266 H. C. COOKE 



the conglomerate at its most easterly observed contact rests upon 

 garnetiferous mica schists, which are believed to be the metamor- 

 phosed equivalent of a once widely distributed sediment, the 

 Nemenjish series, while farther to the west it rests directly upon 

 basic lavas. In addition, the presence of the thick basal conglom- 

 erate indicates that the erosion of the underlying series was great 

 before the deposition of the upper. In the undeformed conglom- 

 erate of the Kenoniska area, none of the basic pebbles possess 

 schistosity; so that the lava series could not have undergone 

 serious deformation before the conglomerate was laid down. 



The similarity of the relations which prevail among the dif- 

 ferent patches of younger sediments toward the older series of 

 lavas and sediments still further strengthens the probability that 

 these patches originally formed part of a single widespread forma- 

 tion. The unconformity appears to be one of erosion mainly, 

 with little or no structural discordance. 



As regards the relations of the sediments to younger forma- 

 tions, the only younger rock in contact with all of them is the 

 granite, which in each case intrudes them. As there is no sedi- 

 mentary rock younger than the granite in the region, by which 

 its age may be checked, the granite does not afford any adequate 

 datum plane for correlation purposes. 



Summary. — It has been shown that in the patches of sediments 

 now known under the local names of Pontiac, Mattagami, Broad- 

 back, Lucky Strike, and Brock series, there are several striking 

 similarities: (i) The character of the sediments shows that they 

 were in each case derived by the disintegration and erosion of a 

 land area, which went on with such rapidity that the component 

 minerals of the eroded rocks were not highly decomposed; as a 

 result, the sediments are not characterized by a normal sandstone- 

 shale alternation, but are of a greywacke or arkose composition 

 throughout. (2) The absence of the metamorphosed equivalents 

 of hmestone or shale, of which latter sediment there must have 

 been some quantity, indicates fairly shoal water or subaerial con- 

 ditions during the deposition of each series. (3) The succession 

 in each case is similar; a conglomerate lies at the base, overlain 

 by finer-grained material, arkose or greywacke, of essentially the 



