442 COPPER BEARING ROCKS OF LAKE SUPERIOR. 
tance, I add three cross-sections of the gorge (constructed from Mr. Sweet’s descrip- 
tions), whose southwest wall is represented in Fig. 10, with the design of bringing out 
more distinctly the relations of the exposures here seen. 
we 
Fic. 37.—Cross-sections of gorge of Black River, Douglas County, Wisconsin. I, at about 4 of Fig. 10; I, 
at 5o0f Fig. 10; III,at 7 of Fig. 10. Scale natural, 200 feet to the inch. 
These cuts will serve to make plainer Mr. Sweet’s reading of the structure at this 
point. If the reading is correct, of which I have no doubt, it is evident not only that 
we have to do here with an unconformable contact, but also that the newer sandstone 
is here deposited within the sinuosities of the old coast-line. 
NGO MEE 2-1. 
(Page 316.) 
DIABASE-PORPHYRITE OF THE GREAT PALISADES. 
The compact diabase-porphyrite of the layer immediately beneath the quartz- 
porphyry of the Palisades contains 47.9 per cent. of silica. 
NOM E22; 
(Page 350.) 
GEOLOGICAL POSITION OF THE COPPER-BEARING ROCKS. 
The question of the equivalency of the Copper-Bearing Rocks with geological 
formations of other regions is not directly touched upon in the discussions of Chapter 
VIII, in which I have contented myself with an attempt to demonstrate their complete 
distinctness, structurally, from any of the immediately associated formations and their 
consequent right to a distinct name, of at least local significance. I have shown that 
they are not Huronian, and that at the same time they are separated by a great uncon- 
formity from the overlying fossiliferous Cambrian sandstones, with which they come in 
contact. Heretofore most of the differences of opinion in this connection have been upon 
these very points. A number of writers, and especially Messrs. Foster and Whitney, 
maintaining the unity of the Keweenaw Series and the Cambrian sandstones above re- 
ferred to, and maintaining at the same time the equivalency of these sandstones with the 
so-called Potsdam of New York, have been led to include the Copper-Bearing Rocks also 
with the Potsdam sandstone. On the other hand, those who have maintained the pre- 
Potsdam age of the Copper-Bearing Rocks, including the writer of this volume, accept- 
ing the reference of the overlying sandstones to the Potsdam of New York, have 
