47 



of 191 1, and on his report (7), recently published as a mem- 

 oir of the Survey, is based the following description. 



The only rocks known in the area to be visited are those 

 of the Keewatin, Laurentian, Steeprock, and Seine series. 

 The position of the Steeprock series, well down in the Pre- 

 Cambrian, is of interest for the reason that the limestone 

 of which it is chiefly composed, is fossiliferous. 



Keewatin. — This is the oldest group in the region and 

 consists chiefly of felsites (quartz porphyries), gabbros, 

 diabases, greenstones, and their schistose equivalents, as 

 well as occasional exposures of tuffs and agglomerate 

 schists. The strike of the schistosity varies from place to 

 place, but seems in a general way to accord with the contour 

 of the lake shore. 



Laurentian. — This is primarily a medium-grained horn- 

 blende granite gneiss, showing only a slight foliation in 

 the Steeprock area. Near its contacts with the Keewatin, 

 it not only holds as inclusions large fragments of the older 

 series, but itself becomes quite basic, and grades into a 

 type closely resembling typical Keewatin hornblende 

 schist. In places, however, very sharp contacts of the 

 two series in their normal phases are exposed. On account 

 of its somewhat bleached appearance, and of its association 

 with much sheared varieties in neighbouring localities, the 

 series is correlated with the Laurentian. 



Steeprock Series. — The rocks thus designated include 

 the following formations in descending order: 



4. Green schists, evidently of detrital origin, traversed 

 by dykes and flows of diabase and diorite. 



3. Volcanic ash, highly pyritiferous, schistose rock, 

 often containing fragments of limestone and black cherty 

 material. 



2. Limestone, dolomitic to sideritic, and usually weather- 

 ing brown. The exposures at the sharp bends in the 

 shore-line of the lake are a brecciated variety, consisting 

 of angular fragments of limestone, black chert, and typical 

 Keewatin rocks. 



1. Basal conglomerate, usually fine-grained, and fre- 

 quently of the nature of a quartzite or arkose. 



The rocks of the series are almost in a vertical attitude, 

 the prevailing dips being 70 to 90 S.W. The series is 

 tentatively correlated with the Lower Huronian. 



Seine Series. — Quartzites and quartzose slates of the 

 normal type, striking almost east and west, occur some 



