ORIGIN OF CRYSTALLINE ROCKS. 487 
either alone, or in combination with silicates of alumina and alka- 
lies. They include the following as their chief constituent mineral 
species :— pyroxene, hornblende, olivine, serpentine, talc, chlorite, 
epidote, garnet and triclinic feldspars such as labradorite. The 
great types of this second class are not less well defined than 
the first, and consist of pyroxenic and hornblendic rocks, passing 
into diorites, diabases, ophiolites and talcose, chloritic and epi- 
dotic rocks. Intermediate varieties resulting from the associa- 
tion of the minerals of this class with those of the first, and also 
with the materials of non-silicated rocks, such as limestones and 
dolomites, show an occasional blending of the conditions under 
. which these various types of rocks were formed. 
The distinctions just drawn between the two great divisions of sil- 
icated rocks, are not confined to stratified deposits, but are equally 
well marked in eruptive and unstratified masses, among which the 
first type is represented by trachytes and granites, and the second, 
by dolerites and diorites. This fundamental difference between 
acid and basic rocks, as the two classes are called, finds its ex- 
pression in the theories of Phillips, Durocher and Bunsen, who 
have deduced all silicated rocks from two supposed layers of molt- 
en matter within the earth’s crust, consisting respectively of acid 
and basic mixtures ; the trachytic and pyroxenic magmas of Bunsen. 
From these, by a process of partial crystallization and eliquation, 
or by commingling in various proportions, those eruptive rocks 
which depart more or less from the normal types, are supposed by 
the theorists of this school to be generated.* The doctrine that 
these eruptive rocks are not derived directly from a hitherto uncon- 
gealed nucleus, but are softened and crystallized sediments, in fact 
that the whole of the rocks at present known to us have at one 
time been aqueous deposits, has, however, found its advocates. In 
Support of this view, I have endeavored to show that the natural 
result of forces constantly in operation, tends to resolve the various 
igneous rocks into two classes of sediments, in which the two types 
are, to a great extent, preserved. The mechanical and chemical 
agencies which transform the crystalline rocks into sediments, sepa- 
rate these more or less completely into coarse, sandy, permeable 
beds on the one hand, and fine clayey impervious muds on the other. 
The action of infiltrating atmospheric waters on the first and more 
silicious strata, removes from themi lime, magnesia, iron-oxyd and 
* Hunt on Some Points of Chemical Geology, Quar. Jour. Geol. Soc., XV, 489. 
