14 GEOLOGICAL SURVEY, CANADA. 



of the syucline the conglomerate rests upon the granite gneiss and 

 on the other limb, upon the Keewatin. 



This interpretation of the structure greatly simplifies the geology 

 of Steeprock lake. The Steeprock series is reduced to four forma- 

 tions, viz. : Smyth's I, II, III, and IV. Formation V is identified 

 with II, and VI with I. Formations VII and VIII are Keewatin 

 below the basal conglomerate, and IX is non-existent as mapped. 



With reference to the dykes which traverse the region, it may be 

 said that the greenstone pebbles in the basal conglomerate, which 

 Smyth supposed to have been derived from an earlier system of 

 dykes, are derived from the Keewatin, and that nearly all the dykes 

 belong to his second class, i.e., are later than the Steeprock series. 



A word in regard to the Atikokan series also may not be out of 

 place. I examined, very cursorily, the shores of Margaret lake, and 

 found no reason to segregate the rocks there exposed from those of 

 the basement upon which the Steeprock series rests. Mr. Smyth 

 regards these rocks as of later age than the Steeprock series lying 

 across the edges of the latter. It seemed to me that following the 

 Steeprock formations southerly along their strike they abutted 

 upon the rocks of Margaret lake by reason of a N.E.-S.W. fault with 

 resultant down-throw on the N.W. side; the formations of the Steep- 

 rock series having been entirely removed on the south side. 



The simplification of the structure, and the reduction of the 

 number of its constituent formations detracts nothing, however, 

 from the interest attaching to the Steeprock series. There is 

 enough of it left to make it a most important member of the 

 Archsean Complex. The fact that it is definitely segregated from 

 the Keewatin, only adds to the interest. But perhaps the most 

 interesting fact connected with this series is that its dominant 

 formation, the limestone, which is estimated by Smyth to be not less 

 than 500 feet, nor more than 700 feet in thickness, is fossiliferous. 

 The rock is, in part, almost an aggregate of fossils, but, in part, it 

 is also composed of calcareous detritus derived from the waste of 

 organisms, as may be clearly determined from the cross-bedded 

 structures which are quite apparent in it. 



In part the fossils are wholly calcareous, and in part they are 

 wholly silicified, and there are intermediate conditions due to partial 

 silicification. Where not silicified the fossils appear on the 

 weathered surface of the limestone as radial structures, the rays 

 extending out to a limit which is approximately circular in sections 



