I20 THE GEOLOGY OF TASMANIA— THE PRE-CAMBRIAN. 



The writer has observed a small isolated hill of red- 

 dish quartzite entirely similar to that of the Denison 

 Range on the western border of the central plateau, at 

 a spot just to the north of the Linda Track, and a miile 

 to the eastward of Mount Arrowsmith. 



Moreover, this sedimentary series, which consists of 

 conglomerate, pebbly sandstone, crystalline sandstone, 

 and quartzite, is, apparently, precisely that which is re- 

 ferred to as the West Coast Range conglomerate series. 

 That is to sa}^ the Denison Range is composed of rocks 

 which appear to be identical in all respects with those 

 which contribute so largely to the bulk of Mounts 

 Jukes, Huxley, Owen, Lyell, Sedgwick, Murchison, 

 Farrell, and Zeehan. The lithological resemblance be- 

 tween the rocks from all these places is striking; and 

 the differences between the rocks of this character and 

 all other known sediments in Tasmania are equally well 

 marked. 



These rocks have hitherto been considered to be 

 much younger, but there has been no satisfactory proof 

 of age. The West Coast Range conglomerate has been 

 placed at the base of the Silurian system for the reason 

 that it is clearly older than the tubicolar sandstone. 

 The probable Cambrian age of the latter formation has 

 already been indicated here, and if this be admitted a 

 still greater antiquity must be assigned to the West 

 Coast Range conglomerate series. 



By far the most important information bearing on 

 this matter which has yet been gathered is that which 

 was obtained early in 1908 by Mr. W. H. Twelvetrees, 

 in his westward traverse of the country between the 

 Valley of Rasselas and the Prince of Wales Range. 



The bedrock of the Valley of Rasselas is the Ordo- 

 vician limestone, with a strike of N. 25deg. W. and a 

 north-easterly dip at an angle of 7odeg. This limestone 

 rests unconformably upon the siliceous sediments of 

 the Denison Range (i), which strike N. 3odeg. W. and 

 dip (at the Thumbs and Mount Wright) towards the 

 north-east at 5odeg. 



Since the Gordon River limestone horizon is re- 

 garded as the base of the Ordovician in Tasmania, it 



(i) See Section I. 



