iQoi] Dresser — Note on an Amygdaloidal Trap Rock. i8i 



iron, and various ores of copper, especially chalcocite, bornite, 

 and chalcopyrite are of frequent occurrence. 



The examination of four thin sections under the microscope 

 suffices to show that the rock is of volcanic origin, and hence is a 

 true amygdaloid instead of an argillite. Although it is much 

 altered, crystals of primary plagioclase can be distinguished in it 

 with certainty. In arrangement they suggest the structure of 

 diabase, but as the interstitial material is wholly secondary, 

 chlorite, iron ore, leucoxene, &c., further evidence is needed to 

 determine its precise original character. This was probably variable, 

 as fibrous hornblende occurs in some quantity at a point about 

 three miles west of the St. Francis river. Also near the same 

 place the rock contains nodular masses three or four inches in 

 diameter, which are composed chiefly of concentric layers of 

 quartz and epidote. 



The copper ores, as far as seen, occur in connection with 

 either calcite or quartz, in which cases the latter minerals do not 

 appear to form veins having either uniform width or well defined 

 edges, although they frequently do so in other cases when the 

 veins are much smaller in size. The copper-bearing masses of 

 calcite and quartz, however, seem rather to occupy crevices and 

 fissures, such as might have been produced by the intense dynamic 

 metamorphism by which the entire region has been greatly dis- 

 turbed, and as the copper, from its position, must have been 

 deposited contemporaneously with these gangue materials, it must 

 like them be regarded as of secondary nature, probably deposited 

 by infiltration. 



In its mode of occurrence this rock seems to conform to the 

 stratification of the region, and shares in the foliation which the 

 adjacent rocks have suff'ered. It lies in the pre-Cambrian, as 

 recently divided by Dr. Ells (Annual Report Geological Survey of 

 Canada, Vol. VII, N. S., Part J, 1894), appearing, wherever it 

 has been observed, between the Cambrian on the northwest 

 ard pre-Cambrian strata, generally dolomite or quarzite, on the 

 southeast. 



As it has resisted denudation better than most of the asso- 

 ciated rocks, it usually forms a rather conspicuous feature of the 

 landscape. The width varies from one to two miles, and the 

 extent along the strike has not been ascertained.. 



