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- 
sast, 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 maun- 
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 
this direction for a long distance. Finally it re-assumes, in the Carpathians, a similar 
course to that which it bad on the southern slope of the Alps. Three very distinct 
reéntering 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. Hach of them has been a center 
of eruptive activity, commencing with the granitic, and continuing through the por- 
phyritie down to the voleanic 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 
porphyritie 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 activity. 
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 voleanic 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) 
