BASC03I.] MELAPHYRES AND AUGITE-PORPHYRITES. 77 
rarely some fresh feldspar still remains, while the augite is always 
decomposed, indicates that the decomposition products of augite have 
acted upon fresh feldspar, thus also developing epidote. But the feld- 
spars and augite of the porphyrites under discussion will not account 
for all the epidote and quartz present in them, composing, as they do, 
the amygdules, and in many cases the entire rock. The dip of the 
foliation planes of the basic ernptives indicates a thickness formerly 
much greater. A large amount of erosion of the igneous rocks has 
occurred since they were elevated to their present position. The water 
which percolated through this great thickness of igneous material 
brought with it the lime and alumina. 
It is plain that these processes of epidotization and silicitication took 
place not only while the porphyrites were being elevated, but have con- 
tinued since the cessation of all dynamic? action. The filling of vents 
and cracks by these materials, the fresh, unschistose character of the 
epidote and quartz in vesicles which themselves show the effect of 
squeezing, the presence of granular epidote in the schists and slates, 
all lead to this conclusion. While this is true, there are, on the other 
hand, amygdules of epidote where the fan-shaped radiating crystals 
of epidote have been broken and pulled apart in consonance with the 
alteration in the shape of the vesicle, and the spaces thus formed filled 
with silica. 
The nonvesicular character of the acid ernptives has saved them 
from so extended an epidotization as characterizes the basic ernptives. 
In the case of the amygdaloidal aporhyohtes of the Bigham copper 
mine, the same conditions which obtained with the porphyrites have 
operated to effect with them an extended development of epidote. 
While there is so complete an alteration in mineral constituents, there 
is surprisingly little change in structure. 
In this respect the South Mountain basic eruptives are a contrast to 
similar greenstones of the Menominee and Marquette regions. A com- 
parative study of the greenstones of the two regions shows that the 
Lake Superior rocks, while more altered in structure, possess feld- 
spars less altered than do the South Mountain greenstones. Calcite is 
much more abundant in the former rocks, and epidote in the latter, as a 
secondary product. 
Acessory minerals. — Copper occurs in microscopic quantities in the 
amygdules of the basic eruptives, just as it did in the amygdaloidal 
aporhyolites. This is true only of the amygdaloids from the various 
copper- mine localities described on pages 25-27. At these localities the 
carbonates of copper, malachite and azurite, occur as thin stains. The 
former sometimes forms crystals of considerable size in the vesicular 
cavities (one-fourth inch). A silvery-green asbestos occurs in some 
abundance in quartz veins penetrating the basic eruptives. It is 
plainly a secondary product. A finely divided red hematite is some- 
times quite conspicuous in the amygdules as a cementing material for 
