BY W. H. TWELYETREES AND W. F. PETTERD. 37 



intrusive mass, but this may be with equal probability the 

 exposure of a thick lava mass. A common characteristic 

 of metamorphosed felsites and their tuffs in schist areas is 

 the indefinable nature of the boundary line separating them 

 from the schists. This seems to be the case in this region, 

 and is a fact in support of their contemporaneity. We 

 fhave seen the same felsite in the shaft at Mount Black 

 Mine, and still further north on the Tasmanian Copper 

 Company's property between Rosebery Township and the 

 Pieman River. Further south, too, it occurs in the direc- 

 tion of Red Hills and Mount Darwin ; and a very coarse 

 granular chlorite-stained variety of the same series is found 

 on the White Spur between Moore's Pimple and Mount 

 Read. This zone therefore extends in a N. and S. direc- 

 tion for about twenty miles, Avhile E. and W. its breadth 

 is comparatively small. The zone of felsites seems to 

 mark the upturned edges of sheets of lava roughly parallel 

 with the axis of the present West Coast range. These 

 lavas were probably geologically contemporaneous with 

 the argillaceous sediments now converted into schists, and 

 with them were folded, crumpled and rolled out into the 

 schistose, banded conditions in which we now find all the 

 rocks of this belt of country. At least this interpretation 

 is the one which seems to us the most feasible in the 

 present state of our knowledge of this difficult piece 

 of country. 



Tlieir irlations to the Ore Beds. 



This fact confronts us : Whenever the felsite appears in 

 ■tunnels driven through the metal-bearing phyllites or 

 schists, ore is no longer found ; the felsitic rock is barren. 

 The occurrence of a band of this felsite in an adit is 

 suggestive, at the first blush, of an intrusion ; but the 

 absence of sharply-defined walls is against the idea, and it 

 can be explained quite satisfactorily on the supposition 

 that it is an intercalated sheet. It cuts ofi" the ore simply 

 because the ore is not contained in a lode fissure, but has 

 been deposited by a process of segregation, or has replaced 

 the original rock by metasomatic substitution. The ore 

 bodies on Mount Read form lenticular masses in the 

 argillitic schists parallel with the plane of foliation, and 

 disposed at irregular intervals in directions jjarallel to each 

 other. The deposition took place probably subsequently to 

 foliation, judging from the parallelisms with the enclosing 

 schists. This is seen on a small scale in some of these 

 mines, where the ore follows a minute arching and folding 



