NELSON ON THE BAHAMAS. 205 



all the rain* that falls passes directly down, percolating through 

 it to nearly the sea-level. Here it meets with a body of salt water 

 that permeates the lower portions of the rocky structure, and, from 

 its lighter specific gravity, the fresh water floats upon this sea 

 water, rising and falling with the tide, but not contemporaneously 

 at all points, but at a time peculiar to each spot, according to its 

 distance from the sea and the more or less porous condition of the 

 intervening stone. In digging wells, therefore, the shaft is not carried 

 lower down after the first appearance of the " spring" than is necessary 

 to give room for the bucket, so that the salt water may not follow the 

 fresh water into the well. Both salt and fresh water may thus be 

 most certainly met with by sinking a pit at any spot, or in any 

 variety of the rock, — from the denser kind in the large islands, to 

 the mere isolated sand-bank, such as at the S.W. of Memory Rock 

 (Little Bahama Bank), where the wells are mere pits in the sand a 

 few feet deep. 



There are large caverns f in Long Cay and Rum Cay ; and pro- 

 bably caverns are as numerous in the Bahama Islands as in the Ber< 

 mudas ; but so few extensive excavations have been made, that this 

 cannot be positively affirmed. 



In their present submerged condition, such banks as those of 

 Great Bahama, Cay Sal, &c., can only be regarded as tabular rocky 

 expansions, of which the upper surface and sides are covered with 

 sand to various depths. These tabular spaces are marked out to a 

 great extent by a periphery of Cays externally, and by innumerable 

 sunken rocks ("boilers," "breakers," or "ledges") internally. 



One of the most striking objects in the topography of the Bahamas 

 is the very deep submarine valley, forming the gulf known as " the 

 Tongue of the Ocean," which runs into the Great Bahama Bank 

 from its northern end. The colour of the water around the islands 

 is usually that of the aqua-marina variety of beryl ; but the water of 

 the Tongue of the Ocean has the deep blue colour of oceanic depths. 



3. Coi'al Reefs, Coarse Limestone Rock, and Calcareous Sand 

 Rock. — The author describes a coral-reef as consisting of masses of 

 numerous species of Madrepora, Astrcea, Dcedal(sa, Oculina, bases 

 and axes of Gorgonia, Millepora, Nullipora, Corallince, &c. &c., 

 growing confusedly together without any other apparent order than 

 that of accidental succession and accretion, both laterally and vertically. 

 These are at times aided or even superseded by Serpulce, &c., as seen 

 in the serpuline reefs J. In the cavities of the mass, fragments of 

 corals, shells, and other organic remains (perfect or broken), sand, 

 comminuted shells, &c. and chalky mud find their way, and the 

 whole becomes sohdified into a compact rock § by the aid of cal- 



* From Mr. Lees' Meteorological notices already referred to, it appears that tlie 

 approximate annual quantity of rain in an average of seven years is 4 feet 4 inches, 

 giving an average of 1 inch per week. 



t Some of these are remarkable for the rude ancient Indian pictures diawn on 

 their walls. 



J See Bermuda Memoir, loc. cit. pp. 116, 117. 



§ See also Lyell's Principles of Geology, ed. 7. p. 760-765, for detailed ac- 



p 2 



