272 • H. C. COOKE 



fault, which might have brought the two series into intimate 

 contact. It would appear therefore as if there was no discordance, 

 structural or erosional, between the sedimentary series and the 

 underlying extrusive rocks. 



The problem presents itself, however, of explaining why the 

 Nemenjish sediments here rest upon basalts, which have been 

 stjown to form elsewhere the basal rocks of the lava series, rather 

 than upon its higher members. On the creek flowing from Little 

 Nemenjish Lake to Obatogamau Lake, five miles to the east, the 

 sediments overlie andesites, although the contact is not exposed. 



Two hypotheses present themselves. There may have been 

 a period of erosion intervening between the extrusion of the lavas 

 and the deposition of the sediments, so that where the sediments 

 rest on the lower members the younger lavas may have been 

 removed by erosion; or the younger lavas may never have been 

 extruded in these localities. The first hypothesis appears inad- 

 missible, on account of the perfect conformity and lack of any 

 trace of basal conglomerate or coarse basal sediments. If the 

 second hypothesis be true, then some of the lower beds of the 

 Nemenjish series must be equivalent in age to the upper lava 

 flows, unless conditions were such that no sediment was deposited 

 during the period intervening between the extrusion of the earliest 

 and latest lavas. We might therefore expect to find at some 

 point an interbedding of the sediments with the higher lava flows. 

 This is actually the case on Kaopatina Lake, where a thin flow 

 of rhyolite porphyry in interbanded with the sediments. In this 

 case, the beds with which the flows are interbanded are not of 

 true clastic material, but appear to be made up in part of cherty 

 material, perhaps a chemical sediment, and in part of fine-grained, 

 rather basic material, perhaps volcanic dust or the first products 

 of erosion of the lava fields. In the critical section of the Opawika 

 River above Kaopatina Lake, beds of similar composition were 

 observed to lie at the base of the Nemenjish series, and to grade 

 up into it by a gradual increase in the proportion of normal clastic 

 material. 



It seems probable therefore that the whole region was not 

 covered by uniform sheets of lava of the same composition, but 



