IIOKN EXPEDITION SUMMARY. 109 



Miocene."* Mes.si.s. Hall .-uid Pi-itcliurdf lioNWNer liavc shown t,li;it (.ertaiii jilaiiL 

 lieui'iiii; lieds ill Victoria- lie hclow the niariiic E(X-ciic, "and tliis," say.s Pr'ot'es.sur 

 Tate, " accord.s well with the general fact that wherever the ha.se of the marine 

 Eocene is readied lacustrine and plant-bearing beds succeed in depth." 



Til the section dealing with Post-Crc^taceous Phenoinoiia the question of the 

 sibcification of the Upper and Suj)ra-Cretaceous rock is dealt witli. Messrs. Jack 

 and Etheridge,J in referring to the deposition of the Robing Downs and Desert 

 Handstone, point out that the hitter must at one time have occupied at least three- 

 quarters of the present surface of (Queensland, though now its denuded lemnants 

 only cover aljout one-twentieth of their origin;d area. After tlie Rolling b)owiis 

 formation had been laid down a considerable upheaval took place. " Tiie denuda- 

 tion of tiie Rolling Downs formation followed and must have gom; on for some 

 time. Unequal movements of ilepressioii then brougiit about lacustrine conditions 

 on porti(^ns of the now uplifted bottom of tlie old sea strait, and in other portions 

 permitted of the admission of the waters of the t)cean. Finally a genera! uplieaval 

 phiced the deposits of tiie period just concludeil in lu^arly tlie positions in which 

 we now tiiid them." 



Messrs. Tate and Watt point out that after the deposition, tirst oi the Upper 

 Crcitaceous, and then of the Supra-Cretaceous (Desert Sandstone), both series und(;r- 

 went a considerable amount of denudation before tlie silicitication, wliicli is now so 

 characteristic a feature of the latter, took place. " In every example of siliciti- 

 cation of the sediments of Upper Cretaceous age there is no covering l)ed, a,nd 

 when the Desert Sandstone is present the alteration is limited to that formation. 

 it may therefore be inferred that denudation of the Cretaceous plateau preceded 

 the process of silicitication, which acting from above downwards atiected whatever 

 seilinient chanced to be at the surface." The greatest amount of silicitication is 

 seen between the Stevenson River and Charlotti; Waters in which district also the 

 the largest number of obsidian bombs and unrolled agates are found. 



The origin of the silicitication is very dithcult to account for. At present two 

 theories have been advanced (1) Mr. Eastg has supposed that it is due to ilepositioii 

 from silicated waters, the siliceous material being derived from the decoiiij)osition 

 of the iiietainorphic rocks of the McDonnell Ranges and that the siliciiication took 



* General Geology, p. 07, OS. 



t Auist. Ass. Adv. Sei., vol. v., Adelaiile, lS9;i, p. 'i'ii. 

 X Geoloyy of Queensland, etc., p. 511. 



i) .J. J. East "On the Geological Structnre and riiysical Features of Central Australia." Tran. U.S. S. .\ust., 

 vol. xii., pp. .31-53. 188U. 



