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 ccjnie 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, aifords 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 shoi*e, 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 surfoce, 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, 



