146 BULLETIN OF THE 



the growth of vegetation, and consequently the amount of humus upon 

 the soil, are considerably greater. The effect of the longer presence of 

 the rain-water, and the greater amount of the acids from decaying vege- 

 tation, is at once shown in the development of a great number of sink- 

 holes. Portions of the reef are so thickly set with these depressions, 

 that nearly all the rain-water appears to find its way by underground 

 channels to the sea, where we can note its emergence in great springs. 

 Descending from the summit of the reef towards the Everglades, I ob- 

 served that with each foot in height of descent the corrosive action of 

 the land water increased in amount. All portions of the reef which 

 were so situated as to be exposed to the waves of the lake which in the 

 rainy season covers this district, were very deeply corroded. Such sur- 

 faces often presented broad areas of rock so far eaten away by the 

 action of dissolving waters that, for the depth of a foot or more, the 

 remaining portion of the strata resembled the floor of a cavern covered 

 with stalagmitic materials. These decayed fragments of the rock often 

 assume curiously branched forms, and were so attenuated that the 

 pressure of the foot upon them would cause them to break down in 

 such a manner that it was impossible to walk over the surface. As 

 we approach the Everglades, the number of the sink-holes rapidly di- 

 minishes, probably for the reason that the elevation above the sea level 

 is not sufficient to impel the water to force a passage through the crev- 

 ices of the rock. Wherever the sink-holes occur, it is a noteworthy fact 

 tliat they frequently, if not generally, form the descending shaft which 

 gives exit to the waters in the central portion of some large coral. The 

 imbedded dome-shaped mass of the Meandrinas seem oftenest to be 

 chosen as the seat of these vertical shafts, which lead into the lower 

 lying caverns. 



The quantity of material taken into solution by the swamp waters in 

 the rainy season, when the flooding of this area near the top of the 

 Miami Reef occurs, may be judged by tlie tliick coating of limy mud 

 which is deposited in the occasional closed sink-holes, from which the 

 waters have disappeared by desiccation. In these depressions the layer 

 of sediment, composed in large part of lime, often attains a thickness 

 of one fifth of an inch. As it necessarily i-eprcsents the amount of lime 

 in solution by the waters in a single season of rain, we may fairly take 

 it as a measure of the solutional work accomplished in one year. The 

 facts are not sufficient to permit a qnantitativc determination as to the 

 amount of this corrosion, but I am inclined to think that we are jus- 

 tified in assuming it to be a considerable fraction of an inch in each 



