THE WEST INDIAN BRIDGE. 23 



crossing tlie submerged coastal plains is no greater than that illus- 

 trated by those of the land. 



The resemblance between the terrestrial and submarine valleys 

 is even more striking in the case of their tributaries, which come 

 from various directions. Where there are short tributaries descend- 

 ing from submerged plateaus, they have also the form of amphi- 

 theaters like those dissecting the margins of table-lands. 



The slopes of the submarine channels consist of a series of steps, 

 like the gradation plains descending from high table-lands. These 

 steps represent long pauses in the elevation of the plateaus. For 

 comparison of the declivity of the drowned valleys with those of the 

 land, sections are given on pages 21 and 22 in Figs. 5 to 9, all 

 drawn to the same scale. The declivities of some of these grada- 

 tion plains do not exceed a foot per mile — that is, these slopes 

 are as low as those of continental valleys which are reduced to 

 the base level of erosion. While much is yet to be learned of the 

 detailed features between the submarine steps, we already know that 

 their mean declivity is less abrupt than that of land valleys descend- 

 ing from the Mexican plateaus. The submerged steps appear to 

 have the same origin as those on the border of table-lands — that is 

 to say, they were formed during the pauses in the terrestrial oscilla- 

 tions when the now drowned continental plateaus formed table-lands. 



The deep channels crossing the submerged plateaus for a distance 

 of two hundred or three hundred miles, with a depth of from two 

 thousand to six thousand feet — and among the Bahama banks to 

 even greater proportions — show a close resemblance to the Colorado 

 Valley and Caiion. 



The Geological Yardstick and West Indian Bridge. — From 

 the apparently complete analogy between the characteristics of the 

 land and submarine valleys or channels — namely, (1) the submarine 

 valleys being continuations of those of the continent or islands; (2) 

 both having tributaries converging from every possible direction; 

 (3) both classes of valleys having their magnitude of corresponding 

 proportions, with (4) similar great canons and amphitheaterlike 

 tributaries; (5) both terrestrial and submarine channels with similar 

 gradation plains and steps characterizing their slopes; (6) without 

 obstructing barriers — the conclusion is reached that the depths of 

 the submarine channels may be used as yardsticks to show that the 

 land lately stood nearly as high as the valleys are deep. 



Applied to the West Indian region and adjacent parts of the 

 continent, it would thus appear that these regions stood from ten 

 thousand to twelve thousand feet, or in some localities fourteen thou- 

 sand feet, higher than now. The West Indian bridge reached a 

 height of from two to more than two and a half miles, while the 



