OF VOLCANIC ROCKS. 45 



age. and chiefly compose the central portion of the Alps, shows that it is directed from 

 west to east in Lombardy, but in the vicinity of Lugano bends suddenly to the north- 

 east, then turns as abruptly back to its former direction from west to east. After 

 having passed the granitic mass of the Adamello, the same change is repeated on a 

 grander scale. The boundary is turned again in a perfectly straight line to the north- 

 east, and then resumes its former course, which it follows in an equally direct man- 

 ner, and in which it continues, exceedingly distinct at first — less distinct, by the 

 encroachment on it of more recent formations, farther east — until it turns a third time 

 to the northeast at the sudden termination of the Alps near Vienna, and continues in 

 tliis direction for a long distance. Finally it re-assumes, in the Carpathians, a similar 

 course to that which it had on the southern slope of the Alps. Three very distinct 

 reentering angles are thereby formed. The first of them encloses the country of Lu- 

 gano ; the second comprises the vicinity of Predazzo and Fassa in southeastern Tyrol, 

 and of Belluno and Vicenza in Venetia ; while the third, which is by far the most 

 extensive, comprehends all northwestern Hungary. Each of them has been a center 

 of eruptive activity, commencing with the granitic, and continuing through the por- 

 phyritic down to the volcanic eras, and all three are among the most classical coun- 

 tries for the study of eruptive rocks. There is, however, a conspicuous difference in 

 the mode of manifestation of the eruptive activity in each of the three eras. Little 

 connection exists apparently between the granitic masses of the three countries. They 

 are portions of the generally scattered granitic outbreaks, and differ among themselves 

 probably as much in age as they do in regard to the nature of their rocks. In the 

 porphyritic era, eruptive activity was contemporaneous in the three localities, but 

 scarcely extended beyond them. In the volcanic era, when the southern slope of the 

 Alps and Carpathians formed only a portion of a much more extensive belt, the 

 countries adjoining those three places were chiefly distinguished by the intensity of 

 eruptive activhVy. 



Great as have been the interruptions between the different eras, the continu- 

 ance of the selection of those three nooks at the foot of a prominent mountain range 

 for the manifestation of subterranean energy, from Palaeozoic down to modern time, 

 is evident. Similar instances, though less striking, might be mentioned from other 

 parts of Europe, such as the porphyritic region of middle Germany. Reverting to 

 other parts of the globe, it appears to be a general experience, though it is far from 

 being absolutely proved, that all the principal accumulations of volcanic rocks are 

 encountered in the neighborhood or immediate vicinity of granitic masses. These are 

 scattered over areas where no volcanic rocks occur ; but the distribution of the latter 

 within any of the volcanic belts appears to have been dependent, in a great measure, 

 upon the vicinity of the channels which had in preceding time afforded vent to granite. 



The general law deduceable from these relations is this : that, with the growing 

 thickness of the earth's crust, the systems of fractures which were formed in it at cer- 

 tain epochs, and partly gave vent to the emission of rocky matter, increased in depth 

 as well as in length, and were more and more concentrated to definite portions of the 

 crust, which are recognizable upon the earth's surface by the partly coincident areas of 



k (83) 



