Conglomerate and Gravel Deposits, 397 



these trees to places exposed to the waves, the sea accumulates 

 a beach against them, forming lagoons or lakes*. 



I have observed off Jamaica, that the coral reefs and islands 

 are protected on the side of the prevalent winds and breakers 

 by shingle beaches composed of rounded pieces, and, occasion- 

 ally, large fragments of coralt. 



From what has been above stated, it will be seen that thejsea 

 endeavours to throw back upon the land the detritus it has re- 

 ceived from it, and even, as in the case of corals and shells, of 

 hard substances that have been formed in it, and that the peb- 

 bles or shingles are not likely to quit the coast under ordinary 

 circumstances^ ; indeed the common velocity of tides seems in- 

 adequate to transport them in moderate depths, where the 

 power of the waves on the surface of the sea ceases. 



II. Action of Tideless Seas on their Coasts. 



The principal difference between these and those above 

 noticed, consists in the phenomena attendant on the discharge 

 of rivers into them, which will be noticed under the head of 

 rivers. Shingle beaches are accumulated, and protect lands 

 behind them, but from the want of tide we do not see their 

 bases, and they appear of inferior dimensions to those on tidal 

 seas. From the want of tide, which should successively pre- 

 sent different portions of a cliff to the greatest action of the 

 breakers, the destruction of coasts is not so great, and the 

 spaces of open sea being more or less limited, and the batter- 

 ing power of the breakers is greatly inferior to that of the great 

 ocean swell, discharged on a tidal coast. Still the same rejec- 

 tion of detritus derived from the land will be observed when it 

 does not fall into deep water, beyond the reach of the moving 

 power of such seas; and we know of no current sufficiently 

 strong in tideless seas to distribute the gravel that has been 

 thrown into their deep waters. 



Large lakes present nearly the same phenomena as to shin- 



* Lakes of this description at Albion, sooth side of Jamaica, contain numer- 

 ous alligators (Crocodilus actus, Cuv.) and marine fish. The zoology of some of 

 these lakes would be interesting to geologists, as they become brackish from the 

 heavy rains that drain into them from the mountain sides, and may thus contain 

 marine and fresh-water animals. 



f I remarked one or two curious instances of the growth of land behind such 

 beaches at Old Harbour. There are many islands, some covered by Mangrove 

 trees ; one in particular struck me : on its windward side (here was a beach of 

 coral shingles, evidently on the increase, the older or back part bound together 

 by tropical sea-coast creepers; behind these were the Mangroves, mixed, if my 

 recollection does uot fail me, with some other tropical sea-shore trees near the 

 beach, but alone and advancing into the sea on the leeward side of the island, 

 where, not being exposed to breakers, they accumulated silt and mud about 

 their roots, and thus extended the island in that direction. 



X Even in the case of sands, which do not enter within the scope of this 

 memoir, there is a tendency in the sea to throw them upon the land. Witness 

 the sandy Dune s, so common on various coasts. 



