. Bulletin of the Geological Society of America. Geology. 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,
. Bulletin of the Geological Society of America. Geology. 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 hereâthe gradients of the valleys down the continental slope. As the great descent is usually. â t^r- r r fii 3";-' ^ n- J" i»5 FiGrKE 2.âChart 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 wou
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