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 Daubr6e, 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 ; 19 but this supposition is utterly irreconcilable 



19 It must here he remarked that some of the most eminent writer? on the subject of voleanoes, chiefly Pon- 

 lett Scrope, Prof. Dana and others, have suggested long ago, that the ascending of lava in a volcanic channel must 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 



(92). 



