OF VOLCANIC ROCKS. 49 
conclusions appear to be the only ones which are fully justified. If we venture to 
inquire into the nature of the agencies which caused eruptive activity, and to explore 
their fundamental causes, we have to transgress the limits of legitimate speculation 
and enter the realm of hypothesis, the only way of proceeding in which consists in 
weighing probabilities. Further experience, it may be hoped, will extend the limits 
of induction, in the same measure as it will furnish increased evidence. 
I will attempt, in the first place, to consider, what must be the nature, and 
what the position of the source at which the voleanic and all eruptive rocks have 
originated, in order that their physical and chemical properties and their mutual rela- 
tions may be explained, and then to examine what have been the agencies that have 
had, in all probability, the most immediate influence upon the facts connected with 
the mode of distribution and succession of eruptive rocks in general. We have to 
start in these considerations by a well-known argument. 
If no changes had ever taken place on the surface of the globe, and no sedi- 
ments had ever been deposited on it, but that state was still*preserved which the 
globe must have exhibited when the substance of the rocks was liquid on its surface, 
then the matter nearest to this would probably be of a nearly uniform chemical com- 
position, and, consequently, of a uniform specific gravity. If we imagine the globe to 
consist of concentric layers, it is exceedingly probable, from physical laws, that then the 
substance of each successive layer would be, too, of a nearly uniform composition 
throughout its extent. Proceeding, however, from those layers nearest to the surface 
to those at greater distance from it, the specific gravity of the matter composing them 
severally would necessarily increase gradually towards the interior. This would be 
effected by the tendency of the heavier elements to predominate as the depth became 
greater, while the lighter would increase in as gradual a ratio towards the surface. 
If the layers were infinite in number, then the passage, in chemical composition as well 
as in specific gravity, from the surface towards the center of the earth would be one of 
infinite gradations. This condition is eminently the most probable, if physical laws 
are taken into consideration. If the crust was then allowed to solidify, to any depth 
required, the same interior arrangements would continue to exist unaltered, regardless 
of any changes on the surface. That a condition similar to this actually exists at the 
present time, is rendered evident by the well-known fact, that the specific gravity of 
the mass of the globe greatly exceeds that of the average of the matter which com- 
poses its exterior crust. Hruptive rocks have been carried to the surface from places 
beneath it. The depth from which their material has been derived, the way in which 
it was rendered liquid, and the cause and mode of its ejection, these are the principal 
points of conjecture in regard to them. The most probable place of its derivation 
are those imaginary layers which are beneath the theater of external changes, and still 
occupy their primeval position. This appears to be manifest from the two facts to 
which we have constantly to refer: the ejection of identical chemical compounds in all 
countries and ages, and the exhibition by eruptive rocks, in each country, as well as 
over the entire globe, of a series of compounds the specific gravity of which increases 
in inverse ratio with the amount of silica. The suggestion that eruptive rocks, in 
virtue of these facts, represent the arrangement of matter in the interior of the globe, 
(87) 
