62 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
which it was applied makes up the lower portion of the bed whose upper or 
vesicular portion is the extraordinarily scoriaceous amygdaloid so well 
known on Keweenaw Point as the ‘‘ashbed.” ‘The ‘ordinary type” is alone 
considered under the present heading, the “‘ashbed type,” according to my 
observations, being more closely related to the diabase-porphyrites. While 
the ordinary type diabases, with their amygdaloids, are the most com- 
mon kinds on Keweenaw Point, they do not appear to me, when we look 
the whole ground over, to be any more common than the fine-grained oliv- 
initic kinds, and not much more so than those of the ashbed type. 
The ordinary type diabases make up relatively thin flows, which 
are almost invariably furnished with vesicular or amygdaloidal upper por- 
tions. These vesicular upper portions have always undergone great in- 
ternal changes, both in connection with the deposition of minerals in ves- 
icles, and in theformation of pseud-amygdules, or minerals replacing primary 
constituents in such a way as to present macroscopically very much the 
appearance of the true vesicular fillings. The latter change, with others, 
has usually affected also the lower non-vesicular portions of the beds in 
greater or less degree. So general are these alterations that an account of 
these rocks has to be taken up with the internal changes that they have un- 
dergone more than with the nature and arrangement of the original constit- 
uents. Professor Pumpelly’s studies have been so exhaustive that I can do 
no better than quote largely from him in this connection. His microscopic © 
studies were chiefly made on specimens from the Eagle River section of 
Keweenaw Point (that is, on beds lying betwgen the Great Conglomerate 
and the base of the Greenstone Group), and on specimens from the lower 
horizons at Portage Lake. I have examined, in addition, a large number 
of specimens from still higher and lower horizons on Keweenaw Point, 
and from all other parts of the extent of the formation. For the most part, 
my work has merely served to extend the geographical range to which most 
of his conclusions are applicable. 
“Externally”, according to Pumpelly, ‘‘the different varieties of these 
diabases are dark in shade, varying from almost black in unaltered speci- 
mens to dark-green or dark-brown, or varying, minutely subdivided mix- 
tures of these colors, according to the relative proportions of chlorite and 
ferric oxide among the decomposition products. They vary in texture from 
