ORIGIX OF THE SUBMARINE VALLEYS 



223 



mined off our own coast. Thus on the chart between Jamaica and 

 Central America there is a submarine plateau rising almost to sealevel, 

 and in the accompanying illustration this may be seen in places incised 

 by narrow channels, and, again, these, uniting from the opposite side of 

 the submarine plain, divide it into separated banks or islands. These 

 suggest, not merely a moderate elevation that formerly obtained, but 

 also one of considerable amount, as, for example, those seen between 

 Jamaica and Haiti, where the lower plateaus are indented by the 500 

 and 1,000 fathom contours. 



One other point may be again referred to here — the gradients of the 

 valleys down the continental slope. As the great descent is usually 



■t ^r- r r f ii 







3";-' ^ n- J" i»5 







FiGrKE 2. — Chart of Area between Haiti and Central America. 

 Showing dissection of banks by narrow channels and valleys. Soundings in fathoms. 



restricted to a comparatively short distance, the mean declivity of the 

 valleys at first seems too great for comparison with those of the land, 

 but, as we have already found, these are often characterized by abrupt 

 steps, with more gentle gradients between, similar to the valleys descend- 

 ing from the high plateaus of Mexico and Central America or the tribu- 

 taries of the Colorado canyon, which descend 3,000 feet in perhaps 10 

 miles. But, in order to reveal their true character, the soundings must 

 be made close together for this purpose, as the mean slope gives us no 

 information whatever; and so, for the present, the best the writer can 

 do is to compare them with land valleys from high plateaus, which is 

 justified in the study of the Floridian channel, which descends by long 

 stretches, with gradients of a foot or less per mile, as small as that of 

 the Mississippi, succeeded by precipitous steps like those from one sub- 

 marine plateau to a lower. 



To cover all the questions raised would far exceed the limits of this 



XXXII— BcLL. Geol. Soc. Am.. Vol. 14, 1902 



