1864.] Geology and Paleontology. 123 
feldspars as albite and oligoclase, would be thus a sand-compound of 
quartz and the less destructible potash-feldspar. The mechanical 
agency might be air or water; if the latter, there would be found sus- 
pended in it a fine clay, consisting mostly of the partially decomposed 
soda-feldspar. Now this process of destruction is evidently one 
which must go on in the wearing away of rocks by aqueous agency—just 
the agency which is the most important in such inquiries as the pre- 
sent, because most intimately associated with the deposition of the 
sedimentary rocks. It is easy to see how, by such partial destruction 
of the primal rocks, quartz is for the most part wanting in those 
which contain a large proportion of alumina, while it is abundant in 
those in which potash-feldspar predominates. So long as this decom- 
position of alkaliné silicates is sub-aérial, both the silica and alkali 
are removed in a soluble form. But as immense quantities of unde- 
composed fragments of the primal rock are detached by atmospheric 
causes, and carried down to the sea, which acts upon these with more 
power even than upon the surfaces of the rock directly exposed to its 
influence, the process is often sub-marine and beneath the sea-bottom 
in the midst of sediments containing the carbonates of lime and mag- 
nesia. When the silicate of soda is, under such circumstances, set 
free, it reacts upon those earthy carbonates, or upon the chlorides in the 
sea-water, and forms in either case_a soluble soda-salt, and insoluble 
silicates of lime and magnesia. | 
The sources of the carbonates of lime and magnesia in sedimentary 
strata, have been the decomposition of silicates containing those bases, 
such as lime-feldspar and pyroxene, and the action of the alkaline 
carbonates formed by the decomposition of feldspars upon the 
chlorides of lime and magnesia originally present in sea-water, but 
which have been by this process in subsequent ages, in great part 
replaced, by the resulting chloride of sodium, A curious result as 
showing the sea to have not been originally as salt in the primeval era 
as it is at the present epoch, and giving, if the total could be ascer- 
tained, the clay or aluminous silicate of the earth’s crust, as a measure 
of not only the quantity of salt added to the primitive ocean, but of 
the amount of the carbonic acid removed from the air, and of the 
carbonates of lime and magnesia precipitated. As the coarser sedi- 
ments in which quartz and orthoclase prevail are permeable to 
infiltrating waters, their soda, lime, magnesia, and oxide of iron will 
be gradually removed, leaving at last only silica, alumina, and potash 
—the elements of granite. On the other hand, the finer marls and 
clays, resisting the penetration of water, will retain their soda, lime, 
magnesia, and oxide of iron, and having an excess of alumina, will by 
their metamorphism give rise to basic lime and soda-feldspars, to 
pyroxene and homblende—the elements of diorites and dolerites. In 
this way, the long-continued operation of chemical and mechanical 
causes would naturally tend to divide all the crystalline silico-alumi- 
nous rocks of the earth’s crust into two types, exactly corresponding 
to the two classes of so-called igneous rocks, the trachytic and 
pyroxenic, which geologists have supposed to have been derived from 
two distinct imaginary magmas beneath it. When, however, ordinary 
