SUCCESSION OF THE SEMI-CRYSTALLINE ROCKS. 383 



2. Porphyrellite and chloritic schists and other conditions into which they 

 graduate ; also the porodites, agglomerates, and tuffs of nondescript character. 



1. Graywackes. 



(Underlain by hornblende and mica schists.) 



In his description of the corresponding system of rocks on the shores of 

 the Lake of the Woods Dr. Lawson groups them as follows : 



" Felsitic, serieitic, and other glossy fissile schists of a hydromicaceous or chloritic 

 character, with some carbonaceous schists and limited occurrences of limestone. 



'•Mica or hydromica schists, clay-slates, and quartzites. 



" Hornblende schists, with associated trap rocks, principally altered diabases and 

 diorites." [Afterward excluded from the Kewatin.] * 



In the vicinity of Rainy lake he gives : 



" Felsitic schists (quartz porphyries and their tuffs) and agglomerates. 

 " Altered traps and green hornblendic schists." f 



Dr. Lawson's hydromica schists are my serieitic schists. His agglomerates 

 are embraced in my No. 2, and so, I think, are some of his felsitic schists. 

 While the general character of the rocks studied by him is plainly the same 

 as that of the rocks described by the Minnesota Survey, the correlations in 

 detail have not yet been completed. 



The approximate thickness of this system of rocks in Minnesota, as deduced 

 from four sections between the Basswood and White Iron Areas, is about 

 15,000 feet. In the Rainy lake region Dr. Lawson has calculated thick- 

 nesses of 10,200 and 13,200 feet. 



Structural and Mineralogical Relations of the Crystalline 



and Semi-Crystalline Schists. 



Wherever the crystalline and semi-crystalline schists are seen in juxtaposi- 

 tion their stratification is strictly conformable. Wherever the crystalline 

 schists are wanting, the semi-crystalline schists are found in conformity with 

 the gneisses. Moreover, whether the semi-crystalline schists occur in jux- 

 taposition with the crystalline schists or the gneisses, there exist frequently 

 those transitions by alternation which characterize the passage from the 

 crystalline schists to the gneisses. This mode of transition, however, is much 

 the most characteristic of the passage from the semi-crystallines to the crys- 

 tallines ; but this passage is simultaneous with mineralological changes which 

 must also be mentioned. 



It was early remarked that the Minnesota graywackes contain always 

 some proportion of fine mica scales. As we descend to the neighborhood of 



* Canadian Geological Report, Doc. CC, 1886, pp. 12, 29, 106, etc. 

 t Canadian Report, 1888, Doc. F, p. 46. 



