60 RICHTHOFEN NATURAL SYSTEM 



tic ingredient, while silica enters into their average composition probably in a 

 much lower proportion than it is contained in ordinary granite. The varieties known 

 as granitite are of rare occurrence. In another respect, namely, the mode of geograph- 

 ical distribution, the Jurassic granite of the Andes marks an advanced stage, if com- 

 pared with the Permian and Triassic porphyries of Europe, inasmuch as the area of 

 its distribution excels the porphyritic regions of the latter continent in regard to their 

 extent and the unity of their inner relations, while it does not come up, in these 

 respects, to the properties of the great volcanic belts. 



2. Origin of Volcanic Action. 



I have tried to demonstrate in another chapter, that volcanic activity is inti- 

 mately connected with massive eruptions in a three-fold way, namely, in respect to 

 the epoch of its commencement, the mode of its distribution, and the nature of the 

 rocks ejected. Basaltic volcanoes occur in ranges built up by the massive eruptions of 

 basalt, or in their vicinity, in such connection as to make obvious the nearly contem- 

 poraneous origin of both. Andesitic volcanoes are found in the neighborhood of those 

 masses of andesite which had been ejected without being attended by volcanic action ; 

 while volcanoes which have emitted rhyolitic or trachytic lava are so situated as to 

 justify the inference of a close connection between the first opening of their vents 

 and the origin of neighboring accumulations, either of rocks of similar character or of 

 andesite, but which are due to massive eruptions. 21 



No distinct line of demarcation can be drawn between the two modes of mani- 



21 This affinity in regard to petrographical character and geographical distribution is probably the cause why both 

 agencies have ordinarily been confounded. It has been a current notion, strongly advocated at times, that all volcanic 

 rocks have come to the surface in a similar way to that in which lava is being ejected from volcanic vents, and that the 

 cause of the ejection has been the same in all cases. In regard to the fact that extensive mountain ranges are completely 

 built up of volcanic rocks, it, was argued that it is by no means necessary, and in fact erroneous, to suppose them to have 

 originated in events surpassing in magnitude those of the present day, as the length of geological time would explain how 

 they could be built up by the gradual accumulation of innumerable currents of lava. Where traces of former volcanic 

 vents could not be found, the easy destructibility of the matter of which the sides of craters are usually composed, afforded 

 a convenient and apparently just argument for explaining their absence by denudation. The same doctrine was applied for 

 explaining the mode of formation of ancient eruptive rocks where we see mountain masses made up of them, while others 

 have arrived at the conclusion that they were the " roots " or " cores " of volcanoes ; granite itself is by them considered 

 to have had formerly, and to have now this function, even in the case of those volcanoes the lava of which consists of basalt. 

 Both these doctrines have been adopted the more readily, as they appear to be in harmony with the favorite hypothesis, that 

 at no time have any changes on the surface of the globe been more violent than those going on at the present day, nor 

 different from them in mode. However ably this theory, which contributed so much to the advancement of science by 

 checking the phantasmagorias of former time, has been advocated, an unbiased comparison of the grand manifestations 

 recorded in the geological structure of an andesitic mountain range, with the mode and degree of activity of present vol. 

 canoes, must lead to different conclusions. In the endeavor to sustain the a priori assumption of the equality of force in all 

 ages, too little stress has been laid on the circumstance, that, however nearly equal the aggregate amount of force acting on 

 the globe may have been, its modes must have undergone a change, chiefly by the partial conversion of the heat of the 

 globe into other forces. At the same time, the degree of the intensity of the manifestations of subterranean forces must 

 have varied in a two-fold way. With the continuous increase of resistance, the aggregate motions of the crust must have de- 

 creased, since a growing amount of these forces'was required for overcoming the resistance. On the other hand, the manifest- 

 ations of the same forces had to become periodical and paroxysmal, and periods of violent action had to be separated by 

 others of repose. The violent activity of the Tertiary period has passed, and our present volcanoes appear to mark the transi- 

 tion into another period of repose. I will try to demonstrate that the causes of their action are utterly inadequate for ex- 

 plaining the grander phenomena and correlations of massive eruptions. 

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