58 RICHTHOFEN NATURAL SYSTEM 



Let us now direct our attention at once to the volcanic era. The conditions 

 of the globe must have been very different in the Tertiary from what they had 

 been in the Paleozoic period. A longer time of comparative repose had in most parts 

 of the globe preceded the inauguration of the violent manifestations of vulcanism in 

 the Tertiary period than had ever before elapsed between any two eras of eruptive 

 activity. The globe had cooled down. Volumes of sedimentary matter had accumu- 

 lated, and added externally to the thickness of its crust, while it had increased in a 

 vastly greater measure by the crystallization of liquid matter below. Those silicious 

 compounds especially, of low specific gravity, which had formerly yielded the material 

 of the vast accumulations of quartziferous eruptive rocks, would have been consoli- 

 dated, and the limit as it were between the solid and the viscous state of aggregation 

 receded into regions where the matter would be of a less silicious composition and of 

 greater specific gravity. The similarity in distant countries of the rocks first ejected 

 (propylite and andesite) goes to show that the recession of that limit into greater 

 depth must have proceeded in a nearly equal ratio in all those regions where volcanic 

 rocks are distributed. When the tension below had increased sufficiently to overcome 

 the resistance, it would now no longer manifest itself in the formation of small and 

 differentiated systems of ruptures. In the direct ratio of the increase of the resist- 

 ance the fractures would have to be of greater extent, and those elongated belts of 

 them would be formed which even now are partially distinguished as the belts of vol- 

 canic activity. The first rocks ejected would necessarily be of a more basic composition 

 than the predominant rocks of the granitic era, while the repetition, at a later 

 epoch, of the process of fracturing would give rise to the ejection of rocks in 

 which silica would be contained in a still lower proportion. The greater portion 

 indeed of the ejected rocks consisted of propylite and andesite, in the first, and of 

 basalt in the second half of the volcanic era. A notable but only apparent anomaly 

 in the regular order of succession has been the emission of trachyte and rhyolite 

 between the andesitic and basaltic epochs. But if it is considered that these rocks were 

 ejected partly from the same fractures through which andesite had ascended, and 

 partly from others in their immediate vicinity, while the distribution of basalt has 

 been independent, to a certain extent, of all foregoing eruptions, it is evident that 

 the occurrence of trachyte and rhyolite is closely dependent on that of andesite, and 

 bears only a very remote relation to basalt. It appears that after the ejection of the 

 chief bulk of andesite, when other processes ending in the opening of fractures into 

 the basaltic region were being slowly prepared in depth, the seat of eruptive activity 

 ascended gradually toregions at less distance from the surface. There is, within the 

 limits of conjecture based on physical laws, no lack of processes which could cooperate 

 to that effect. The consolidation of the ejected masses within the fissures would prob- 

 ably proceed simultaneously, by loss of heat, from the surface downwards and, by 

 pressure, from below upwards. The opening of new branches from the main frac- 

 tures, the remelting (by the aid of the heat of the molten mass within the latter, and 

 of water finding access to it) of solidified matter adjoining the fracture, the emission 

 of that remelted matter through those branches : all these are secondary processes 

 depending on the first almost necessarily. The supposition that to these is due the 

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