THE 
AMERICAN GEOLOGIST 
Vol. XIII. MAY, 1894. No. 5. 
A MULTIPLE DIABASE DYKE. 
By Andrew G. L.wvsox, Berkeley, Gal. 
(Plate VII.) 
On a small island opposite the mouth of White Graved river 
on the northeast coast of lake Superior there are displayed 
certain phenomena of a somewhat unique character. The is- 
land is not more than about 200 yards in length and is per- 
haps 100 yards wide. It is composed of common Laurentian 
granite of a reddish color. The constituent minerals are red 
and white feldspars, both orthoclase and plagioclase, quartz 
in abundance, biotite more or less chloritized, hornblende and 
a little epidote. The rock is of medium texture and has the 
usual granular structure. There are occasional large pheno- 
crysts of orthoclase, attaining a diameter of half an inch. The 
rock is entirely devoid of any trace of foliation. 
The island is traversed by three diabase dykes. These are 
part of a great system of similar dykes which traverse the 
region. The dominant dyke of the three has a strike about 
S. S. E., or parallel to the general trend of the coast. It fills 
a (dean cut fissure in the granite about 75 feet wide, with ver- 
tical, straight parallel walls. This dyke has in a large meas- 
ure conditioned the topography of the island. The dyke has 
disintegrated more rapidly than the granite, so that a longi- 
tudinal trench has been formed, dividing the island into two 
parallel hummocky ridges. At each end of the dyke is a cove, 
forming a sharp indentation in the shore contour. The island 
has an elevation above the lake of 50 or 60 feet. Its longitu- 
