Vol. XXVII. 



igio 



1 Armitage, Country about West Essendon. oi 



an underground circulation has been established. Even after 

 these conditions exist, and the conditions are favourable to a 

 vigorous underground circulation .... cementation is 

 still very slow." He further states that the time necessary to 

 form quartzite from sands is enormous, and that, on the average, 

 it is necessary that 260,000 parts of water should pass through 

 a sandstone formation to deposit one part of quartz. Therefore, 

 quartzites formed by ordinary underground circulation of waters 

 are of very slow formation. As a corollary to this it follows 

 that the most extensive quartzite formations are found in 

 Palaeozoic and Mesozoic strata, where there are beds of such 

 material of enormous extent. No extent of Tertiary quartzites 

 comparable with the more ancient formations seems to exist. 



In Egypt, near Cairo, are found Tertiary quartzites, the 

 method of formation of which has led to a great amount of 

 discussion. Professor Dawson (13) thought that siliceous springs 

 were responsible for the cementation of the sands ; but Pro- 

 fessor Zittel (14) says (trans.) : — " Hot springs are not necessary 

 to account for the silicification of the fossil trees [and the sand- 

 stones]. . . . One seeks in vain for siliceous sinter, and of 

 this product of geysers no one has yet seen anything." Others 

 have endeavoured to account for the quartzite of the area on 

 the assumption that volcanic activity was the agent that formed 

 it ; but Captain Lyons connects the presence of vast numbers 

 of silicified trees with its formation. He writes (15) : — " There 

 is no doubt that the siliceous cementation of the sandstones, 

 and the molecular replacement of the woody structure of the 

 fossil trees by silica, are results of one and the same action. 

 . . . . Over the whole of this area, at three points only 

 . have eruptive rocks been recorded. . . . Con- 

 sidering the large amount of decaying vegetable matter there 

 must have been in the sands of an estuary into which such 

 numbers of trees were drifted, I would suggest the action of 

 water holding natron (sodium carbonate) in solution as a possible 

 explanation." He then details the process whereby the vege- 

 table matter would cause the silicification of the sandstone. 

 Sir A. Geikie (16) mentions Eocene quartzites, known as grey 

 wethers, in Wiltshire, which occur also over the north of France 

 towards the Ardennes : — " They have been used for the huge 

 blocks of which Stonehenge and others of the so-called Druidical 

 circles have been constructed ; hence they have been termed 

 Druid-stones. Other names are Sarsen stones (supposed to 

 indicate that their accumulation has been popularly ascribed to 

 the Saracens), and Grey Wethers, from their resemblance in 

 the distance to flocks of (wether) sheep." In the south of Cape 

 Colony, at a few places, probably Lower Tertiary in age, 

 quartzites and siliceous conglomerates occur, forming cappings 



