44 AQUEOUS AGENCIES. 



British Channel and Straits of Dover (Fig. 35). These two currents 

 coming from opposite directions meet and make still water, and there- 

 fore deposit their sediment and form banks. Again, the tidal current 

 is concentrated in the British Channel, and runs with great velocity, 

 scouring out this channel, and in addition gathering abundant sediment 

 from the rivers emptying into the channel. Thus loaded w r ith sedi- 

 ment it rushes through the narrow Straits of Dover, and, coming in 

 contact with the still w r ater of the German Sea, forms eddies on either 

 side, and deposits its sediments. Besides the banks thus formed, there 

 are, of course, bars formed at the mouths of the rivers emptying into 

 this shallow sea. By a combination of all these causes, w r e explain the 

 numerous banks which render the navigation of this sea so dangerous. 

 But great banks far away from shore are usually formed by oceanic 

 currents. Thus the Banks of Newfoundland are evidently formed, 

 partly at least, by the meeting of the polar current (e, Fig. 34), bearing 

 icebergs loaded with earth, and the w r arm current of the Gulf Stream, 

 perhaps also bearing its share of fine sediment. Again, the Gulf 

 Stream, rushing at high velocity (four miles per hour) through the 

 narrow Straits of Florida, coming in contact with the still water of 

 the Atlantic beyond and forming eddies on each side, and depositing 

 sediment, has certainly contributed to form, if it has not wholly formed, 

 the Bahama Banks on one side, and the bank on which the Florida 

 reefs are built on the other. It is probable that many other peculiari- 

 ties of the Atlantic bottom in the course of the Gulf Stream may be 

 similarly accounted for.* 



Land formed by Ocean Agencies. — Upon submarine banks, however 

 these may be produced, are gradually formed islands. These islands 

 are always formed by the immediate agency of waves. As soon as the 

 submarine bank rises so near the surface that the w r aves touch bottom, 

 and form breakers, these commence to throw up the sand or mud until 

 an island is formed, which continues to grow by the same agency, until 

 it becomes inhabited by plants and animals, and finally by man. The 

 height of such islands above the sea will depend upon the height of the 

 tides and the force of the waves. They are seldom more than fifteen 

 feet above high water. Thus, we find that extensive banks are always 

 dotted over with islands. In this manner are formed the low islands so 

 common about the mouths of harbors and estuaries, also the narrow 

 sand-spits all along our Southern coast, separating the harbors and 

 sounds from the ocean. Fig, 36, which is a map of the North Carolina 

 coast, will give a good idea of these sand-spits. In the course of time 

 such sounds, being protected in some measure by the sand-spits from 



* See the author's views on this subject, American Journal of Science, vol. xxiii, p 

 46, 1857; Nature, vol. xxii, p. 558, 1880; Science, vol. ii, p. 764, 1883. 



