30 PHYSIOGRAPHIC GEOLOGY. 



within 150 miles of the coast, and thus influences the extent of the Pacific 

 border river systems. The western drainage of the Eocky Mountains, rising 

 partly in the Yellowstone Park, and partly just south of it, has its outlet 

 to the ocean through the Colorado and Gulf of California, and along the 

 Columbia Eiver and streams farther north, the Colorado and Columbia 

 reaching salt Avater at points 1200 miles apart. Thus it is that the " Great 

 Basin " is Avithout drainage. Again, a subordinate range of this chain, that 

 of the Coast Range, 2000 to 4000 feet high, is a barrier, for 800 miles, to 

 most of the drainage waters of the Sierra Nevada and Cascade Mountains ; 

 and consequently the Sacramento and Joaquin rivers, and not the ocean, 

 receive all the Sierra waters for 500 miles, and the Willamette, the waters 

 of the Cascade Eange for 150 miles. 



South America has an arrangement of interior river systems parallel to 

 that of North America ; the Amazon flowing eastward, like the St. Lawrence ; 

 the La Plata flowing southward, like the Mississippi ; the Orinoco and other 

 streams northward, like the Mackenzie. This adds a fourth to the charac- 

 teristics exhibiting parallelism in structure betAveen two continents, North 

 and South Ainerica (page 22). Africa, on the opposite side of the Atlantic, 

 has the arrangement reversed as regards the east and west streams : the 

 great Niger empties into the western ocean, the Atlantic ; the Nile is the 

 northward-flowing stream ; but the south Avard-flowing interior waters are 

 divided between the Congo draining to the southwestward and the Zambesi 

 to the southeastward. 



The lengths and drainage areas of some of the largest of rivers are as follows : Amazon, 

 length (L.) = 3545 miles, drainage-area (D.) = 2,264,000 square miles ; La Plata, L. = 2400, 

 D. = 1,250,000 ; Mississippi, L. = 2800 (but from its mouth to the head of the Missouri 

 4200), D. = 1,285,000 ; Nile, L. = 3815, D. = 1,049,000 ; Congo, L. = 2900, D. = 1,540,000 ; 

 Yenisei, L. = 2800, D. = 784,500 ; Amur, L. = 2380, D. = 583,000 ; Obi-Irtish, L. = 

 2320, D. = 725,000 ; Lena, L. = 2400, D. = 594,000 ; Yang-tse-Kiang, L. = 2800, D. = 

 548,000 ; Hoang Ho (Yellow River), L. = 2280, D. = 537,000. 



The lengths of the valleys, excluding the minor beds, are : the Amazon, 2600 miles ; 

 the Mississippi, 1164 ; the Nile, 3100. 



II. SYSTEM IX THE RELIEFS OR SURFACE FORMS OF THE 



CONTINENTS. 



Law of the system. — The mountains, plateaus, lowlands, and river regions 

 are the elements, in the arrangement of which the system in the surface form 

 of the continents is exhibited. The law at the basis of the system depends 

 on a relation between the continents and their bordering oceans, and is as 

 follows : — 



First. The continents have in general elevated mountain borders and 

 a low or basin-like interior. 



Second. The highest border faces the larger ocean. 



A survey of the continents in succession with reference to this law will 

 exhibit both the unity of system among them and the peculiarities of each, 

 dependent on their different relations to the ocean. 



