524 



CENOZOIC TIME. 



geosynclinal of the Cretaceous, was upturned and made into mountain 

 ridges along the Coast region, parallel with the Cretaceous ridges and 

 Sierra Nevada. Again, over the eastern slope of the Rocky Moun- 

 tains, there was, at the close of the Miocene, a great contraction of 

 the lake region ; for the Pliocene lacustrine beds have, according to 

 Hayden, a much more limited distribution. This is evidence that the 

 elevation of the Rocky Mountains had gone forward during the 

 period. There is proof that mountain-making pressure, from the Pa- 

 cific direction, had acted with energy against the continental crust, in the 

 occurrence of extensive areas of igneous rocks over the Pacific slope 

 and part of the summit region. The vast areas of trachyte and doler- 

 yte show that immense regions were flooded by outpourings from frac- 

 tures, at successive times. These eruptions continued to take place 

 over those regions, at intervals, from the close of the Miocene even into 

 the Quaternary age ; and they have not even now altogether ceased ; so 

 that it is not easy to decide the particular date of the successive out- 

 flows. The beds form ramparts of basaltic columns, in several ranges, 

 along the Snake River, or upper Columbia, and have a thickness there 

 of 700 to 1,000 feet (King) ; in the cut through the Cascade range 

 the thickness is over 4,000 feet (LeConte). As these eruptions far 

 exceed all those of earlier time, they may be looked upon as the results 

 of mountain-making pressure, after the crust had become so stiff, from 

 its successive thickenings and the consolidations of the superficial de- 

 posits, that it could not bend, and hence broke. The rocks of the 

 eruptions after the close of the Miocene, included both trachytic and 

 dolerytic kinds. 



On the Atlantic Border, the elevation of the coast, which placed the 

 Miocene beds above the sea-level, may have taken place at this time, 

 as above remarked. There is probable proof of elevation contem- 

 poraneously with the Rocky Mountain movements of this era, in the 

 present height of the Tertiary in parts of Georgia and Alabama; for, 

 while in general the beds on the Gulf Border are but one hundred to 

 two hundred feet above the sea, near Milledgeville, Georgia, they are 

 now six hundred feet, and near Montgomery about eight hundred feet. 

 The position of the region, in a line with the general trend of Florida, 

 suggests that its elevation may have been connected with that of the 

 Peninsula of Florida itself. Moreover, the northwestward trend cor- 

 responds with that of the Rocky Mountain region, and not with that 

 of the Alleghany range, which was raised soon after the Paleozoic. 

 In San Domingo, according to Gabb, the Miocene has an elevation of 

 two hundred to two thousand feet. 



The elevation of the Rocky Mountains, which took place in the 

 course of the Tertiary, and which had reached fully its present limit 

 by the close of the age, amounted to not less than eleven thousand 





