222 Hides Illustrative of Geological Phenomena. 



out its whole extent by a line of sand-banks and islands of 

 very various forms and outlines, but very uniform in their 

 mineralogical character, being composed, for the most pai*t, 

 of a fine white and very quartzose sand. On the coasts of 

 the Southern States (the Carolinas and Virginia), they form 

 a chain of low islands, separated from the coast by a series 

 of lagoons, which give a peculiar character to the navigation 

 of those districts. 



Higher up, on the southern coasts of New England, they 

 occur as submarine ridges, parallel to the coast, and sepa- 

 rated from each other by wide channels. Farther north 

 these deposits are more extensive, and form vast submarine 

 plateaux, such as the St George's and Newfoundland Banks. 

 Finally, deposits analogous to these are formed at the bot- 

 tom of the bays, but in a state of more complete trituration. 

 These are known under the name o^ flats. 



Mr Davis, after having devoted several years to the study 

 of these various species of banks, has arrived at this result : 

 that their forms, extent, and distribution, are principally de- 

 tennined by tides, — the wind and the waves playing but a 

 subordinate part in their formation. 



One of the first points on which Mr Davis insists, is the 

 relation that exists between the strength of tides and the 

 distribution of sand-banks. On both sides of the Atlantic 

 we invariably find sand-banks most numerous where the tides 

 are slight, or where their force is exhausted after having 

 been considerable. Mr Davis accounts for this in the fol- 

 lowing manner : — According to the researches of Mr Whe- 

 well, the tidal wave, on entering the Atlantic Ocean, passes 

 onward in the form of an arc, the convexity of which is turned 

 toward the north. In its progress northward, this wave 

 strikes against the coasts of the two continents of Africa and 

 America. From this shock proceed the various local cur- 

 rents, the direction and rapidity of which are determined by 

 the shape of the coasts. Their rapidity is, in general, in pro- 

 portion to the directness of the obstacles opposing them, and 

 the narrowness of the channels through which they run. 

 These tidal currents, in running with great rapidity along a 

 coast, raise up and carry with them the moveable deposits 



