OF VOLCANIC ROCKS. 87 
been gradually turned from their channels, overflowing their banks at several places in 
succession, and taking a course down the slope, until an entirely new system of water- 
courses was created, at right angles to the crest, the steep ravines and gorges of which are 
now one of the characteristic features of the Sierra Nevada. It appears that the changes 
of level since the inauguration of the voleanic era have also been progressing on a grand 
scale in the eastern part of the Great Basin, where the eruptive activity had probably 
even more gigantic proportions than in the west. The central portion of the Rocky 
Mountains was raised, according to the estimate of Dana, about seven thousand feet since 
the Cretaceous ; and its eastern part from one to two thousand feet, since the Miocene, 
according to the observations of Hayden. 
These figures render evident the great elevation which the mountain mass 
between the Sacramento and the Missouri must have undergone since the commence- 
ment of the volcanic era. ‘Twice in the history of that country, since the Triassic 
period, may there be recognized an extraordinary intensity of all those changes which 
we must ascribe to subterranean agencies. The first instance was in or about the 
Jurassic epoch, when strata, which appear to have been quietly deposited during pre- 
ceding eras, were elevated, and a gigantic intrusion and ejection of granitic masses was 
attended by an intense and wide-spread metamorphism, by which disturbances and 
plications were promoted; the second, in the volcanic era. It is probable that similar 
phases to those mentioned will be recognized throughout the entire range of the Andes, 
the geological structure of which appears to offer much similarity in different parts. To 
the events of the volcanic era, chiefly, will have to be ascribed the connection of both 
parts of the continent, though it may have been prepared by that preceding era of in- 
tensified actions, which manifested itself in the ejection of the granite of the Sierré 
Nevada, and appears to have left no less distinct traces in other portions of the range 
of the Andes. We may still note a peculiar difference in the mode of the changes of 
level if we proceed across the continent from west to east. It appears that the narrow 
strip of land adjoining immediately the western coast has been subjected rather to 
periodical oscillations than to any lasting changes of level, while the great elevation of 
the mountain ranges and highlands must be chiefly ascribed to the circumstance that 
all changes have acted there essentially in the same direction, producing the elevation 
of extensive regions. This would explain why the western descent of the Andes has 
been periodically increasing in steepness and the strip between them and the coast is of 
little width. The latter appears to correspond to the boundary between an area of 
elevation to the east and an area of subsidence to the west, one of which was especially 
subjected to the manifestations of vulcanism, while in the other it has left no recogniz- 
able signs. It is quite different on the eastern side of the range of the Andes, where, 
in both parts of the continent, a slow rise has taken place, which has connected, during 
the volcanic era, that mountain range with other ranges farther east, by those exten- 
sive low-lands which are so important a feature as regards the extraordinary produc- 
tivity of both parts of the continent. 
Similar to these are the relations presented by the European continent, in regard 
to which we will only mention a few prominent facts. During the porphyritic era and 
the time immediately succeeding it, great changes of level had taken place on that 
