Crosby’s description of Cuba. 309 
The second reef rises steeply, often vertically from the 
inner edge of the first, and along the north coast varies 
from 200 to 250 feet, being more affected by atmospheric 
domination. It is older than the other, and the organic 
structures therein are in part obliterated by crystallisation, 
but of their identity of origin there can be no doubt. The 
third has an altitude of about 500 feet, and is yet older, 
more solid and more crystalline. A fourth reef has an ele- 
vation probably of not less than 800 feet. These ancient 
coast-reefs, with slight interruption, extend round the whole 
coast of Cuba. Moreover, the limestone plateau of El 
Yunque is considered by Mr. Crosby to be an old coral reef. 
Its top is about 1,800 feet above the sea ; its sides for the 
upper 500 or 600 feet are an almost continuous wall of 
cliffs. 
Now, these terraces, of which the lowest is the most 
recent, obviously prove that Cuba has been elevated, and 
they mark stages in the uprising. But there is also evi- 
dence that, at the present day, the coast is sinking. This 
is indicated by the condition of the lower part of the valleys, 
which are invaded by the sea and are filled to a consider- 
able depth with laud detritus. Moreover, if El Yunque 
be an ancient reef, it is even now, after undergoing con- 
siderable denudation, more than 1,000 feet thick, and in any 
case, the third reef, mentioned above, consists of not less 
than 400 feet (in vertical thickness) of coral rock. But the 
reef-building corals do not flourish, generally speaking, in 
water deeper than about 25 fathoms. Hence, the maximum 
thickness of a coral reef would be about 150 feet, and to 
obtain even this we must assume that, from the time when 
its growth became possible, till it reached the surface, the 
bed of the sea remained at rest. Thus the conclusion seems 
inevitable that the elevation of the island of Cuba was inter- 
rupted and diversified by periods of movement in the 
opposite direction. 
