442 



COPPER BEARING EOCKS 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. 



-Diaiait 



'\'-\ijriJv K 

 jHrcit/on or' thi Sir-ike) 



\2p''-f$*$f**'*£* * 





Fig. 37.— Cross-sections of gorge of Black Eiver, Douglas County, Wisconsin. I, at about 4 of Fig. 10; II, 

 at 5 of 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. 



NOTE 21. 



(Page 316.) 



DIABASE-PORPHYRITE OF THE GREAT PALISADES. 



The compact diabase-porphyrite of the layer immediately beneath the quartz- 

 porphyry of the Talisades contains 47.9 per cent, of silica. 



NOTE 22. 



(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 Hurouian, 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 



