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 Paleozoic 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 voleanic 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 preéxisting 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 
voleanic 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 OriGiIn oF Votcanic 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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