MUSEUM OF COMPARATIVE ZOOLOGY. 145 



pouring foi'th theii' 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 bo 

 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 vcrsaut 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 sui'f\\ce, the rain-water obtains but little 

 carbonic dioxide. 



When we pass from this relatively steep slope to tlie flat top of the 

 reef, where the watei's are less rapidly drained away, — where indeed 

 dui'ing the rainy season the surface is, as I am informed, very wet. — 



VOL. XVI. — NO. 7. 10 



