22 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
on the line of the latter stream being almost continuous from the head of 
Bladder Lake to the contact with the overlying fine-grained diabases—that 
there can be no doubt as to its occupying the whole of the width indicated. 
It is in this vicinity that the coarse gabbro appears to occupy a position of 
unconformity to the underlying Huronian slates.’ 
A few miles west of the Brunschweiler the coarse gabbro exposures 
begin to have scattered among them others of the usual fine-grained dia- 
bases, and beyond range 6 west the gabbro is no longer met with. It thus 
terminates to the westward much as it does to the eastward. 
The entire length of the belt occupied by these coarse gabbros is some 
40 miles, its width ranging from 14 miles to 45 miles. Three principal 
phases of the rock occur, viz: orthoclase-free gabbro, orthoclase-gabbro 
and hornblende-gabbro. The first of these is bluish-gray to black, and 
ranges from below medium-grained to very coarse-grained, the crystals 
reaching several inches in length. The usual constituents are a very basic 
feldspar—which, judging from the angles, is commonly near anorthite—dial- 
lagic augite or true diallage, titaniferous magnetite, and olivine. There 
are sundry alteration-products often present, but the rock on the whole is a 
very fresh one. The second variety of gabbro is found especially in the 
more northern portions of the gabbro belt, forming apparently continuous 
bands, which have in some cases been traced for a number of miles. It 
is a red- and black-mottled, or red-, black- and gray-mottled rock, and com- 
monly quite coarse-grained, though never reaching the extreme degree of 
coarseness sometimes shown by the orthoclase-gabbro of the Saint Louis. 
It is also marked by very abundant and noticeably large grains of titan- 
iferous magnetite. Oligoclase and orthoclase, both much altered and red- 
stained, diallage commonly largely altered to greenish uralite, and beyond 
this to chlorite, titaniferous magnetite and very abundant apatite are the 
ingredients. 
Still a third variety, forming a belt or belts some 15 miles in length, 
near the junction with the Huronian, belongs to what I have called horn- 
blende-gabbro in Chapter III. This rock is peculiar in containing much 
deep-brown intensely dichroic hornblende, which, however, I think can be 
1See pp. 144, 156. 
i 
