70 THREE CRUISES OF THE “ BLAKE.” 
ining the great Alacran Reef, an excellent plan (Fig. 44) 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 width, 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 palmata form a nearly compact level wall, with its top flush 
with 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, Bull. M. C. Z., V. No. 1, April, 1878. 
