FORMER VIEWS AS TO ORIGIN OF CONGLOMERATES. 4 
drawn upon liberally for accounts of those regions I have not myself seen, 
more especially of the Nipigon Lake basin, and for data for mapping 
away from the lake shore, although I have here again departed from the au- 
thor in some important points. The descriptions and maps of Messrs. 
Pumpelly and Marvine, in the first volume of the Geological Survey of 
Michigan, have furnished a large part of the material from which I have 
made up my account of the structure of Keweenaw Point. Pumpelly’s 
microscopic descriptions of the “trap” and amygdaloid of Keweenaw 
Point have formed the basis for my own microscopic studies, whose 
results I give especially in Chapter III, in which I have occasion to quote 
freely from his memoirs above mentioned. The general reports on the 
geology of Northern Wisconsin included in the third volume of the Geology 
of Wisconsin, by Messrs. Chamberlin, Strong, Sweet, and myself, have sup- 
plied all of the material for the description of that region which I shall 
give in Chapter VI. Upon N. H. Winchell’s reports I have depended for a 
few locations of rocks in the country back of the Minnesota coast and 
along the national boundary line. 
It is desirable that I should give here a brief account of the different 
views that have been held as to the origin and relations of the copper-bear- 
ing rocks, although this will involve reference to a number of points whose 
explanation must be deferred to subsequent pages. This series of rocks 
is described by all writers as made up of reddish sandstones, conglomerates, 
“traps,” and amygdaloids, while a few of the authorities recognize also the 
local development of reddish felsitic or ‘jasper”-like rocks. As to the ori- 
gin of these several kinds of rocks, however, as to their mutual structural 
relations, and as to their position in the geological scale, there has been 
the widest divergence of views. 
The sandstones of the series appear to have been taken by all as of the 
usual sedimentary origin, but the conglomerates into which they shade have 
been regarded both as eruptive and as sedimentary. Messrs. Foster and 
Whitney, whose work, having been for thirty years the most widely recog- 
nized authority upon Lake Superior geology, may appropriately be men- 
