44 RICHTHOFEN—NATURAL SYSTEM 
tion. But this may be due in part to their concealment, in the spaces between those 
lines, by sedimentary rocks. More signs of the separation of distinct ‘‘ regions of eruptive 
action,” as they may be called, are exhibited in the porphyritic era. Eruptive activ- 
ity has been intense within them, but appears not to have spread beyond certain boun- 
daries. Each of these regions embraces a number of the former granitic districts, 
while it leaves others excluded. Some resemblance with the peculiar features of the 
granitic era is afforded, inasmuch as each porphyritic region has been independent 
from others in regard to the epoch of its eruptive activity. We mentioned before 
that one of the porphyritic regions comprises the middle part of Germany, while 
another stretches along the southern slope of the Alps and Carpathians.—If we 
proceed to the volcanic era, it presents to us the reverse of the individuality 
peculiar to granitic districts, in the wonderful unity exhibited in regard to time 
and space over the whole area of extensive belts. In reference to unity of time 
alone, we might call the greater part of the continent of Europe, and even the 
entire surface of the globe, one great region of eruptive action, during the vol- 
canic era, since the first emission of rocky matter has been nearly cotemporaneous 
in widely separated countries, while its culminating epochs have probably varied but 
little in them separately, and the rocks have been ejected everywhere in a similar 
order of succession. In regard to local distribution, however, we have to distinguish 
certain belts, far exceeding in area the porphyritic regions. Each of them extends 
over a number of preéxisting mountain ranges, and the eruptions in each have fol- 
lowed, in their distribution, chiefly the lines of former elevation and ancient sea- 
coasts. But there is unmistakably to be recognized a tendency of the agencies which 
caused the eruptions, to connect these separated ancient belts of elevation ; either lon- 
gitudinally, when ranges superior in extent to the preceding ones would be formed, as 
appears to have been the case in the Andes; or, as it were, ina lateral sense, when 
the connection of neighboring mountain ranges into table lands would be either 
initiated or promoted. One of these belts, consisting of parts which had previously 
been disconnected, may be traced from Armenia to the Rhine, though I will try to 
show in the sequel that it is only a part of another belt which is of far greater extent. 
We mentioned before, that porphyritic rocks are encountered chiefly in those 
places where granitic rocks had preceded them. As regards the voleanic belts, eruptive 
activity has been particularly violent in certain portions of them. It is worthy of 
note that, wherever this has been the case, either granite or both granitic and por- 
phyritic rocks had been ejected before. This fact leads us to consider the correlation of 
the three classes of eruptive rocks in reference to their peculiar modes of distribution. 
Little information exists in regard to this subject. Only one instance” shall be related, 
which is highly suggestive for the existence of such a correlation, though it is of slight 
value as long as it is not corroborated by corresponding facts observed in other coun- 
tries. A survey, on a geological map of the Alps and Carpathians, of the southern 
boundary-line of those highly metamorphosed formations which preceded the Trias in 
16 Referred to in Richthofen, Geognostische Bechreibung der Umgegend von Predazzo, Sanct Cassian und der 
Seisser Alp in Siid-Tyrol : Gotha, 1860. 
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