OF ARTS AND SCIENCES. 253 



XX. 



METASOMATIC DEVELOPMENT OF THE COPPER- 

 BEARING ROCKS OF LAKE SUPERIOR. 



By Raphael Pumpelly. 



Presented Jan. 9, 1878. 



Lake Superior is divided into two structurally distinct basins by the 

 long peninsula of Keweenaw Point. The western basin lying between 

 this point and the north-western shore of the lake is a geocllnal trough, 

 bordered, and perhaps bottomed, by an immense development of volcanic 

 rocks, in the form of great beds or jflows. The south-eastern lip of this 

 trough consists of these beds associated with conglomerates and sand- 

 stones, both of which consist essentially of porphyry detritus. These 

 rocks, for which Major Brooks has proposed the name of the Kewee- 

 naw series, have a minimum actual thickness of more than two miles, 

 and a linear extent, in Michigan and Wisconsin alone, of between two 

 and three hundred miles. As a rule, they preserve a very marked 

 uniformity of character throughout this area. Messrs. Foster and 

 Whitney and Owen considered them as eruptions of the Potsdam 

 epoch, basing their opinion chiefly on the external resemblance of the 

 sandstones interbedded with the eruptive rocks to the closely adjacent 

 Potsdam sandstone, and on a supposed conformability between the 

 two series. 



The State Survey, and the private surveys of Major Brooks and 

 myself in Michigan, and later, in Wisconsin, those of Professor Irving, 

 of the Wisconsin State Geological Survey, have discovered abundant 

 evidence of non-conformability, and of the greater age of the Keweenaw 

 series. This is, in fact, much more nearly conformable to the under- 

 lying highly tilted Huronian schists.* 



They are thus the product of the earliest eruption of basaltic rocks 



* Both these eruptive rocks and the sandstones have been referred to the 

 Triassic period by C. T. Jackson, Jules Marcou, and R. Bell. 



