HISTORY OF CRYSTALLINE ROCKS. 37 
country-rock, and thereby showing the survival of the original chemical conditions of solu- 
tion and deposition after one, and even after two, movements of displacement and disrup- 
tion in the region. 
§ 69. We have thus endeavored in the present essay to bring together, in the first 
place, a number of facts which serve to throw light upon the generation of mineral silicates 
by aqueous processes, especially in later times, subsequent to the formation of the great 
series of crystalline schists, and thereby help to a better understanding of the crenitic 
hypothesis. We have next considered the two plutonic hypotheses as to the origin of 
erystalline rocks, and have discussed the question of stratiform structure in rocks whose 
eruptive character is undisputed. This has led us to consider the process of differentiation 
in such masses through partial crystallization and eliquation, and, farther, to a discussion 
of the possible relations of water to the process. The secular changes which may be 
wrought in igneous masses by aqueous percolation were next discussed, with reference at 
the same time to the crenitic process. From this we were led to a discussion of the strati- 
form structure seen in vein-like masses for which an igneous origin is inadmissible, and 
which, it is maintained, are endogenous deposits of crenitic origin. An account of these, 
as they have been observed in the ancient gneissic rocks of North America, leads to a 
farther consideration of the crenitic hypothesis, alike in relation to the genesis of the 
silicates, carbonates, and non-silicated oxides of the crystalline rocks, and also to the general 
plication of the ancient crystalline strata. 
§ 70. The conclusions from this extended study are, briefly, as follows : The quartzless 
basic material which is supposed to have constituted the primary plutonic mass, and is 
the direct source of basaltic and doleritic rocks, has been subject to modifications from 
three agencies :— 
1. The solvent action of permeating and circulating waters, which, from parts of it, 
have removed alumina, with preponderating proportions of silica and potash,—the 
elements of granitic, trachytic, and gneissic rocks,—and also silicates of alumina and 
other protoxyds, which have been more or less directly the source of the other silicated 
species of the oxyds, and in part also of the carbonates of the crystalline schists and vein- 
ston: s. 
2. The farther action of the same circulating waters in carrying down from the surface, 
alike in the condition of carbonates, formed by subaerial action, and of sulphates and 
chlorids, large portions of calcium, magnesium, sodium, and potassium, all of which, by 
interchange and replacement, have variously modified the composition of the plutonic 
material. 
3. The process of differentiation in portions of the plutonic mass by partial crystalliza- 
tion and eliquation, thereby giving rise to more chrysolitic and more pyroxenic aggregates 
on the one hand, and to more feldspathic aggregates on the other,—a process in which it is 
conceived water may intervene, giving to the material an igneo-aqueous fluidity. 
All of these agencies, it is believed, have, from the earlier ages, been at work on the 
plutonic substratum, causing secular changes alike in the crenitic products derived there- 
from, and in the residual portion, from which have come, and are still derived, the basic 
eruptive rocks. 
