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 volcanic 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 Sierra 

 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 



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