BY L. KEITH WARD, B.A., B.E. 



141 



an interesting comparison may be drawn between the 

 Tasmanian occurrences and those of Canada. This 

 matter is discussed in a later portion of this paper. 



In estimating- the thickness of the sediments repre- 

 sented in the region surrounding the Frenchman's Cap, 

 it is evident that the structural features, referred to 

 here, must be duly considered. Enough detailed infor- 

 mation has not yet been acquired to give even an ap- 

 proximate idea of the true thickness of the upper and 

 lower series of which mention has been made. 



The writer claims that there is a marked uncon- 

 formity, above which several hundred feet of sediments 

 exist, and below which a very much greater thickness 

 is represented by the more schistose members of the 

 Pre-Cambrian. About 2,000 feet of the latter schists 

 are visible in the immediate neighbourhood of the 

 Frenchman's Cap, if we calculate only the vertical dis- 

 tance between the lower limit of the upper quartzite on 

 the top of the range and the bottom of the river gorges 

 carved in the foliated schists. But it is more than prob- 

 able that the vertical thickness of the schistose strata in 

 this region differs from that of the original beds. No 

 account has been taken of the effects of compression 

 and foliation, nor can these be quantitatively estim.ated 

 in the present state of our knowledge. 



The recognition of two distinct horizons in the Pre- 

 Cambrian necessitates a very careful treatment of the 

 structural features of strike and dip, unless it is per- 

 fectly plain which horizon is under examination. 



In the region traversed by the Franklin River and 

 its tributaries the upper horizon is, on the whole, as in- 

 dicated above, nearly horizontal, but the variations in 

 dip and strike of the lower members are very marked. 



A general absence of regularity of structure is 

 noticeable in 'every district, and the axes of the main 

 foldings do not appear to have extended for long dis- 

 tances. 



Nor have the folds exerted any appreciable effect 

 upon the topography. No traces of ancient fold ranges 

 have remained in those areas where these rocks have 

 been examined by the writer. And the outlines of the 

 present mountain ranges are noticeably independent of 

 the rock structure. 



