bascom.] SUMMARY OF CONCLUSIONS. 85 
Chief among the processes of alteration which have been going on 
since that time is devitrification. Out of glassy and lithoidal rhyolites 
devitrification has been developing aporhyolites. This process consists 
in the replacement of whatever glassy base was present in the original 
rock by a uniform quartz -feldspar mosaic. Sometimes the alteration is 
carried still further, and the original spherulitic crystallization is also 
replaced by this secondary granular crystallization. Rarely, if ever, 
does the secondary crystallization completely obscure the former char- 
acter of the rock, while often all of the structures peculiar to a fresh 
glassy lava are retained. 
The other processes of alteration, which all of the original rock 
types have undergone in some slight degree, and some of them in an 
extreme degree, are the processes of sericitization and epidotization. 
The former process, has been a chief factor in the development of slates 
from massive porphyrites and aporhyolites. 
These three processes of alteration — devitrification, sericitization, 
and epidotization — represent statical metamorphisin, dynamic meta- 
morphism, and weathering, respectively. 
BASIC IGNEOUS HOCKS. 
In contrast with the acid rocks, the basic rocks have their original 
constituents so completely replaced that it is not easy to determine the 
original type or types. The original constituents were plagioclase, pyr- 
oxene, olivine, ilmenite, and magnetite. The original structures were 
the microophitic, the porphyritic (inconspicuous and mostly confined to 
the olivine-bearing type), and the amygdaloidal, a universal structure. 
In view of these constituents and structures, these rocks have been 
regarded as members of the diabase or augite-porphyrite group and of 
the olivine-diabase or melaphyre group. The augite-porphyrites resem- 
ble the spilites in their scanty porphyritic crystals, their ever-present 
inclination to the amygdaloidal structure, and their susceptibility to 
weathering. 
The character of the alteration which has taken place in these mela- 
phyres and spilites varies with the amount of shearing which has 
occurred. Where shearing has been a factor in the alteration, chlorite, 
actinolite, and quartz replace the pyroxene and plagioclase. 
In the absence of shearing, epidote has resulted from the interaction 
of plagioclase and pyroxene. Olivine has altered to epidote, serpen- 
tine, and iron oxide ; ilmenite has altered to leucoxene, and magnetite to 
hematite, when altered at all. In the absence of shearing the ophitic 
structure is preserved in outline, although sometimes the micropoiki- 
litic is added to it through the infiltration of silica; with the presence 
of shearing the development of chlorite and actinolite has obliterated 
the original structure and producer! the schistosity characteristic of 
chlonte-actinolite rocks. 
