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 crey- 
ices of the rock. Wherever the sink-holes occur, it is a noteworthy fact 
that 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 the thick 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 represents 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 quantitative 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 
