and detritus. The physical conditions of the 

 country (luring the period of the Diprotodon, Nototheriuni, and 

 associated fauna, diff'ered materially from that which now sub- 

 sists, for the structure of the larger quadrupeds would render 

 them incajrable of obtaining a subsistence from the short herba^iie 

 now existing in the same localities, and it is evident that their 

 food was of a large succulent growth, such as is found only in 

 moist climates and marshy land or lake margins. This view is 

 also supported by the fact that on the Darling Downs and Peak 

 Downs the associated fossils include crocodile and turtle, so that 

 what are now open grassy plains nuist have been lakes or 

 swam])s, into which the streams from the adjacent basaltic hills 

 flowed, and, gradually filling the hollows with iletrilus, formed 

 level plains. 



Enormous Rainiai.i.s. 

 That this gradual filling up of lakes .actually occurred is sho\\ n 

 by the beds of drift which are found in sinking wells and in 

 sections exposed by erosion of watercourses ; but in all these 

 instances there is evidence that the ancient rainfall was excessive, 

 as even our present wettest seasons are in.adequate to the removal 

 of the quantities of drift which have been the result of a single 

 flood in the ancient period. (Jn the ridges around the lakes 

 there existed a forest growth, as many species of opossum have 

 left their bones .as evidence ; but the timber evidently differed 

 from the present .scanty growth of eucalypti. Whether the same 

 abundant rainfall extended far into the western interior is uncer- 

 tain, but the rivers evidently maintained a luxuriant vegetation 

 adapted to the sustenance of these gigantic animals, as the dis- 

 covery of a nearly complete skeleton of Diprotodon on the shore 

 of Lake Mulligan, in South Australia, shows that these animals 

 lived in this locality, as it is not probable that their bodies could 

 have floated down the Coeat River which drained the interior of 

 the continent through Lake Eyre. 



.Vnothek Cham.k. 

 It is evident that the climate gradually became drier, that the 

 rivers nearly ceased their flow, and the lakes and marshes 

 became dry land, while the vegetation was reduced to short 

 grasses that no longer sufficed for the subsistence of the huge 

 Diprotodon and gigantic kangaroo, though some of the smaller 

 maystill survive to keep conn>any with the dingo, who, while he 

 left the inqiressions of his teeth in the bones of the Dijirotodon, 

 has shown a greater facility for adapting himselt tii altered con- 

 ditions. Is this the survival of the fittest ? It was in these days 

 that some of the rivers flowing direct to the coast cut through the 

 .sandstones into the softer shales beneath, and by their erosit)n 

 formed considerable valleys bounded by rocky clift's, and when 

 the land was subsequently depressed the sea flowed in anil 

 forme<l inlets, of which .Sydney Harbour and the entrance 

 to the Hawke.sbury River on the east coa.st. Port Darwin and 

 Cambridge Gulf on the north-west, and the Pallinup River on 

 the south-west of the continent may be cited as examples. 



Conclusion. 

 • Thus Australia, after its first appearance in the form of a 

 group of small lands on the east, and a larger island on the west, 

 was raised at the close of the Pahvozoii; period into a continent 

 of at least double its present area, including Papua, antl with a 

 mountain range of gre.at altitude. In the Mesozoic times, after 

 a grand growth of vegetation which formed its c.ial beds, it was 

 destined to be almost entirely submerged in the Crel.aceous sea, 

 but was again resuscitated in the Tertiary period with the 

 geographical form it now presents. Thus its climate at the time 

 of this last elevation maintained a nuignificent system of rivers, 

 which drained the interior into Spencer's Culf, but the gradual 

 decrease in rainfirll has drieil up these watercourses, ami their 

 channels have been nearly obliterated, and the country changed 

 from one of great fertility to a comparatively desert interior 

 which can imly be partially reclaimeil by the deep boring ol 

 artesian walls. 



