144 



THE GEOLOGY OF TASMANIA— THE PRB-CAMBRIAN. 



Orogenic movements, which have tilted the Cam- 

 brian rocks till they stand vertically in some places, 

 must have affected the subjacent Pre-Cambrian rocks 

 as well. 



After the close of the Cambrian period no schistosit}^ 

 of any moment appears to have been developed in any 

 of the Tasmanian rocks. 



The Ordovician sedini'ents indicate a deep submer- 

 gence of Western Tasmania beneath the ocean, and 

 traces of the marine limestones still occupy the beds of 

 some of the western rivers. These are not deposited 

 conformably upon the sediments of Cambrian age, and 

 their position in deep troughs carved in the schists 

 argues for a mature erosion of the areas covered by the 

 Pre-Cambrian rocks before submergence in Ordovician 

 tim^ 



Since the exposed peaks and ridges of Pre-Cambrian 

 rocks appear to attain altitudes which are approximately 

 the same, the idea suggests itself that a peneplain may 

 have been developed in late Pre-Cambrian or Cambrian 

 time. This peneplain may have been deeply dissected 

 before the Ordovician period. However, this matter 

 demands much more detailed investigation over the 

 whole of the Pre-Cambrian terrain. 



The area lying between the Raglan Range and the 

 Prince of Wales Range, examined by the writer, appears 

 to have regained many of the fundamental outlines 

 which it possessed at the beginning of the Ordovician 

 period. These outlines were masked by the deposition 

 of Ordovician and Silurian sediments, and then, after an 

 interval, of those of Permo-Carboniferous and Mesozoic 

 age. 



The only igneous invasion which was sufficiently 

 widespread to deserve mention here is that of the upper 

 Mesozoic diabase. This diabase still remains in the form 

 of outliers capping the mountains built of Cambrian and 

 Pre-Cambrian rocks. Its distribution argues a much 

 wider extent than is now apparent, and its existence 

 postulates a cover of sedimentary rocks, since removed 

 by subaerial denudation. 



Since the close of the Mesozoic era progressive de- 

 gradation of the whole of Western Tasmania has con- 

 tinued almost without interruption ; and in the final 

 stages of this 'long cycle of erosion the' physiography of 



