154 The American Geologist. March, isoi 
and various vicissitudes have since then interfered with the work. 
However, as some of the facts observed are of interest it is pro- 
posed to give them here without attempting to elaborate them so 
fully as was originally contemplated. 
The dykes of the Rainy Lake region show in a marked degree 
that variation from a fine texture at the dyke walls to a coarse 
texture in the middle of the dyke, which is more or less charac- 
teristic of dykes the world over. In the abstract referred to it 
was stated that ' ' On examination of the dykes in question, it be- 
came apparent that this variation in the physical appearance of 
the dykes is not simph^ one of texture or degree of coarseness of 
the constituent minerals, but it is rather the incidental concomi- 
tant of important structural, mineralogical and chemical variations 
which appear very constantly in the same way in different dykes. " 
It will be the object of the present paper to establish this general 
statement by giving some account of the facts upon which it is 
based. 
A dyke about 150 feet wide which traverses Stop island on 
the south side of Rainy lake, in which the variations alluded to 
are strongly accentuated, will be first described somewhat in de- 
tail. From this dyke a series of four specimens was taken, viz. : 
I at the contact with the dyke wall. 
II at four feet from the contact. 
III at fifteen feet from the contact. 
IV at seventy-five feet from the contact (middle of d}'ke). 
Textured variation. — To the unaided eye there is apparent a 
very distinct gradation in texture from that of an aphanitic rock 
at the contact to that of a coarse gabbro or diorite in the middle 
of the dyke. The gradation is rapid in the first four feet, less so 
from four feet to fifteen, and scarcely perceptible from fifteen feet 
to the middle of the cbyke ; the rock on which the observations 
were made in the field presenting a continuous, clean, fresh, glac- 
iated surface. In order to arrive at some definite information 
regarding the gradation in texture, careful measurements were 
made of the constituent minerals in thin sections of the different 
specimens. The following are the results of these measurements : 
I Ground mass. 
Pyroxoni — largest grains 031:") mm 
Ave. diameter 030 mm 
Feldspar in slender needles. 
Ave. size 004 X .052 mm 
