200 Lawson on diabase dykes of the (Rainy lake region. 
common strike throughout the region from N. W. and S. E. to 
N. 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 char- 
acter of the country rock ; the absence of inclusions of the country 
rock, or of apophyses of the dyke running into it, except in very 
occasional instances; their generally uniform width under dif- 
ferent 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, aphaiiitic, black rock at the immediate 
contact with the dyke walls, by insensible gradations to a very 
coarse-grained, mottled, dark gray rock in tlwe middle of the 
dyke ; an occasionally observed peculiar pitting of the weathered 
surface, arranged in straight, more or less imiformly 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 moutofinecs of the schists and gneisses; 
their assumption of brownish tints on surfaces aerially weath- 
ered; (surfaces beneath high water mark of the lakes are gener- 
ally quite fresh and black.) 
These dykes have as yet received only a preliminary study, 
and it will require a much more extended examination of the 
country in which they occur and a much more elaborate inves- 
tigation of their petrographical 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 
of what is being done in this line of investigation in the field 
west of lake Superior.' 
One of the most characteristic of these dykes is one that trav- 
erses the coarse granitoid gneiss of the west arm of Jackfish 
lake, which lies to the north-west of Rainy lake. Its width 
is 135 feet and its contact with the country rock is well exposed 
as a sharp line. From a macroscopic examination the gneiss 
1 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 ex- 
press his grateful acknowledgments. 
