Tides Illustrative of Geological Phenomena. 223 



and the detritus of all sorts which the waves and atmospheric 

 forces have detached from the beaches. These currents, 

 however, 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 de- 

 parture 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 favourable conditions for the formation of 

 sand- deposits exist where the tidal current, after passing a 

 promontory, is deflected laterally into a wide bay, where i* 

 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 neighbourhood of coral reefs. This is the rea- 

 son 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 example 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 currents, 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 with- 

 out sand-banks, unless it be in their lateral coves. 



A remarkable phenomenon takes place when the tidal cur- 

 rent flows with a moderate rapidity along a coast, so as to 

 deposit a bank of sand against the cliff^s. In this case, it is 

 not unusual to see the bank stretching 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 



