INSTITUTE OF SCIENCE, PHILADELPHIA. 33 



and there exposes the older fossiliferous deposits beneath ; these, how-- 

 ever, are practically all concealed beneath the water's level. 



The fresh-water limestone forms the bed-rock of the beautiful "prairie " 

 or meadow land which opens out immediately above Fort Thompson, 

 and which soon passes off into the region of endless swamps and ever- 

 glades that continue to the Okeechobee wilderness. There can be little 

 question, it appears to me, that this vast area of scattered ponds and 

 swamps marks the site of an ancient continuous, or nearly continuous, 

 body of fresh water, which covered the region in the form of a vast shal- 

 low lake, and whose origin is probably to be traced back to the period 

 when the land gradually emerged from the sea. The general configura- 

 tions of the country, and the broad extent over which the limestone (or 

 its remains) is spread, leave little doubt in my mind as to a former union 

 of the present scattered waters, whose isolation may have been brought 

 about principally as the result of vegetable growths, or of this in combina- 

 tion with actual desiccation. 



The limestone has been traced eastward, as reported by Captain 

 Menge, the officer in charge of the dredging operations connected with 

 the Okeechobee Canal, for a considerable number of miles, disappearing * 

 at a depth of five feet two inches beneath the canal surface, about three 

 miles west of Lake Hikpochee. We, ourselves, traced the extension of 

 the limestone for nearly this distance by means of the scattered shell 

 remains (fossils), which at intervals were dredged up from the bottom of 

 the canal. All the molluscan forms occurring in the limestone are 

 identical with species now living in the river, and consist mainly of 

 Planorbis (Pliysa) scalaris, innumerable shells of which, evidently distrib- 

 uted at a period of recent high-water, are scattered over the open tracts, 

 and in crevices on the trunks of trees. I obtained specimens from tree- 

 trunks at an elevation certainly not less than 10 or 12 feet above water- 

 level, but the high-waterline marked on the palmetto trunks the traces 

 of a recent overflow was still much above this, probably fully six or 

 eight feet. 



That is to say, had not been traced further, hut there can be no question as to its 

 extension beyond this point. 



