of extensive Conglomerate and Gravel Deposits. 165 



illustrated by Mr. R. C. Taylor, in his Geology of East Nor- 

 folk. After observing that the land encroachment at Lowes- 

 toft Ness had been effected at distinct and distant intervals ; 

 that its form had been influenced by the direction of the cur- 

 rents in the channel, and the position of the adjacent shoals, 

 and that the lines of growth are indicated by a series of con- 

 centric ridges or embankments, inclosing certain areas, he 

 observes, " a rampart of heavy materials is first thrown up to 

 an unusual altitude, by some extraordinary tide, attended with 

 a violent gale. Subsequent tides extend the base, and heap 

 up lighter substances on its summit. Sand is blown from the 

 beach and fills the interstices. The Ariindo arenaria and other 

 plants, by degrees obtain a footing ; creep along the ridge, 

 give solidity to the mass, and in some cases form a matted 

 covering of turf: meanwhile another mound is forming ex- 

 ternally, and by the like process rises and gives protection to 

 the first. Occasionally the sea forces its way through one of 

 the external and incomplete mounds, but it is singular to ob- 

 ' serve how soon the breach is repaired*." 



In tropical countries the advance of the Mangrove trees 

 outwards from sheltered situations at the bottoms of ci'eeks, 

 bays, or the mouths of rivers, has a great tendency to increase 

 land at the expense of the sea. So long as the sea continues 

 sheltered, it throws up no barrier against the Mangroves; but 

 when their strange stilt-like roots have advanced the mass of 

 these trees to places exposed to the waves, the sea accumu- 

 lates a beach against them, forming lagoons or lakes f. 



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, occa- 

 sionally, large fragments of coral :{:. 



* On the Geology of East Norfolk, p. b2, and the highly instructive 

 plate 6. 



t Lakes of this description at Albion, south side of Jamaica, contain nu- 

 merous alligators (Crocodilus acutus, 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. 



X 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 there 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 not fail me, with some other 

 tropical sea-shore trees near the beach, but alone and advancing into the sea 

 on tlie leeward side of the island, where, not being exposed to the breakers, 

 they nccunuilated silt and mud about their roots, and thus extended the 

 island in that direction. 



From 



