174 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



possible connection witli the bedded traps that form so large a part 

 of the two latter geological series. With the question of the possible 

 identity of character and age of these dykes with the traps of the 

 Animikie or Keweenawan, or of both, is associated the equally 

 interesting one of the extent of the earth's sui-face, over which, in 

 early geological times, were in simultaneous operation, those parti- 

 cular volcanic forces which appear to have had their focus in the 

 Lake Superior basin. 



The more notable field characters of these dykes are : their common 

 strike throughout the region from N.W. and S.E. to IST.N.W and 

 S.S.E. ; the sharp, well defined nature of the gash or fissure which 

 they fill, no matter what may be the character of the country rock ; 

 the absence of inclusions of the country rock, or of apophyses of the 

 dyke running into it, except in veiy occasional instances ; their 

 generally uniform width under different conditions of occurrence in 

 different localities, the limits being as a rule 60 and 150 feet; their 

 continuity for one or several miles where exposures permit them to 

 be traced ; their passage from a very compact, aphanitic, black rock 

 at the immediate contact with the dyke walls, by insensible gradations 

 to a very coarse-grained, mottled, dark gray rock in the middle of 

 the dyke ; an occasionally observed peculiar pitting of the weathered 

 surface, arranged in straight, more or less uniformly spaced lines 

 transverse to the strike of the dyke ; their prominent, steeply 

 rounded, or domed, glaciated surfaces in contrast to the more gently 

 inclined roches moutonnees of the schists and gneisses ; their 

 assumption of brownish tints on surfaces aerially weathered ; (surfaces 

 beneath high water mark of the lakes are generally quite fresh and 

 black). 



These dykes have as yet only received a preliminary study, and 

 it will require a much more extended examination of the country 

 in which they occur and a much more elaborate investigation of their 

 jjetrographical characters before a comprehensive statement of their 

 geological relations can be formulated. A few notes regarding the 

 microscopic features of these dykes, taken together with what has 

 been said of their field occurrence, may however, be of interest, and 

 will serve as a report of progress pf what is being done in this line 

 of investigation in the field west of Lake Superior.*' 



* These rocks were studied microscopically in the laboratory of the Johns Hopkins University, 

 Baltimore, under the guidance of Prof. G. H. Williams, for whose kind advice and assistance 

 the writer desires to express his grateful acknowledgments. 



