76 BULLETIN OF THE 



northward over the shallow plateau (the Blake plateau) extending 

 north of the Bahamas to Cape Hatteras. It is this part of the Blake 

 plateau which, if I am right in tracing its past history, has been worn 

 away by the unceasing flow of the Gulf Stream. 



Thus the Gulf Stream now flows north of the Straits of Bemini 

 upon this comparatively shallow submarine Blake plateau,* of an 

 average depth of about 450 fathoms, and finally pours into tlie 

 deep water of the Atlantic over the edge of the steep slope south of 

 Capt Hatteras. At the same time it precipitates on this slope all the 

 silt it has carried along on its bottom, and which represents for the 

 greater part the wearing action of the Gulf Stream in its course north- 

 ward. 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 along the above-named slopes all the silt held in suspense, and 

 whatever materials are picked up along its course due to its action upon 

 the shallow banks and reefs of the great Bank itself 



"VVe have, unfortunately, no very definite data regarding the wearing 

 action of water charged with silt to the degree indicated by the im- 

 mense quantity of it 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. Now what may be the 

 wearing action of a mighty river like the Gulf Stream, having perhaps 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 difficult to say. 

 Supposing, however, that this wearing action is no greater than aerial 

 denudation over the area of the Mississippi drainage basin, ■ — that is, at 

 the rate of one foot in six thousand years (it certainly is not too much 

 to assume the same amount for the grinding action of the Gulf Stream), — 

 this would give us fv period of about ten millions of years since the ter- 

 mination of the cretaceous period. This estimate is probably far too 

 high, judging by what we know of the wearing action of water in 

 hydraulic sluices ; we probably have a safer estimate in a period of five 

 millions of years as indicating the time which has elapsed since the 

 beginning of the Tertiary. If we assume with Ramsay that this repre- 

 sents about one tenth of the time which has probably elapsed since 



* The different shades on the map correspond with the respective velocities 

 of 1, 2, 3, 4, and 5 knots per hour. 



