28 FOKCE OF SUBMARINE CURRENTS 



which the velocities shall vary from at least 5 or 

 6 to 15 or 20 miles an hour, provided the depth of the 

 sea do not exceed 800 or 1,000 feet." In considering 

 the magnitude of the blocks which might be moved, 

 he found that the force exerted on a surface of given 

 magnitude increases as the square of the velocity, 

 and that it varies as the sixth power of the velocity 

 of the current. But the movements must be repeated 

 for large blocks to travel beyond short distances. 



It is evident that we have in this form of dis- 

 turbance an engine of vast power, and, though our 

 hypothesis does not deal with the greater and more 

 rapid movements and powerful currents contemplated 

 by Mr. Hopkins, we may infer what the results 

 might be with changes having even only a fraction 

 of such magnitude. Movements of this character 

 would, like Nasrnyth's hammer, be capable at times, 

 when the uplift though short was rapid, of exerting 

 enormous force ; while at other times, when the 

 uplift was slow, the action might be of the most gentle 

 description. It follows from these premises that the 

 character of the deposits formed under such circum- 

 stances will afford an approximately relative measure 

 of the velocity and duration of the currents under 

 which they were accumulated. Where, for example, 

 the sediment is fine and thin, we may conclude that 

 the velocity was slow, and the rise which gave origin 

 to it small. When, on the contrary, the materials 

 are coarse, we may suppose the rise to have been 

 more rapid and the velocity of the current greater, 

 though the movement might have been continuous. 



