GEOGRAPHY. 569 



east, south, and southwest, from the original dry land of the Azoic 

 (p. 136). 



The mass of the earth above the ocean's level was increased two 

 or three fold between the beginning and end of the Tertiary 

 period. 



B. Post-tertiary period. — In the Post-tertiary, the great events, in 

 America at least, were (1) the excavation of valleys over the lifted 

 mountains and plains, and the shaping of the lofty summits ; (2) the 

 distribution of earth and gravel, covering and levelling the rugged 

 surface of the earth, laying the foundation of prairies, and filling 

 the broad valleys with alluvium ; (3) the finishing of the valleys 

 and lake-borders with a series of plains or terraces, and the exten- 

 sion of flats along the sea, — a work completed in the age of Man. 



The excavation of valleys by running water began with the first appearance 

 of dry land, and increased with its extent. But the greatest augmentation 

 took place after the lofty mountains had risen in the course of the Tertiary 

 period. The great gorges and canons over a large part of the Rocky Moun- 

 tains below a level of C00O or 7000 feet, and most of the deep channels occupied 

 by rivers in other regions, then had their beginning. 



The canon of the Colorado, between 111° and 115° W., is one of these gorges; 

 and though possibly earlier in its commencement than the Tertiary period, it 

 could have made little progress before the elevation of the mountains after the 

 Cretaceous ; for the present height of the plateau is but 6000 to 7000 feet. 

 According to Newberry, the canon is 300 miles long, and has walls of rock 3000 

 to 6000 feet high. These walls are sections of nearly horizontal strata, ranging, 

 for the principal part of their extent, from the granite to the top of the Carboni- 

 ferous, and higher up the stream to the top of the Cretaceous ; and the whole 

 bears undoubted evidence, according to Newberry, that it was made by running 

 water. The granite has been excavated in some places to a depth of nearly 

 1000 feet; above this there are 2000 to 2500 feet of Palaeozoic sandstones, shales, 

 and limestones, 1000 feet of probably Subcarboniferous limestone, and 1200 feet 

 of Carboniferous sandstones and limestone. A view of one part of the gorge 

 is given in fig. 940, furnished the author by Newberry ,• and another of the side 

 canons, in fig. 941, from the Report of Lieut. Ives, the commander of the Colo- 

 rado Exploring Expedition, of which Dr. Newberry was geologist. 



There were great oscillations of level in the Post-tertiary as well 

 as Tertiary; but (1) the Post-tertiary were mainly high-latitude oscil- 

 lations, being most prominent over the colder latitudes of the globe, 

 the cold-temperate and Arctic ; (2) they were movements of the 

 broad areas of the continents ; (3) they brought no mountain-ranges 

 into existence. 



According to the view presented in the preceding pages, there 

 was an upward oscillation in the Glacial epoch, a downward in the 

 Champlain epoch, and an upward of moderate extent in the Terrace 

 epoch. It submerged the region about Montreal and the Ottawa, 



