THE THROES OF THE CONTINENT. 269 



These, in the succeeding ages, were at times enlarged, at 

 times contracted, through the orographic movements taking 

 place, and finally filled or drained (see Talks XVIII and 

 XLIV). During the Miocene epoch a great lake covered the 

 region of the Great Plains, as far as the Gulf of Mexico a 

 region which appears to have been mostly land during the 

 Eocene. Meantime, vast volcanic eruptions were taking place 

 along the Pacific border, burying thousands of square miles 

 under lava, and supplying ashes which filled some of the 

 western lakes four thousand feet deep. 



The interior lake history continued of similar tenor till the 

 close of the Pliocene, when the grand movements occurred 

 which impressed on the broad Cordilleran region its present 

 surface features. The rocky sheets of the Great Plains were 

 tilted into a position which secured drainage of the great lake 

 which had covered them. In the far west, the Sierra Nevada 

 and the Wahsatch were rent logitudinally by great faults 

 along their crests, and the continental mass between the faults 

 sank down one or two thousand feet, forming the Great Basin, 

 and returning it to the depressed condition which it had held 

 through the whole of Palaeozoic time. On its eastern and 

 western borders, gathered two lakes, each as large as Huron, 

 the eastern of which has shrunken to Great Salt Lake, Utah, 

 and Sevier; while the western exists only in the remnants 

 known as Pyramid, Winnemuca, Carson, Walker, and Hum- 

 boldt Lakes. Later mountain ranges have risen here and 

 there in the Basin, and volcanic outbursts have contributed to 

 diversify the topography. These final disturbances, followed 

 probably by some later ones all embraced within the Quater- 

 nary period shattered the Plateau Province to a destructive 

 extent. Great fractures ran through it from end to end. On 

 one side of each, the rocky sheet is generally upraised, and 

 on the other, depressed. Volcanic mountains have been built 

 up here and there, and earthquakes have shattered the blocks 

 shaped by the meridional faults. Simultaneously, surface ero- 

 sions have perpetually changed the configuration of the surface. 

 Rivers have cut their way through mountains, through lava 



