bascom] SUMMARY. 81 
called ash occurs as a light-green schist. It is not banded, but pre- 
sents under the microscope the same structureless character as the 
rocks abovp described. It consists of actinolite blades and needles, 
epidote granules, and some chlorite, magnetite, and leucoxene. 
SUMMARY. 
The basic igneous rocks display but little variety of structure or 
mineral constitution. The former is that common to porphyrites and 
melaphyres — the microophitic — and in spite of great alteration in the 
mineral constituents of the rocks it still remains a marked structure. 
Shearing obscures it, but in the extreme form of the sheared porphy- 
rite — the slate — it is still discernible. 
The formation of chlorite and actinolite tends to confuse outlines; 
hence some of the. slates in which there is not much of these minerals 
preserve their original structure better than the chlorite-schists. The 
original mineral constituents, i)lagioclase, feldspar, augite, and olivine, 
have almost completely disappeared. No augite remains; olivine crys- 
tals are well preserved in outline, and sometimes a core of the original 
mineral remains. There is considerable feldspar still unaltered. It is 
always striated, but the crystals are too small to allow of an accurate 
determination of their character. That they belong to the basic end 
of the series is shown by their extended alteration to epidote, and by 
the chemical analyses of the rocks. 
The vesicular character of these rocks has aided in the extended 
replacement of their original minerals, and the amygdules are an index 
of the character of that replacement. The presence of vesicles has also, 
doubtless, been a factor in preserving the internal structure of the rock 
in spite of dynamic action. Silicification, epidotization, and chloritiza- 
tion are the processes of alteration which have been most active. 
The source of material is twofold — from the rocks in which these pro- 
cesses have been described, and from the overlying rocks which have 
been removed by erosion. 
Bull. 13G 6 
