GEOLOGY. 243 



channels through which they run. These tidal currents, in running 

 with great rapidity along a coast, raise up and carry with them the 

 movable deposits and the detritus of all sorts which the waves and at- 

 mospheric forces have detached from the beaches. These currents, how- 

 ever, soon lose their force, unless new obstacles come in their way ; and 

 in proportion as they abate, the substances held suspended, begin to be 

 deposited. Any inequality of the bottom is then sufficient to form the 

 nucleus or point of departure of a sand-bank, the direction of which will 

 be parallel to that of the current. Such, for instance, is the origin, of 

 the narrow banks bordering the island of Nantucket, and known under 

 the names of Bass Rip, Great Rip, South Shoal, &c. 



But the most favorable conditions for the formation of sand deposits 

 exist where the tidal current, after passing a promontory, is deflected 

 laterally into a wide bay, where it can expand freely. Not only the 

 heavy materials, but also the more minute particles, are then deposited 

 at the bottom of the bay ; no longer under the form of narrow ridges, 

 but as broad continuous strata or flats, generally composed of very fine 

 sand, or of calcareous mud, where the deposit takes place in the neigh- 

 bourhood of coral reefs. This is the reason why the most extensive 

 and regular deposits are found at the bottom of wide bays. Cape Cod 

 Bay, on the coast of Massachusetts, is cited by Mr. Davis as an ex- 

 ample of this mode of deposition. On the contrary, when the bay is 

 narrow, as the fiords of Norway, or when it lies in the direction of the 

 current, so as to allow the tide to rush in without obstacle and rise to 

 a great height, as, for instance, the Bay of Fundy, the ebb and flood 

 are too violent, and occasion too rapid currents to allow the water to 

 deposit any of the materials which it holds suspended. Hence it is, 

 that such bays are generally without sand-banks, unless it be in their 

 lateral coves. 



A remarkable phenomenon takes place when the tidal current flows 

 with a moderate rapidity along a coast, so as to deposit a bank of sand 

 against the cliffs. In this case, it is not unusual to see the bank stretch- 

 ing out into the sea, but, instead of following the direction of the coast, 

 it inclines, from the pressure from without, towards the interior of the 

 bay, so as to describe a bend, which the seamen of this country call a 

 Hook. Sandy Hook, in the bay of New York, is of this character. 

 Such, also, are the Hook of Cape Cod and the Hook of Holland. The 

 direction of the Hook is invariably that of the current. 



The coasts of Europe offer numerous examples of these various forms 

 of alluvial deposits. Lines of narrow banks, like those on the coasts 

 of New Jersey and the Carolinas, occur on the southwestern shores of 

 France. On the northwestern coasts of France sand-banks are rare, but 

 no sooner do we quit the Channel, than we find them scattered through 

 the North Sea. Holland is, itself, formed in a great measure of alluvial 

 sand. These deposits are formed precisely on the spot most favorable 

 to the formation of alluvial deposits, namely, where the tidal current, 

 having passed through the Channel, enters the vast basin of the North 

 Sea. 



Considered in their general connection, the alluvial deposits of a 

 continent should be looked upon as the product of a series of currents 



