200 H. C. COOKE 



Toward the northern edge mashed arkose, greywacke, and con- 

 glomerate are found. The mica schist is a fine-grained rock con- 

 sisting essentially of quartz and biotite usually with some feldspar. 

 The arkose and greywacke are of much the same composition, 

 though coarser in grain; they differ from each other only in the 

 relative proportions of acid and basic fragments they contain. 

 Both pass into conglomerate by the gradual addition of pebbles. 



The composition of the conglomerate apparently varies a great 

 deal from place to place. Wilson states that in the more western 

 portions of the area studied by him he found pebbles only of granite, 

 rhyolite, and quartz porphyry and none of the more basic rocks. 

 Bancroft, on the contrary, working in the more eastern parts of the 

 area, reports the majority of the pebbles to be of greenstone, 

 although pebbles of granite and diorite are not uncommon. Meta- 

 morphism has squeezed and flattened all the pebbles more or less, 

 converting the softer of them often into mere strings. 



The Pontiac series overlies the volcanic complex unconformably 

 according to Bancroft, as most of the pebbles of the conglomerate 

 can be matched in the rocks below. Wilson, who did not find out- 

 crops containing the more basic pebbles, doubts the uncomformity 

 and is inclined to believe that the sedimentary series is older than 

 the volcanic, since the general dip is steeply to the north. To 

 secure further evidence, if possible, on this question in dispute, the 

 writer visited Lake de Montigny in 191 7 to examine the contact 

 there. The contact itself has been exposed here in workings of 

 some prospectors, and the lavas to the north are well exposed. 

 The examination by the methods outlined in the beginning of this 

 paper indicated that the south side of the flows is the uppermost. 

 This evidence therefore confirms Bancroft's conclusion that the 

 sediments overlie the flows, and some further support to it is given 

 by the similarity of the Pontiac series to the Brock and Broad- 

 back series, both of which are undoubtedly younger than the 

 volcanics. The northward dip must be ascribed to overturned 

 folding. 



The general trend of the strike of these rocks is east- west, and 

 the dip, as stated, steeply north. Bancroft is inclined to believe 

 that they form a tight syncline, but against this hypothesis lies the 



