RELATION OF THE CONTCHICHING AND KEEWATIN SERIES. 341 



an enormous thickness. Without recapitulating these reasons here, it 

 will be sufficient to state that in his opinion the Contchiching series 

 nowhere in the Rainy River district attains a greater thickness than 

 9,000 feet, the greater apparent thickness being due to multiple 

 folding. 



Clastic in Origin. — -The clastic origin of the gneisses and mica schists of 

 this series can hardly be doubted by any one familiar with them in the 

 field; their fine and even lamination and their bedded appearance affords 

 in itself almost conclusive evidence and the microscopic descriptions of 

 them by Lawson strongly support this view. Their mineral composition 

 indicates derivation from the denudation of a granitic floor. 



Structural Conformity between Contchiching and Keewatin. — -Between the 

 Contchiching and the Keewatin rocks there is everywhere a strict con- 

 formity of structural relations. In rocks which have suffered such great 

 mechanical deformation, however, it must always be borne in mind that 

 much of the original structure may have been obliterated and replaced 

 by subsequent cleavage, so that it is hazardous to state that because 

 there is now strict parallelism in the existing schistose planes of contig- 

 uous rock series that this necessarily indicates original conformity. Dr 

 Lawson argues an interval of erosion between the Contchiching and 

 Keewatin series from the presence at the base of the Keewatin of a con- 

 glomerate on the Seine river and on hat Raot bay of Rainy lake. The 

 former of these conglomerates is for the most part an integral portion of 

 Hie Keewatin series and for only a small proportion of its development 

 does it occupy a strictly basal position, so that it rather marks a local 

 break in the Keewatin itself than one between the Keewatin and Cont- 

 chiching, serving in a measure rather to bind these series together 

 than to separate them. The so-called conglomerate of Rat Raot bay is 

 mapped as lying wholly between the two series, but from what the writer 

 has seen of it he regards it as rather of volcanic than of detrital origin. 



Tlie Contchiching and Keewatin lithologically Distinct. — That the Cont- 

 chiching series is lithologically distinct from the Keewatin, and marks a 

 period subsequent to which a profound change in the conditions of rock 

 formation took place, cannot be denied, and the term is useful and appro- 

 priate as designating a well-marked and perhaps the most important for- 

 mation of an extensive series; but the Contchiching cannot be regarded 

 as in any respect coequal or coextensive with the Keewatin. There is no 

 stronger evidence of unconformity between the two than there is between 

 any two distinct horizons of the Keewatin, particularly where conglom- 

 erates are developed, and the Contchiching would seem to be therefore 

 essentially the basal portion of the Keewatin. 



LI-Bur,r,. Gkoi-. Soc. Am., Vol. 4, L892. 



