Professor Spencer — Continental Elevation. 33 



there are deep vallej'^s, often of great length, extending from the 

 mouths of the existing rivers, and crossing the American coastal 

 plains, over deeply-buried channels. These are plainly recognizable 

 in soundings upon the submarine coastal plateaux, and amongst the 

 banks and islands of the neighbouring West Indian seas, to depths 

 of 12,000 feet or more, before reaching the oceanic floors. The 

 drowned valleys radiate from the continental margins and extend in 

 a direction across that of the coast, and the mountain ranges to the 

 back of it. Their courses do not usually coincide with those of the 

 mountain folds. These submarine vallej's are often recognizable for 

 hundreds of miles in descending to the floors of the ocean-basins, as 

 may be seen amongst the Bahamas. Frequently the divides between 

 diff'erent systems are themselves submerged, as in the Straits of 

 Florida. The submerged valleys are no broader than those of 

 existing rivers, such as those of the Amazon and the St. Lawrence, 

 nor indeed are they usually as wide. The Colorado canon, from five 

 to twelve miles across, between walls of 2,000 feet in height, is wider 

 than some of the drowned valleys, which in part are canon-like. 

 Both the submarine plateaux and the floors of the valleys are like 

 comparatively level plains or base-levels of erosion, which represent 

 pauses when the streams and atmospheric agents could not further 

 deepen their valleys, but only broaden them out into plains, until 

 a subsequent elevation of the region permitted the streams once 

 again to deepen their channels. 



Gradients of Submarine Valleys. 



The gradients of the submerged valleys (except along the reaches 

 crossing extensive plains, now below sea-level) can only be com- 

 pared with those of plateau regions, and not with the slopes of such 

 a river as the Mississippi, which flows over great plains at low 

 elevation. The manner in which the valleys descend from one 

 platform to another is illustrated in the plateau region of Mexico 

 and the West. An example of the declivity of such valleys may 

 be seen along the Mexican Railway, back of Vera Cruz, and another 

 above Monterey. The land valleys are made up of a series of steps 

 with greater declivities between them than occur between those 

 submerged. The various platforms represent the rise of the land 

 during the excavation of the valleys. The gradients of the sub- 

 merged plateaux are frequently as small as, or smaller than, those of 

 such plains as the Mississippi, while the declivities at their margins 

 are less abrupt than those of the land valleys descending from 

 tablelands, as may be seen by comparing them with the Mexican 

 valley sections. The gradient of the Colorado river, in its caiion 

 3,000 feet deep, is greater than that of the submerged platforms. 

 Besides the greater valleys, descending from the high plateaux, 

 there are many short tributaries, heading in amphitheatres, -where 

 the slopes may be from 200 to 600 feet per mile; the whole 

 resembling gigantic "wash-outs." So also similar short drowned 

 valleys occur on the edges of the submarine plateaux. The data 

 concerning these comparative declivities were not obtained when the 



DECADE IV. VOL. V. NO. I. 3 



