46 RICHTHOFEN NATURAL SYSTEM 



eruption ; that further, simultaneously with the increase in extent, each area manifested 

 a growing complexity in regard to the distribution of eruptive activity within it, and 

 that this distribution depended in a great measure upon that of the outlets of more 

 ancient eruptive matter. The first recognizable stage of this development, which, 

 however, was probably a far advanced one, is the individualization of those nu- 

 merous districts of fractures, the narrow limits of which are made manifest by the 

 mode of occurrence of Azoic and Palaeozoic granite. A growing development from that 

 stage, in the different directions mentioned, is conspicuous in the porphyritic era in 

 Europe ; when, besides the greater extent of the regions in which subterranean agencies 

 manifested themselves by the fracturing of the crust, every such area had become 

 more definite concerning its boundaries towards those which were not fractured during 

 the same era. On the other hand, however, each porphyritic region was more complex 

 than the granitic districts had been, inasmuch as the former were composed of certain 

 areas of greatest activity, which, as we had occasion to remark, appear to have been 

 chiefly dependent upon the distribution of the granitic districts enclosed within each 

 porphyritic region, while intermediate portions were contemporaneously affected by 

 disturbances merely, but not by any eruptions. A more advanced stage in this gradual 

 development seems to be exhibited by the Jurassic granite of the Andes. The belt 

 distinguished by its eruption appears to have been distinctly bounded, and the area 

 over which contemporaneous manifestations of like character took place, to have been 

 even more extensive than the porphyritic regions of Europe. All these features, how- 

 ever, are conspicuous on a much grander scale in the volcanic era. The growing 

 definiteness of the boundaries of the volcanic belts towards large areas which were 

 entirely free of eruptive activity, the increasing complexity of their interior arrangement, 

 and its dependency upon preexisting lines of elevation, and particularly on the 

 distribution of granitic and porphyritic rocks, are evident from what has been said before. 

 We may even trace a development from the andesitic to the basaltic epoch. It is 

 known how far superior in extent is the range of basaltic outbreaks to the area occupied 

 by other volcanic rocks. If considered with attention, it will be found that there is a 

 tendency in the former to connect within each belt the subordinate ranges of the latter, 

 longitudinally as well as laterally. The only connection, for instance, between the 

 volcanic districts of Hungary and those on the lower Rhine, is occasioned by the chain 

 of isolated basaltic hills which extends through the central part of Germany. 



ON THE ORIGIN OF VOLCANIC ROCKS. 



The facts established in the foregoing chapters, and the inferences drawn there- 

 from, may assist us in considering some questions of wider bearing. We have seen that 

 the volcanic rocks of several, and probably of all parts of the globe, are connected by 

 simple and definite relations, which together comprehend the main features of their 

 natural system ; and we found that correlations of a similar nature ally the volcanic 

 with all the ancient eruptive rocks, justifying, to some extent, the supposition that the 

 eruptive rocks of all ages and places form one harmonious whole, and that we may be 

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