HORN KXPKDITIOX — GKXERAL OKOLOfiY. 69 



2. Domulatioii of tho Upper Crotacpous and Dpsoit Sandsiono was considiM- 

 al)lo, l)otli ill (loptli and .suppilieial oxtout, lifforc siliciliralioii sii]ior\('iu'd. Thus 

 the marly clays at Dalhousie lieconie porcelainiscd as they are traced northward ; 

 and in the neighhoiuhood of Charlotte Waters a white hard kaolin, Mdiicii is, liow- 

 ever, not so hard as to resist tlie knife, is at one spot couveited into a poi'celain of 

 a hardness approaching that of (juartz. It was fioni associated beds thnt Mr. 

 Byrne obtained silicitied casts of Ci'ioceras and a coral. Tn every example of 

 silicification of the sediments of Upper Cretaceous age there is no covei'ing lied, and 

 wlien the Desert Sandstone is present the altei'ation is limited to that, foi-mation. 

 Tt may therefore be inferred that denudation of the Ci'etaceous plateau preceded 

 the process of silicification, which, acting from above downwaids, ailected whate\er 

 sediment chanced to be at the surface. The texture of tJie original iJesert Sand- 

 stoiie permitted doubtlessly greater penetration in depth of the silicifying agent 

 than was possible in the case of the argillaceous deposits. In the foimer the 

 alteration is by infiltration into the interstices of the original coarse-grained .sand- 

 stone, producing a rock which simulates a porphyrite in texture — a clear \iti<'o- 

 resinous colloid cpiartz, compacting sharp (piartzose sand grains. Tn the latter the 

 change seems mainly to be that of substitution. 



Area Affected by Secondary Silicificatiiui. — ^^'(■ have not the data to deliminate 

 the area over which this cliemical alteration has taken place, but that it is vast 

 may lie inferred from the following occurrences : — 



Tiu()Ughi)ut the Lake Torrens Ijasin and the western section of the Lake Eyre 

 basin tlie Desert Sandstone is chalcedonised, e.xtending to the most northern of its 

 outliers in the vicinity of Henbury. The Ordovician limestones about the sources 

 of Alice Creek seem also to have been subj(^cted to the same influences, as along 

 the bedding planes of the somewhat Hssile limestones chalceilonic sul)stitution 

 has taken place for a thickness of an inch or so. An extreme phase of this 

 structural alteration is the development of noble opal in the Cretaceous I'ocks at 

 White Ciifl's, about sixty miles in a north-westerly direction from Wilcanni.i, and 

 thence passing into South Australia near Cooper Creek. Mi'. Ander.son* writes 

 that the chief source of the opal is the Desert Sandstone, in which it occurs in 

 three following positions : — 



1st. Disseminated in minute fragments throughout the sul)stance of the rock. 

 2iid. C(jating the joints and fractures. 



* Rfcords Geol. Surv. X.S.W., vol. iii., p. "o, lSil2. 



