142 



THE GEOLOGY OF TASMANIA— THE PRE-CAMBRIAN. 



Where the Pre-Cambrian rocks are penetrated by 

 igneous intrusions, the latter do not, in general, appear 

 to acquire forms .which are moulded by the structure of 

 the schists. The only exception which may be cited is 

 that of the small dykes or veins of pegmatite in the 

 valley of the Collingwood River. 



VI.— THE GENESIS, HISTORY, AND PRESENT 

 PHYSIOGRAPHICAL FEATTJRES OE THE 

 PRE-CAMBRIAN IN TASMANIA. 



Having come to the conclusion that the great bulk 

 •of these Pre-Cambrian rocks represent aqueous sedi- 

 ments, more or Less altered subsequently to their de- 

 position, but accumulated under conditions similar to 

 those existing on continental borders at the present 

 time, we naturally look for the sources of the fragmental 

 material which built up these ancient beds of con- 

 glom^erate, sandstone, and shale. 



The composition of these beds gives some idea of 

 the nature of the rocks whence the fragments were de- 

 rived. For the vast accumulation of clay and silica must 

 Iiave resulted from the disintegration of quartz and alu- 

 minous silicates. By the action of the many processes 

 involved in the weathering of such rocks, the transport 

 of the fragmental matter to the sea, the sorting distri- 

 bution and final deposition of this material on the sea 

 floor, these Pre-Cambrian sedimentary beds were built 

 up. 



But there have not yet been seen in situ in Tasmania 

 any rocks of greater age than these sediments. Detached 

 boulders (i) have been found which present analogies, 

 in both composition and structure, with the lower Pre- 

 Cambrian rocks of other parts of the world ; but these 

 l)Oulders have not yet been traced to their source. 



The quartz and aluminous silicates nmst have been 

 derived from some still older primary rocks, probably 

 now hidden beneath the sea. 



(i) One of these boulders, of a coarse gneiss, was found 

 by Mr. G. A.. Waller at the 29-milc peg on Innes' Track to 

 Barn Bluff. Garnetifcrous gneiss boulders also occur in the 

 permo-carboniferous glacial beds whichi outcrop on the North 

 Coast at Wynyard. 



