54 RICHTHOFEN —- NATURAL SYSTEM 
would oppose, and, to a certain degree, destroy each other, if the state of aggregation 
of subterranean matter allowed of a free conduct of motion. 
; These slow and continuous agencies, chiefly those among them which give origin 
to the process of elevation, must have been, too, instrumental in causing the massive 
eruptions of rocks. At least, no other force supposed to act beneath the surface can 
account as fully for certain facts connected with them, such as the gradual changes 
in the mode of their geographical distribution, or the connection of the manifesta- 
tions in any certain system of fractures, or the fact that ages of comparative 
repose have been interrupted by paroxysmal actions of great violence which have 
taken place during certain eras in the history of each separate country. The 
latter circumstance points clearly to the assumption, that during the eras of repose, 
or at least their later portion, a constantly increasing amount of potential energy 
must have been accumulated under the crust of the globe; since the subterra. 
nean agencies called into existence by the cooling of the globe did of course never 
rest, nor can they be reasonably supposed to have been more intense in the Tertiary 
than they were in preceding periods. It is self-evident that the increase of tension 
must have been much more considerable under areas of elevation than under the 
more extensive regions of subsidence. In the latter case, when a downward tendency 
is caused by contraction, the weight of the crust must come in its aid ; the essence of 
the resistance to that tendency may therefore be concisely expressed as cohesion minus 
weight ; while weight added to the cohesion will give approximately the resistance to 
elevation by expansion. The tension under a crust of great thickness must therefore, 
in the latter case, increase to a stupendous intensity, until it is sufficient to overcome 
the resistance, and will then be able to result in such paroxysms as that by which 
the volcanic era has been inaugurated, and the main feature of which consists in the 
formation of fractures which, by their mode of association, constitute distinct systems 
or belts, separated by areas in which the existence of fissures in the crust cannot be 
recognized on the surface. 
Although the processes suggested may furnish us the cause for the prime condi- 
tion to the emission of rocky matter from below, namely, the periodical opening in the 
earth’s crust of such fissures as widened with the approach to the surface; yet 
they fail completely to explain, by themselves alone, the more immediate causes and 
the mode of that emission. It may be presumptive to extend speculation upon this 
topic beyond those limits which we have reached ; but a safer guide to conjecture on 
the same than had been known at any former time was given during the last few 
years by the experiments of Daubrée, and the microscopic examinations of the texture 
of rocks by Sorby. It has been held quite generally till of late, that the opening of 
a fissure to a liquid mass below, would be sufficient by itself to cause the ascending 
of the liquid through it to the surface ;” but this supposition is utterly irreconcilable 
19 Tt must here be remarked that some of the most eminent writers on the subject of voleanoes, chiefly Pou- 
lett Scrope, Prof. Dana and others, have suggested long ago, that the ascending of lava in a voleanie channel mast be 
due to both the fluidity and expansion imparted to it by the globular state of the water which finds ingress to the chan- 
nels of the lava and enters into its composition. They have anticipated, by this suggestion, in some measure, the results of 
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