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MUSEUM OF COMPARATIVE ZOOLOGY. 145 
pouring forth their waters at about the level of mean tide. Wherever 
these underground passages are formed (and they are very numerous 
upon all the keys), the area about the neck of the orifices takes on the 
shape of the sink-holes so well known in the great cavern districts of 
Kentucky. Though these depressions in the Florida keys are never 
so large nor so shapely as the characteristic forms of a similar nature 
in older rocks, they are in many cases a conspicuous feature. Good 
examples of such structures occur on Indian Key. 
It is evident that the tendency of this corrosive action effected by 
the rain-water is to lower the central portion of each island, for the 
reason that in this interior field this water dwells longest upon the sur- 
face, and becomes most charged with carbonic dioxide. If the process 
were long enough continued, the effect would be to degrade the interior 
region, leaving the marginal portion where the rain-waters are quickly 
drained+ away in the form of a ring-like elevation. That this effect is 
not more conspicuous in the case of the Florida keys is perhaps to be 
attributed to the fact that the sea cuts back the margin of the islands 
so rapidly that time is not allowed for the development of such a topog- 
raphy. If the shores of these reefs had been protected from the action 
of the waves by the presence of the living coral, the islands would 
probably exhibit a distinct internal depression. 
The most interesting observations which I had an opportunity to 
make concerning the erosion of the elevated reefs rest upon the sec- 
tion from the western shore of Biscayne Bay into the Everglade district. 
As before remarked, the western margin of this bay is formed by a 
ridge of coralline material, which rises quickly from the surface of 
the water to the height of about twenty-two feet above the surface, 
attaining that elevation at an average distance of about half a mile 
from the shore. This versant of the Miami reef, on account of the 
steepness of its slope, provides a tolerably ready drainage for the rain- 
water, which in no case rests for any time upon the surface. Owing to 
the prevailing dryness of this well drained area, the low growing vege- 
tation is small in amount, and the scattered pine blades afford little 
woody material by the fall of its trunks, branches, and leaves. The 
result is, that, owing to the absence of the decaying vegetable matter 
‘and their brief sojourn on this surface, the rain-water obtains but little 
carbonic dioxide. 
When we pass from this relatively steep slope to the flat top of the 
reef, where the waters are less rapidly drained away, — where indeed 
during the rainy season the surface is, as I am informed, very wet, — 
VOL. XVI.—NO. 7. 10 
