FORMER VIEWS AS TO NATURE OF THE FELSITIC ROCKS. 11 
a metamorphic origin for them. Pumpelly and Marvine first recognized 
definitely that the amygdaloids are but the upper portions of the beds 
of “trap,” and the distinction between the true vesicular amygdaloids and 
the non-vesicular pseud-amygdaloids, which distinction the latter subse- 
quently made still plainer by his microscopic investigations, to which we 
are indebted for our first knowledge as to the nature of the so-called 
“traps.” Marvine’s conclusions from his examination of the beds of the 
Eagle River section of Keweenaw Point, made before the use of the micro- 
scope in the study of these rocks, deserve mention as containing the first 
plainly and thoroughly worked-out argument, from structural characters 
alone, in favor of the lava-flow origin of the “traps,” and of the connection 
with the trap beds of the amygdaloids. The results of the work of the 
Wisconsin Survey, as given in the third volume of the Geology of Wis- 
consin, by Chamberlin, Sweet, Strong, and myself, fully sustained the con- 
clusions of Pumpelly and Marvine. 
The most recent writers on the copper rocks of Lake Superior have 
been N. H. Winchell and Wadsworth. Winchell’s views as to the sedi- 
mentary and metamorphic origin of the amygdaloids of the Minnesota 
coast have already been mentioned. In the same category he appears to 
include a large part of the bedded traps, the more highly crystalline kinds 
only being regarded as of eruptive origin.” Wadsworth’s paper maintains 
the lava-flow origin of the traps, although no points are advanced in favor 
of this conclusion that had not already been fully covered by Pumpelly 
and Marvine. It is worthy of note that while maintaining that Foster and 
Whitney are the only authors who have written correctly on Lake Supe- 
rior geology, Wadsworth should yet depart from them in several very im- 
portant points—such as the origin of the conglomerates—and this without 
a word of comment. 
The reddish, acid, eruptive rocks, which I describe in subsequent 
chapters as constituting so important a feature of the copper-bearing series, 
have been almost wholly overlooked heretofore. A number of writers have 
1Metasomatic Development of the Copper-Bearing Rocks of Lake Superior. Proc. Am. Acad. 
Sci., 1878, XIII, pp. 253-309. 
2Ninth Annual Report of the Natural History and Geological Survey of Minnesota, 1880, Prelimi- 
nary list of rocks, pp. 10-114, 
