308 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
and cuts obliquely across the strike. The fine-grained rock shows in the 
thin section a mixture of rounded augite grains and tabular oligoclases, 
with some magnetite. It is close to the so-called ashbed-diabases. At B 
of the map the rock exposed is a reddish-weathering, medium-grained ortho- 
clase-gabbro, with all the characteristics of the orthoclase-gabbros, as de- 
scribed in Chapter III, strongly developed, viz: presence of much ortho- 
Fic. 19.—Section on Beaver River, Minnesota. 
clase; the plagioclase oligoclase; much secondary quartz; much coarse 
titaniferous magnetite; diallage and augite with a viriditic and ocherous 
alteration, and large apatite crystals. The rock is strongly contrasted with 
the neighboring olivine-gabbros. Similar rocks show at K and J and again 
at D. At the latter point a fine-grained black diabase with augite filling 
the interspaces of the labradorites, and not in rounded grains, and con- 
taining small olivines, is involved with the orthoclase-gabbro, coarse anor- 
thite-rock and pink felsite in the most confusing manner. The same con- 
fused appearance is met with at E, where fine-grained olivine-diabase 
has with it red granitic porphyry in a thin seam, and coarse anorthite-rock 
in irregular masses. At F an excessively fine-grained ashbed-like diabase 
is intermingled with granitic porphyry. 
As indicated above, it is very difficult to reduce these involved expos- 
ures to an intelligible order. The prevailing black rock of the vicinity is 
plainly enough one of the usual lakeward-dipping flows. The quartz- 
porphyry of Cedar Island seems as plainly part of a similar flow. Possibly 
all of the acid rocks belong to one great belt overlying and in part involved 
