52 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
miles back of Grand Marais, I obtained, some years since, over ten per 
cent. of titanic acid. 
The augitic ingredient of the orthoclase-gabbros, like that of the more 
ordinary orthoclase-free gabbros, runs from augite to diallage, and in some 
sections the two, quite distinct from each other, are present. In one pecu- 
liar class of the orthoclase-gabbros met with at a number of points in the 
country back from the Minnesota coast, and especially in what I have called 
the “Duluth” and “Lester River” Groups, the augite is in long radiating 
blades, which are very pronounced macroscopically, and which in the thin 
section are seen to be twinned. By far the most of the sections, however, 
show a true diallage, much more highly fibrous than in the orthoclase-free 
gabbros, and nearly always more or less extensively changed to uralite. This 
uralitic alteration, which, in cross section, shows the true hornblende cleav- 
age, is finely displayed in the very coarse gabbro of Duluth. 
Among the accessory ingredients apatite is always very prominent, in 
crystals which at times reach an eighth of an inch in length. Chlorite, as 
an alteration of augite, or more commonly of uralite, and biotite are often 
present, and iron and copper sulphides are often to be met with in small 
particles. 
In the mass the orthoclase-gabbros do not present any features differ- 
ent from those of the orthoclase-free kinds, like which they occur chiefly in 
heavy flows, without amygdaloids. They may also occur as intersecting 
masses, but this needs proof; that is to say, it is not always possible to be 
certain as to the structural relations of rocks met with in isolated exposures 
in the woods. 
As instances of the occurrence of orthoclase-bearing gabbros may be 
mentioned the coarse syenite-like rock of the Bohemian Mountain, on the 
north shore of Lac la Belle, Keweenaw Point; the bed numbered 94 by 
Marvine in his description of the Eagle River section of Keweenaw Point;* 
the rocks of a belt or belts running for many miles through the Bad River 
region of Wisconsin;* and the rocks exposed on the Aminicon River, in 
1 Geological Survey of Michigan, Vol. I, Part II, p. 134. 
? Geology of Wisconsin, Vol. III, p. 170. 
