50 RICHTHOFEN — NATURAL SYSTEM 
those which are rich in silica and of little specific gravity bemg derived from places 
nearer to the surface than those in which a smaller proportion of silica is attended by 
a higher specific gravity, was first distinctly made by Sartorius von Waltershausen, 
who attempted to prove this theory by mathematical calculation, and has at least suc- 
ceeded in demonstrating its adaptation to the nature of eruptive rocks. The assump- 
tion, hypothetical as it may still appear, has an eminent degree of probability, and is of 
the highest importance for the geology of these rocks, for the reason that it is capable 
of explaining numerous observed facts which have failed to be satisfactorily explained 
in any other way. ; 
We may still draw a line of negative argument in corroboration of this theory. 
It is perfectly clear that the source of eruptive rocks must either have been below the 
lowest depth at which rocks of sedimentary origin occur, or above it. We found that 
the former assumption gave a satisfactory explanation of prominent facts, and it is 
just as conspicuous that the second fails to give any explanation at all. If eruptive 
rocks had had their original seat within the crust of sedimentary rocks, and had been 
generated from their substance, they must be analogous to them in chemical compo- 
sition, that is to say, they would vary in this respect within very wide and complex 
limits in any single country, and we should encounter no lesser differences when com- 
paring the totality of eruptive rocks in one part of the globe with that in other parts. 
The fact that no arithmetical relations can be recognized as connecting the various 
sedimentary rocks in their composition, would hold equally good among eruptive 
rocks, since it is impossible to conceive a progress from the indefinite to the definite, 
from that which is void of any recognizable relations in respect to the composition of 
matter to that in which such definite relations are plainly evident, by the mere 
influence of such agencies as would cause fluidity, (that is, the combined action of 
heat, pressure, and water.) As no agency, indeed, is known to which such a result 
may be ascribed, this argument removes beyond the limits of probability the 
assumption, that the original seat of eruptive rocks has been above the foundation of 
the gradual accumulation of those rocks which have been produced by external 
changes, and therefore allows only the other alternative, that it has been below that 
boundary. 
While the theory of Sartorius will explain the causes of the prominent mutual 
relations existing among eruptive rocks in regard to their composition and general 
similarity in all countries, we have still to trace those causes which effected their ejec- 
tion from a deep-seated original place to the surface. This question is more abstruse 
than the foregoing, inasmuch as the manner in which forces have formerly been acting 
beneath the earth’s crust, is a subject more involved, and allowing a larger range of 
hypothetical explanation, than we met on a field where the observed properties of 
matter offered a comparatively safe guide. Complicated as the processes appear to 
have been, to the codperation of which massive eruptions were due, it seems that 
they may severally be traced back to one common cause of a higher order, of which 
all, or most, of them are but different modes of manifestation. Probabilities accumu- 
late to point out as this fundamental cause, the process of the gradual cooling of the 
earth and the solidifying of its crust toward its interior. 
(88) 
