Lawson on diabase dykes of the (Rainy lake region. 203 
less hexagonal sections and in slender prisms with rounded termi- 
nations. Water-clear quartz, with inclusions of apatite microlites 
and liquid inclusions with dancing bubbles, forms a considerable 
proportion of the mineral constituents of the rock and is charac- 
terized by having a common orientation for isolated sections 
over a wide area of the microscopic field, as in the micropeg- 
matite structure. A few colorless garnets are also present. The 
rock, such being its characters, may be classed as a uralitic 
quartz diabase. 
At 20 feet from the contact the rock is very similar to that 
at 60 feet but is much less coarse in texture. It differs from the 
latter in mineralogical composition in that fact that there is 
present an abundance of white or colorless garnets, all perfectly 
isotropic. They have a well defined border indicative of a high 
index of refraction and a perceptibly rough surface. Their 
shape is for the most part rounded, or, when rectilinear outlines 
are observable, they are hexagonal sections of the rhombic do- 
dfecahedron. The larger grains have a curved parting which 
ma}' be demarkation lines between different individuals. The 
treatment of the slide with hydrochloric acid cold or hot, leaves 
them unaffected. The occurrence of garnets in basic dykes is 
by no means unique. They are however regarded as a product 
of contact metamorphism within the dyke. Speaking of the 
"Iron District of Lake Si'iperior, " Wadsworth says, "Most of 
the "cliorites" (uralitic diabases) here (at Republic Mt.) contain 
garnets, this mineral being found principally along the edge of 
the intrusion while the centre \vas nearly if not entirely free 
from it. The schist in like manner near the "diorite" frequently 
contains garnets, both rocks appearing to have mutually reacted 
upon each other. "^ The garnets in the Jack Fish lake dyke 
do not appear to be a product of contact metamorphism since 
they are found in the middle of the dyke and very much more 
abundantly at 30 feet from the contact than at 6 feet from it, or 
immediately at the contact, where their presence has not been 
detected. Beyond the abundance of garnets, the dyke at 20 feet 
has the same characters as at 60 feet. The polysomatic structure 
of the augite is j^ronounced. 
' Notes on the Geology of the Iron and Copper Districts of Lake 
Superior. Bill. Mus. Conip. Zool. Harvard, iSSo, jip. 45, 46, 47. 
