58 RICHTHOFEN —NATURAL SYSTEM 
Let us now direct our attention at once to the voleanic 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 vulecanism 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 voleanic 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 codperate 
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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