294 
The American Geologist. 
May, 1894 
Fig. 1. 
-Island at the mouth of White Gravel 
river, lake Superior. 
dinal profiles are thus those of the hummock, while the trans- 
verse sections show a combination of the hummock and nega- 
tive dyke profiles. The general form of the island is indicated 
in the accompanying dia- 
gram, fig. 1. The other two 
dykes of the island are con- 
fined to the more easterly, 
or lesser, of the two por- 
tions into which it is thus 
divided by the dominant 
dyke. The larger dyke is 
near the S. E. point of 
the island and is remark- 
able only for the irregular character of the containing fis- 
sure w r alls. In this respect it differs from the great majority of 
the diabase dykes of the region, since these as a ride are con- 
tained in remarkably straight fissures. The average width of 
this dyke is perhaps fifty feet. Its strike is S. S. W., so that 
its projection beneath the waters of the lake would intersect 
that of the dominant dyke of thelsland. About fifty feet from 
the margin of this dyke and occupying a jutting point on the S. 
E. extremity of the island is our third dyke — the multiple 
dyke, to which this note has especial reference. 
The rock surface upon which the phenomena to be described 
were observed is at the water's edge. Its area is only about 50 
x20 feet, and it passes beneath the surface of the lake on three 
sides. The waves break over the entire area so that it is per- 
fectly clean and bare. The surface is also smooth and pol- 
ished by glacial action. This surface is formed of the edges 
of over fifty thin sheets of granite mid diabase in alternating 
sequence. These sheets are in vertical attitude. The diabase 
sheets are d} 7 kes cutting the granite. The evidence on this 
point is absolutely conclusive. Although none of the diabase 
sheets exceed 6.1 inches in thickness and many are not over 3 
inches thick, yet there is a very apparent differentiation 
in the texture of the sheets from their middle portions to- 
wards their containing walls. In the middle portions the 
crystalline texture of the rock is readily observable. At the 
margins it is a dense aphanite. The diabase sheets were in 
several instances observed to cut obliquely across the granite 
