OF VOLCANIC ROCKS. 75 
logically absurd to assume the non-existence of a foundation consisting of rocks differ- 
ing in origin from sedimentary rocks ; and the conclusion is inevitable, that it must be 
composed of rocks which were generated by the solidification of such masses as formed 
part of the primordial substance of the globe, next to the surface. Probabilities 
accumulate to point towards the assumption that very silicious granite composes that 
foundation, together with such gneissoid rocks as must be supposed to have been formed 
in an incipient sea of very high temperature, and under the pressure of the superin- 
cumbent atmosphere of aqueous vapors, at a time when the cooling of the globe had 
advanced far enough to allow of their first condensation, and when pressure, water, 
and heat could codperate to produce on its surface similar effects to those which the 
same agencies are supposed to have wrought in later periods at an ever-growing dis- 
tance from it. The most probable mode of these ancient processes has been pointed 
out by Daubrée. The true nature of the foundation rock can, of course, not be 
positively known, but must remain a matter of conjecture. No solution of this 
problem will, however, be more satisfactory than that which is based on the hypoth- 
esis of Sartorius, because it is in harmony with all the phenomena of vulcanism. 
The probabilities in favor of a granitic foundation are, therefore, very great, while 
no valid objection has yet been raised against its having the nature indicated. 
It is further argued that, the position of granite being always in the midst of 
sedimentary, or of such foliated crystalline rocks as are connected with the former by 
gradual passage, and often conspicuously above such rocks, the only material from 
which it could be generated are the sedimentary rocks themselves. The arguments 
brought in evidence of these assertions are among the most potent which can be 
adduced against them. Granite, in Norway, is superposed on Laurentian rocks turned 
upon their edges, and, as no channel can be found by which it ascended, it is argued 
that it must have been generated by metamorphism 7m situ. But as, on the other hand, 
it has justly been remarked, that no position is more favorable for metamorphic action 
than that of upturned strata, it is difficult to comprehend why the Laurentian rocks, 
which were evidently nearer to the source of heat, should not have been metamor- 
phosed in a much higher degree than the overlying granite. The same objection may 
be made in other instances, as in the case of the Huronian and Laurentian rocks in 
Canada, which were found to be overlain by granite thousands of feet in thickness. 
The negative evidence, apparently afforded by the fact that no channels have been dis- 
covered through which the granite could have protruded, is of very slight value, if it 
is taken into consideration how rare are the instances where the conducting channels 
can be seen, in the case of overflows of volcanic rocks, or even in that of currents of 
lava. There are a few more instances known where granite can be distinctly observed 
to overlie the upturned edges of stratified rocks, and must, therefore, have overflowed 
the same in a liquid state. In some other cases, as in those of the extensive granitic 
areas of Bessarabia and Western Australia, the true geological position of granite 
escapes observation ; while, ina number of others, it has the appearance of an intrusive 
mass. 
The strongest objection which may be made to the eruptive nature of 
granite relates to its lithological characters, which are different from those of the 
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