ON THE GEOLOGY OF BARBADOS. 233 



From the preceding description it is clear that the rain whifh 

 annually falls on the island is sufficient to saturate the rock of which 

 it is composed, and that deep swallow-holes are in the process of 

 formation. One can readily understand that if further elevation 

 took j)lace, a suhterranean drainage would at once be established, 

 the water would be drained off from the swallow-holes, and some 

 of these would become choked with earth and rainwash, like many 

 of those in Barbados. 



Another interesting feature is the existence of an outer line of 

 reefs that completely encircle the island, and are separated from it 

 by a strip of shallow water of varying width. On the north side 

 this reef comes very near the island, but on the south side it is 

 generally two or three miles distant from it, and is in fact a kind of 

 barrier-reef, with anchorage-ground inside for small vessels in 2| 

 fathoms of water. 



The existence of this encircling reef is a demonstration that sub- 

 sidence is not a necessary factor in the formation of such a reef, 

 •which only differs from a true barrier-reef in the depth of the inside 

 channel, a factor which is quite as likely to depend on the original 

 conformation of the bottom as on subsidence. Moreover, we are not 

 left without evidence of the extremely recent date of the final ele- 

 vation which produced the low shelves that form the southern beach 

 of the island, for Schomburgk says that " on landing the beach is 

 found everywhere coated with a grey, siliceous, and calcareous sub- 

 stance (the predominant ingredients in which are clay, fragments of 

 limestone, and vegetable fibres) which seems to be deposited by the 

 waters; and as the tide retires, hardens, and assists slowly in increas- 

 ing the island .... There can be little doubt that, excepting on 

 the extreme weather face, it once covered the whole island : the 

 impression of feet and birds' claws being distinctly visible in many 

 places now overgrown with underwood and grass ; the first being 

 believed to have been left by the Indians on their occasional visits 

 already noticed, the others being recognized as those of birds which 

 still frequent the island." 



As a whole, Anegada is interesting as exhibiting an early stage 

 in the develo])ment of a raised coral-island. The coral-rock of 

 which it is composed must be based on some older formation, and its 

 aspect is so different from that of the neighbouring Virgin Islands, 

 which are steep-sided masses of volcanic rock, that its basis is more 

 likely to be a fragment of some Tertiarj' formation, with or without 

 a covering of upheaved oceanic deposits similar to those of Bar- 

 bados. 



(e) San Domingo. — The raised reefs of San Domingo were described 

 by Prof. Gabb *, under the name of the " Coast Limestone ; " and this 

 limestone is coloured separately on his map. It occupies consider- 

 able areas on the southern and south-eastern coasts, the tract east 

 of Santo Domingo city being about 80 miles long by 10 to 20 miles 



* Trans. Amer. Phil. See. 1881, n. s. vol. xv. p. 103, with map and sections ; 

 -Surveyed in 1872. 



