LITHOLOGY. 145 
for their elucidation. Before proceeding to special descriptions, we will 
consider their position among rocks of this class, and what they have in 
common which isolates them as a well defined group. 
The first great division of eruptive masses is into basic and acidic 
rocks. The first are characterized by a content of silica lower than sixty 
per cent. and basaltic rocks are typical of them. The acidic rocks are 
lighter in gravity, contain more silica, and trachyte and quartz porphyry 
are typical examples. These two great classes were instituted by Bun- 
sen, and were by him supposed to represent two great zones or layers of 
fused matter, which originally underlaid the crust of the earth, separated 
from one another by their specific gravities. He supposed that at differ- 
ent periods of the earth’s history matter was erupted from different 
layers of this mass beneath, and thus we have basic and acidic eruptive 
rocks, and also intermediate varieties, which represent the intermediate 
zone of the molten matter. By further study, this once so plausible 
theory has been mostly done away with. It is now considered that the 
shifting of sediments, and vast movements in the earth’s crust, may ren- 
der fluid, or plastic, beds of previously solid matter; and thus, at different 
times and at different places, the crust of the earth may be underlaid by 
beds of molten material, which may be composed of the unstratified, 
original matter of the earth’s exterior, or of sediments of variable com- 
position; and through rifts in the superficial beds we may hence obtain 
most diverse eruptive rocks. The two great divisions of rocks are, 
however, maintained, and we have in our state numerous representatives 
of both classes. It is the basic division that we are now to consider. 
Besides this great chemical distinction, there are certain geological 
conditions which sub-divide eruptive rocks. It is found that those rocks 
which have been erupted in the later periods of the earth’s history pos- 
sess certain characteristics that distinguish them from others. They 
show in general the effect of more rapid cooling, and are either com- 
posed largely of glassy matters, or enclose more or less glassy material 
in their masses. They often possess certain types of microscopic struct- 
ure indicative of free movements in the plastic mass. Of this class the 
basalts are typical among the basic rocks. The older rocks, on the con- 
trary, are completely crystalline, and in their microscopic structure are 
_ entirely granitic. These observations have led the German lithologists 
VOL. IV. 19 
