128 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
ad 
the mass. Magnetite, being the least destructible constituent of the basic 
rocks, is their most common representative. In the case of one belt of sand- 
stone—the Nonesuch belt—which lies far above any basic flow, the basic 
detritus becomes unusually abundant, at times almost wholly excluding the 
usual acidic detritus. 
On the other hand, there are sandstone beds, as those of Black and 
Nipigon bays, on the north side of Lake Superior, in which a large pro- 
portion of the constituent particles have been derived from gneiss and 
granite, and in these cases the quartz particles are unusually abundant. In 
the same region some of the sandstone beds are largely charged with mag- 
nesian and calcareous carbonates, which are even concentrated at times 
in thin seams of limestone, a thing unknown elsewhere in the Keweenaw 
Series. 
As in the conglomerates, so also in the sandstone beds, secondary 
calcite, chlorite and epidote, and even copper, are occasionally to be met 
with. 
The following tabulation includes enough examples to illustrate the 
different varieties of sandstone which occur in the Keweenaw Series. A 
number of other thin sections will be found described in connection with 
the local details of Chapters VI and VII. 
Tabulation of microscopic observations upon sandstones of the Keweenaw Series. 
a 
g g 
a 3 E 
Be Place. ES = Macrasounts descrip- Microscopic descriptions. 
24 8 ails leis ions 
7a 5 sS E i) 
a 5 o| 9° q 
DM [<J m| a i=] 
1943..| Eagle Harbor, Ke- NW 6 | 58 |30W.| Dark purplish-red; | Subangular, or only very slightly 
weenaw Point, medium-grained; rounded fragments, worn from the 
Michigan. firm; gives slight matrix and porphyritic ingredients 
effervescence with of a quartziferous porphyry, along 
hydrochloric acid. with fragments plainly derived from 
an augite-syenite, make up most of 
the rock. The fragments referred to 
the porphyritic ingredients of a 
quartziferous porphyry are almost 
wholly feldspars, both orthoclase and 
oligoclase, quartz particles being al- 
most wholly absent. The particles re- 
ferred to augite-syenite are pieces of 
feldspar affected by the peculiar 
