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 preexisting 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 Ancles ; or, as it were, in a 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 lie 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 volcanic 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 16 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 



K Referred to in Richthofen, Geogno~ti^clie Bechreibnng der Umgegend v. in Predazzo, Sanct Cassian und der 

 Seisser Alp in S (id-Tyrol : Gotha, 1SG0. 



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