264 H. C. COOKE 



'grade toward the base of the series into arkoses, greywackes, and 

 conglomerates, which are of essentially the same composition as 

 the schists above them, and differ only in their coarser grain. 



This greywacke-like composition indicates that in each of the 

 widely separated areas the sediments were derived by fairly rapid 

 disintegration and erosion of older rocks, without extended decom- 

 position of their constituents by atmospheric weathering. Had 

 decomposition been complete and erosion slow, a better separa- 

 tion of the products of disintegration would have taken place, 

 with resultant deposition of a normal sandstone-shale series. The 

 band of quartzite found in the Kenoniska area is the only one 

 observed in which fairly good segregation of the sedimentary 

 material has taken place; yet its significance is not great, as the 

 bed is thin and the quartzite very impure. 



The petrographic similarities indicate that the sediments of 

 the different areas were derived from similar sources, and that 

 similar conditions of weathering and erosion were operative at 

 the time each was laid down. 



The succession of the different rock types also affords some 

 evidence for correlation. At the base of each series is a conglom- 

 erate bed several hundred feet thick. The composition of this 

 conglomerate is in every case similar — pebbles of granite and the 

 underlying series of basic lavas, with their associated tuffs, cherts, 

 and intrusives, imbedded in an impure sandy matrix. Above 

 this, except in the Mattagami area, lies a coarse-grained arkose, 

 greywacke, or grit, which passes upward into finer-grained material 

 of much the same composition, now metamorphosed to mica schist. 

 In the Mattagami area the conglomerate passes into the upper 

 schist by a decrease in the size and number of the pebbles, without 

 the intervening band of arkose; in the Kenoniska area a narrow 

 band of impure quartzite lies between the arkose and the mica 

 schist. 



The succession is one which indicates an almost identical 

 history of deposition in each of the areas under consideration. 

 If the series is of marine origin, the history is one of deposition 

 in a gradually deepening sea or of a subsidence of the land from 

 which the sediments were derived. The absence of shale and 



