114 J. D. Dana—Resulis of the Harth’s Contraction. 
Mallet’s theory presents us with a true cause; but what are 
the limits of its action it is very difficult to decide. It relieves 
the theory of local fire-seas as derivative from the old viscous 
layer of the chief objection urged against it—that such isolated 
fire-regions could not long exist surrounded by cooled rock; 
for, if inadequate to make great undercrust fire-seas one or 
more thousand miles in length, like the Appalachian, the cause 
may be sufficient through the generated heat to keep the old 
fire-seas in prolonged existence. 
2. What are igneous rocks?—From the preceding discussion 
we derive an idea of the distinction between eruptive and meta- 
morphic rocks. Since the larger part of eruptive rocks have 
come from the infra-Archzan region—either the true crust, or 
the fire-seas within or below it—they are igneous in all their 
history, and in no sense metamorphosed sediments, whether 
derived from a second fusion of the rocks where they originated 
n these latter igneous rocks and the metamorphic 
there may be indefinite gradations, as claimed by Hunt. But 
if our reasonings are right, the great part of igneous rocks can 
be proved to have had no such supercrust origin. The argu- 
ment from the presence of moisture or of hydrous minerals i0 
such rocks in favor of their origin from the fusion of sediments 
has been shown to be invalid. 
_8. Source of the ejecting force.—W hen the fractures of the crust 
giving exit to fissure eruptions are a direct sequence to a long 
continued subsidence—as, for example, in the case of the 
Triassico-Jurassic eruptions of the Atlantic border,—there ca 
ardly be a doubt that the lateral pressure causing the subsi- 
dence contributed also to the ejection of the plastic rock from 
beneath. And as the great fissurings of the crust are in all 
cases incidents in the working of lateral pressure, it is unsafe to 
deny that this cause has not aided in the great majority of 
eruptions in non-voleanic regions. 
Another cause of ejection appealed to is pressure from the 
vapors imprisoned in the regions of fused rock. It must have 
often given efficient aid. But such vapors may not exist to the 
extent sometimes supposed about the deep ‘sources of the 
material of fissure eruptions. Non-voleanic igneous rocks are 
