Continental Growth and Geological Periods. 



{Continued from p. 298.) 



Persistency of Drainage Lines. 



THE ideas and considerations dealt with in the preceding part 

 of this article bring us naturally to what is taking place in our 

 own time. Since the Tertiary upheavals new lines of drainage and 

 channels of erosion have been established, by which the waste of 

 the land has been and is conveyed to the lowest levels, which 

 are generally, but not always, in the ocean. That is to say, in some 

 cases the drainage lines terminate in inland seas or hydrographic basins, 

 in which the evaporation exceeds the rainfall. Such are the Aralo- 

 Caspian basin and the Central Asian basin" in which are situated 

 Lakes Balkash and Alakul, the Great Salt Lake and other areas in 

 North America. Mediterranean seas, connected by an outflow or 

 inflow, or both, Hke the Black Sea and the Mediterranean, bays such 

 as the Gulf of Mexico, the Gulf of California, Hudson's Bay, etc., 

 also receive their quota of sediment. 



Though the general levels of the Continental lands have fluc- 

 tuated, the main drainage lines — established and deeply cut into the 

 strata they traverse — seem to be very persistent, so that the waste 

 of a large part of the North American Continent has travelled down 

 the Mississippi to the sea since Tertiary times. Similarly, that of the 

 South American Continent has followed the lines of the Amazons, 

 La Plata, and Orinoco, while that of the western area of both conti- 

 nents, cut off by the divide of the Andes and Rockies, extending from 

 south lat. 50*^ to north lat. 70°, has perforce been cast into the Pacific 

 Ocean or the embayments connected with it. 



In the Continent of Africa the Congo delivers its load into the broad 

 Atlantic, almost opposite to the Amazons, with a flood almost equal 

 to, if not superior to it. The same persistence is seen in the rivers 

 of Europe and Asiatic Russia and the Northern portion of North 

 America delivering into the Arctic Ocean, and in the great rivers of 

 China flowing eastward into the shallower eastern seas. These lines 

 of drainage, as I have sought to show in the " Origin of Mountain 



^'^ See the "Physical Conditions of the Aralo-Caspian Region," Wm. Hewitt, 

 Pres. Address L'pool Geol. Soc, Proceedings, 1892-3. 



Z 



