part 2^ GEOLOGY OB 1 THE TBEFKIW PY KITES DEPOSIT. Ill 



III. The Oee-Bodt. 



This is a finely granular mass of pyrites of considerable purity, 

 The colour is unusually pale for pyrites, but becomes more brassy 

 on exposure. A polished surface examined microscopically shows 

 the p3 r rites in very minute cubes, aggregated together and separated 

 by interstitial dark areas. Treated with hydrochloric acid small 

 areas effervesce briskly, but most of the polished surface is un- 

 touched. This is consistent with the presence of quartz-veins 

 in the ore and also of an efflorescence of gypsum on the floor of 

 the mine, doubtless the result of the action of the oxidized 

 sulphide on calcite in the ore. 



The pyrrhotite, mentioned above as found beneath the igneous 

 rock, when polished shows a similar structure to that of the 

 granular ore, with the difference that we have here pyrrhotite 

 instead of pyrites. 



The mode of occurrence of the ore-body, between the shale 

 and the intrusion, has been already mentioned. Although it 

 resembles a bed about 6* feet thick, and extends over a consider- 

 able distance, it does not seem to be continuous over all the area 

 described. Owing to the fact that the ore has been destroyed 

 by weathering for a distance of several yards from the outcrop, 

 it does not appear at the surface. Its presence, however, seems, 

 to be indicated by small thrusts and disturbances in the shales 

 above the igneous rock, as shown at the entrance to the main 

 (No. 2) adit. These disturbances may very possibly be due to 

 the entire destruction of the pyrites by weathering, causing the 

 unsupported shales above to founder. The presence of chalybeate 

 springs is an indication of the presence of pyrites, and the Trefriw 

 Well obtains its water, which is intensely chalybeate, from such 

 a spring. But, in places where the junction of the strata and the 

 intrusive rock is visible there is no obvious disturbance ; veins of 

 the intrusion are seen to run upwards into the shales, and there is 

 hardening and silicification of the latter. Here the ore is evidently 

 absent, and indeed never existed. 



IV. Ojiigix of the he- Bom. 



The Keport of the Koyal Ontario Nickel Commission, published 

 in Toronto in 1917, points out (p. 126) that, in the case of many 

 ore-deposits throughout the world that have been closely studied, 

 no agreement has yet been reached regarding the source of the 

 ores, or the manner in which they arrived at their present positions. 

 This statement applies to the chief pyritic deposits, such as those 

 of Kio Tinto or Sudbury, with which one would naturally compare 

 the Trefriw deposits. 



F. Berschlag, J. H. L. Vogt, & P. Krusch 1 remark that the 



1 ' The Deposits of the Useful Minerals & Rocks,' transl. by S. J. Truscott,. 

 vol. i (1914) & vol. ii (1916) p. 240. 



