118 THE PROGRESS OF GECLOGiCAL RESEARCH IX TASMANIA, 



well-determined fact that they underlie with a marked un- 

 conformity the West Coast Range Conglomerate series, which 

 was regarded by Ward as of Lower Cambrian age. It has 

 subsequently been demonstrated, however, that this West 

 Coast Range Conglomerate series is the basal conglomerate 

 of the Silurian System, so that the observed stratigraphical 

 succession gives no more definite age determination than that 

 of Pre-Silurian. The only occurrence m situ of rocks of 

 Cambrian Age in Tasmania is that of the Dikelocephalus 

 sandstone near Railton, but as the exact relationship be- 

 tween this series and any other rock series in the vicinity 

 has not been demonstrated, there must be some doubt as to 

 the Pre-Cambrian Age determination. Certainly, the very 

 fresh character of the Cambrian sandstone as compared 

 with the schistose character of the rock series referred to 

 as Pre-Cambrian is suggestive, and the Pre-Cambrian age 

 determination is largely based on lithological character. 



That the reference of this rock series to the Pre-Cam- 

 brian is most probably correct is indicated by the fact that 

 the following is the definitely ascertained succession: — 



Silurian. — West Coast Range Conglomerate Series. 

 Diastrophic Period and Erosion Interval. 



Camhro-Ordovician. — Porphyroid Igneous Complex. 

 Diastrophic Period and Erosion Interval. 



Pre-Camhrian {?) . — Quartzites and Mica-Schists. 



Accepting their Pre-Cambrian age, L. K. Ward refers 

 them to the Algonkian, and subdivides them into an Upper 

 and Lower Series — the upper consisting of a relatively 

 gently folded series of white quartzites, and the lov/er series 

 of intensely crumpled mica and quartz schists, the two being 

 separated by an unconformity. In the vicinity of Point Hibbs 

 the mica-schists contain intercalated beds of dolomitic lime- 

 stones, this being the only locality in Tasmania where cal- 

 careous beds are known to occur in the Pre-Cambrian. 



The areas occupied by these rocks have been indicated 

 in the latest geological map of Tasmania. Their greatest 

 development is in the south-western portion of the Island, 

 although isolated areas of much lesser extent occur on the 

 west, north-west, and north coasts. The late W. H. Twelve- 

 trees estimated the total thickness to be 13,000 feet, but in 

 view ^^ ^\e fact that the structural geology has not been 



