36 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
III. GLASsy, known only as subordinate, vitreous modifications of diabase-porphyrite 
not deserving of a special name. 
TV. GRANULAR PLAGIOCLASE-DIALLAGE ROCKS. 
a. Plagioclase-diallage = gabbro. 
b. Plagioclase-diallage-olivine = olivine-gabbro. 
For the present work I have studied some eight hundred sections made 
from specimens collected from all parts of the entire extent of the Kewee- 
naw Series, including the typical localities of Pumpelly’s Michigan descrip- 
tions. I have also had in my hands the original sections made by Pumpelly 
for his study of the Wisconsin rocks. Although my microscopic work has 
been extended over so much wider a field than his—covering, as it has, not 
only the already partly studied districts of Keweenaw Point and northern 
Wisconsin, but also the hitherto wholly unstudied rocks of the north and 
east shores of the lake, of the Bohemian Range on Keweenaw Point, of the 
Ontonagon, Porcupine Mountain, and “South Range” districts of western 
Michigan, of the Snake and Kettle River region of Minnesota, and of Mi- 
chipicoten Island—its chief result, so far as the basic rocks are concerned, 
has been to extend the application of Pumpelly’s conclusions over the entire 
spread of the Keweenaw Series. I have been able to recognize a number of 
new varieties of his gabbro, melaphyr and diabase, and two new kinds— 
anorthite-rock and diabase-porphyrite—not to be included under any of 
these three, though closely related to them, and have found that there is a 
large class of rocks which are intermediate in point of acidity. 
The classification given below of the basic original rocks of the Ke- 
weenaw Series is based not only on microscopic differences and resem- 
blances, but also upon the characters and relations of the several kinds as 
seen in the mass, and on their prominence and persistence in the field. For 
instance, the first group given of the coarser grained rocks is made to in- 
clude orthoclase-free gabbro and diabase, olivine-diabase, and olivine-gab- 
bro,’ because all of these in the hand specimens bear the closest resemblance 
to each other, and in the field are seen to constitute parts of the same con- 
tinuous mass or bed, forming together one of the most prominently occur- 
ring types. 
1The distinction between diallage and augite is a valueless one, since not only are both often 
found in the same section, but every gradation is found in the rocks of this class from augite to diallage. 
