28 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
plagioclase class, hornblende occurring only very rarely and then always as 
an alteration-product. Pumpelly has heretofore recognized the three types 
of diabase, melaphyr, and gabbro as characterizing the Keweenaw Point 
district, and after a study in the field of the entire North Shore, and the 
examination of large numbers of specimens from all portions of the Lake 
Superior basin, I am able, though having discovered a number of interest- 
ing new varieties, to add only two kinds deserving of distinct names, viz. 
diabase-porphyrite and anorthite-rock, and these are, after all, so closely 
allied to the others as to be hardly more than varieties. Indeed, the three 
kinds first named grade into each other in the field, and are themselves 
merely phases of an ancient class of rocks, for which the science has as 
yet established no common name, but which are the old equivalents of the 
post-Cretaceous basalts. 
The diabase is a plagioclase-augite rock, with or without olivine, and 
without unindividualized base; the melaphyr carries more or less of this 
base with olivine, and is, throughout the Lake Superior region, everywhere 
characterized by the presence of relatively large individuals of augite, in- 
cluding numbers of minute plagioclases; while the gabbro has part or all of 
the augitic ingredient as diallage, is orthoclase-bearing or not, and is either 
olivine-bearing or not. The diabase-porphyrite is an olivine-free diabase, 
with a strong porphyritic development and a more or less thoroughly unin- 
dividualized base; and the anorthite-rock is merely a coarse gabbro, in which 
all ingredients but the feldspar are wanting. The nomenclature adopted for 
these rocks is Rosenbusch’s. There are numbers of peculiar phases of the 
three kinds named, due to amygdaloidal and compact conditions, relative 
abundance of the several ingredients, coarseness of grain, the presence of 
unusual constituents, and, especially, internal molecular rearrangements. 
But the same types constantly recur in the circuit of the lake, and there 
are only one or two subordinate varieties that have not been seen again and 
again, and at points widely removed from each other. Some arrangement 
of the kinds as to horizon also is to be observed. One of the most inter- 
esting results of my work is the finding of gradation phases, not only 
between all the kinds named, but from the most basic kinds, with less than 
