70 HORN EXPEDITION — CENEKAL GEOLOGY. 



3i'd. Occurring irregularly as definitely-shaped pieces, which have resulted 

 from the replacement of fragments of fossil wood, shells, etc. 



Coteniporaneous plioioineiia. — The phenomena of silicitication of the Cretaceous 

 deposits is most fully displayed between the Stevenson River and Charlotte 

 Waters, and here is the metropolis of obsidian bombs and unrolled agates. Bombs 

 and agate.s occur widely dispersed over the Cretaceous area in Central Australia, 

 but we are not aware if any other locality has yielded them in their pristine 

 condition, the former are most frequently found in an eroded state, and the latter 

 in a fragmentary condition, and more or less rolled. The agates which abound 

 between Blood Creek and the River Stevenson, range from the size of one's list to 

 that of one's head, exhibit a black somewhat scoriaceous exterior and in appearance 

 resemble those obtained in volcanic scorije. 



Origin oj the Silicification. — Mr. East (viii., p. 52) attributes the change from a 

 sandy formation to a porcelainised sandstone to immersion in silicated waters, 

 derived from the decomposition of the metamorphic rocks of the McDonnell 

 Range, which have wholly evaporated. That the phenomenon of silicification 

 implies precipitation from solution cannot be denied, but Mr. East's view limits 

 the operation to the final evaporation of the lacustrine waters in which the Desert 

 Sandstone was deposited; whereas we have endeavoured to prove that considerable 

 denudation had taken place before silicitication could have hajapened, whilst the 

 accompanying j^henomena, viz., the formation of agates, and obsidian bombs and 

 Desert Sandstone breccia, seem to demand a common origin. It must be 

 conceded that the process of silicification has stopped ; that the artesian waters of 

 the Cretaceous basin do not, and probably never did, possess silicifying properties. 

 Under the circumstances we hope to be pardoned for speculating upon the origin 

 of this silicification, which shall, at the same time, be in unison with the require- 

 ments to satisfy that of the attendant phenomena. 



In the first place the occurrence of the obsidian bombs and agates on the 

 Desert Sandstone plateaus and their slopes could not have been transported there 

 by water, unless in the form of ice (an hypothesis incompatible with the co-ordinate 

 features). The origin of the Desert Sandstone breccia was certainly not due to 

 fracture of the original l:)ed by failure of support arising from denuding action, but 

 might have been caused by a lava-flow or the deposition of highly heated volcanic 

 ashes when saturated with water. The obsidian bombs demand volcanic action, 

 and agates are not infrequently associated with volcanic ejectamenta ; whilst the 

 silicates of the ash-beds or lava under chemical action would furnish silicated 

 waters as a source of the chalcedonising action on the underlying rock-surfaces, 



