138 THREE CRUISES OF THE " BLAKE." 



the single channel of the Straits of Bernini, and then the whole 

 mass of the Gulf Stream flowed northward over the shallow 

 Blake Plateau extending north of the Bahamas to Cape Hat- 

 teras. It is this part of the Blake Plateau which, if I am right 

 in tracing its past history, has been worn away by the unceas- 

 ing flow of the Gulf Stream. 



The Gulf Stream now flows north of the Straits of Bemini 

 upon this comparatively shallow submarine Blake Plateau,^ with 

 an average depth of about four hundred and fifty fathoms, and 

 finally pours into the deep water of the Atlantic over the edge 

 of the steep slope south of Cape Hatteras. At the same time it 

 precipitates on this slope all the silt it has carried on its bottom, 

 accumulated mainly through the decay of pelagic material and 

 the wearing action of the Gulf Stream in its course northward. 

 A similar action, but on a smaller scale, also takes place on the 

 steep western and northeastern slopes of the Yucatan Bank. 

 The shallow surface waters of a part of the Stream pour over 

 this bank, and deposit on its slopes the silt held in suspension, 

 and whatever materials are gathered along its course by its 

 action upon the smaller banks and reefs of the great bank 

 itself. 



We have, unfortunately, no very definite data regarding the 

 wearing action of water so densely charged with silt as is shown 

 by the immense quantity deposited by the Gulf Stream on the 

 northeastern edge of the Blake Plateau, just south of Cape 

 Hatteras. The Mississippi, with a depth of say five fathoms, 

 and a velocity not much greater than that of the Gulf Stream, 

 has in a couple of years dug out a depth of at least eighty feet 

 a short distance back of its bar. What may be the wearing ac- 

 tion of a mighty river like the Gulf Stream, having an average 

 depth of three hundred and fifty fathoms, and a breadth of some 

 fifty to seventy-five miles, with a velocity of five miles, it is diffi- 

 cult to say. Supposing, however, that this wearing action is 

 no greater than the aerial denudation over the area of the Mis- 

 sissippi drainage basin, — that is, at the rate of one foot in six 

 thousand years, and it certainly is not too much to assume the 



1 The different shades on the map (see Fig. 176) correspond with the respective 

 velocities of 1, 2, 3, i, and 5 knots per hour. 



