144 BULLETIN OF THE 
condition of swamp a large part of the peninsula. The St. Johns River, 
and the extensive swamps in which it heads, are in good part deter- 
mined by the existence of this barrier. In a less complete way, the 
waters of Lake Okochobee and of the Everglades to the south of it are 
likewise prevented from finding a path to the sea by this natural wall. 
Thus, at Cocoanut Grove, the waters of the Everglades at-a distance 
of only three miles from the shore in their time of lowest level lie 
sixteen feet above high tide. In the rainy season they often rise to such 
an altitude that they pour over the reef whenever it is less than twenty 
feet in altitude. A sufficiently wide canal, having a depth of twenty 
feet and a length of not over four miles, would drain the waters of the 
Everglades into Biscayne Bay. The rivers which flow over this part 
of the reef come down to the sea level over a series of rapids formed 
upon the harder layers of the reef, and thus the full escape of the 
Everglade waters is prevented. In the region more to the north, the 
entanglement of the vegetation about the head-waters of the streams, 
even where they have no rapids in their beds, likewise hinders the 
escape of the marsh waters. 
The superficial geology of the elevated reefs which constitute the keys, 
as well as the section to the west of Biscayne Bay, affords an interesting 
subject of inquiry, which, owing to my brief sojourn in this region, was 
but imperfectly followed. All the keys are evidently undergoing a 
rapid corrosion by the action of the rain-water which falls upon their 
surfaces, as well as a considerable marginal erosion by the mechanical 
impact of the waves. On all the shores, it is also evident that the sea- 
waters exercise a considerable solvent influence upon the limestone, but 
this influence is much less manifest than in the case of the rain-water in 
the interior portion of the key. In the strip immediately adjacent to 
the shore, where owing to the steepness of the slope the rain is quickly 
shed from the surface, the solvent action of the fluvial waters is rela- 
tively small; but at a little distance back from the coast, where the 
vegetation is more dense and the surface nearly level, the solutional 
work is much more manifest, and is almost always distinctly traceable. 
In time of heavy rain, the water gathers on the surface, being held there 
in part by the dense mat of low growing vegetation. . While so retained 
on the surface, it doubtless obtains a considerable charge of carbonic 
dioxide, which, as is well known, vastly increases the capacity of the 
fluid for taking lime into solution. In most cases the water is conveyed 
away through narrow crevices which penetrate the underlying rocks, 
and discharge through small caverns communicating with the shore, 
