47 



taken place by the infiltration of silicated waters into an open 

 and porous rock, causing great induration within certain 

 limits, and such indurated portions have persisted, whilst the 

 incoherent portions of the beds have become washed away. 



As the sarsen type of stone depends on the petrological 

 texture of the stone and its mode of distribution, rather than 

 the geological age of the rock, they may occur in any country 

 and of any age when the suitable conditions for their formation 

 exist. Sarsen stones of this type are widely distributed 

 throughout South Australia. They have not attracted the 

 same attention from the public here as those of England, from 

 the fact that in England they are rendered conspicuous 

 because they occur in districts where few other hard stones 

 are found, while in South Australia they occur in places where 

 they are surrounded by outcrops of hard rocks, and in many 

 cases the two classes of rocks are so similar in appearance tfliat 

 only an experienced field geologist can detect the difference. 



In South Australia sarsens are found along the lines of 

 ancient drainage, where the sands and gravels of rivers that 

 no longer occupy these valleys have become intermittently sub- 

 jected to silicification, having some portions altered to a close, 

 compact, siliceous rock, while other portions have remained 

 loose and friable, or entirely removed by denudation. They 

 often form groups in the paddocks, preventing cultivation, or 

 occur as large boulders overspreading some ancient terrace,, 

 like the grey-wethers of southern England. They can be seen 

 from the railway train near Yacka, and at Stone Hut, and in 

 many positions on the Willochra Plains between Melrose and 

 Booleroo Centre, also on the low range facing the sea near 

 Ardrosisan,( 4 ) and in many other situations. 



The English sarsen stones are of a light-grey colour, and 

 in most cases are very fine in the grain, possessing a sacchar- 

 oidal lustre. Examined in thin sections by the microscope, 

 they are seen to consist almost exclusively of very fine sand 

 grains, closely dovetailed and united together by a siliceous 

 cement. On account of this form of structure they exhibit a 

 conchoidal fracture. 



The South Australian examples answer generally to the 

 same descriptions. In most cases a different form of silicifica- 

 tion can be recognized between that of the siliceous quartzites 

 of Cambrian Age and the siliceously-cemented river sediments 

 that have made the sarsen stones. The Cambrian quartzites 

 give evidence of metamorphic action, while the indurated 

 river sediments do not. In the case of the metamorphic 



(4)Howchin: "Notes on the Geology of Ardrossan and Neigh- 

 bourhood," Trans. Roy. Soc. S. Austr., v. 42, 1918, pis. xxii. to 

 xxvii. 



