70 THREE CKUISES OF THE "' BLAKE," 



ining the great Alacran Reef,^ an excellent plan (Fig. 4A) of 

 which is given on one of the British Hydrographic maps of the 

 Gulf of Mexico, from the surveys of Commander Barrett, R. N. 

 It is one of the circular reefs resembling atolls, and I was the 

 more desirous to get an idea of its mode of formation because, 

 according to Darwin's theory of coral reefs, such atolls should 

 not occur in areas of elevation like those in which the Florida 

 reefs, the coast reefs of Cuba, the Bahamas, and the Central 

 American reefs are found. 



The examination of Alacran Reef showed it to be in full ac- 

 tivity. The greatest length of the reef is about fourteen miles ; 

 its ^\idth, eight miles. It consists of a steep semicircular wall 

 rising abruptly on the eastern side, which is also that of the 

 prevailing winds, from a depth of thirty fathoms to ten fathoms, 

 where the corals begin to grow, and the slope becomes much 

 more gradual. The eastern edge of the reef presents a singu- 

 larly regular section, where the huge masses of heads of Madre- 

 pora 2>(ilinata form a nearly compact level wall, with its top flush 

 Avith the level of the sea. This wall is constantly beaten by the 

 sea, and the breakers, pounding upon the coral heads exposed to 

 their action, detach large pieces or break off the dead masses, 

 which are again triturated into smaller fragments until they form 

 a fine sand swept by the wind in a westerly direction ; this sand 

 little by little fills up the channels left between the masses of 

 coral which form the outer wall and extend towards the centre 

 of the reef. The detritus from the outer reef diminishes gradu- 

 ally in quantity, and leaves a channel varying from three to nine 

 fathoms in depth on the eastern edge of the low islands to which 

 the reef dwindles at its western extremity. This channel de- 

 scribes deep indentations in the western edge of that section of 

 the plateau which extends from the eastern edge of the reef. On 

 the eastern edge of the islands the reef is completely choked with 

 sand, while on the gentle western sea-slope of their outer side 

 the corals are thriving, but creep out very gradually seaward in- 

 stead of building a steep, abrupt wall like that of the outer edge 

 of the reef. 



It is probable that, in the case of an atoll of this sort formed 



1 Letter No. 1, BuU. M. C. Z., V. No. 1, AprU, 1878. 



