INTRODUCTION Xlll 



is to the effect that it is widely distributed westward of Gainesville, and is 

 often separated from the ' rotten limestone,' or nummulitic rock, by thin 

 beds of ferruginous sand, which sometimes come to the surface, and contain 

 boulders or nodules of bog-iron ore. 



" The ' phosphatic rock' contains but few fossils ; an echinoderm was 

 found in it by Mr. Johnson, and I found a shark's tooth of small size and 

 uncertain genus, but belonging to the group of sharks with non-serrated 

 cusps to the teeth. It also contains some silicified wood occasionally showing 

 teredo-borings, and from its surface sometimes project heads of silicified 

 corals belonging to the Astraeidae. These are called ' fossil stumps' by the 

 people of Florida. It is unquestionably of marine origin, and consists of 

 silicious grains, varying from very fine sand to small water-worn pebbles, 

 probably derived from the -silicious parts of the older Vicksburg rocks, all 

 coated with a pure white form of limestone containing variable proportions of 

 phosphoric acid. This serves also to cement the grains together, and when 

 of moderate size the rock has a striking oolitic structure. From specimens ob- 

 tained at Hawthorne, Alachua County, Dr. Hawes obtained a mean of sixteen 

 per cent, of phosphoric acid. The rock is now manufactured into a fertilizer 

 at this place. Specimens obtained loose on the surface near Archer showed, 

 according to Professor F. W. Clark, of the Survey, considerably less phos- 

 phoric acid, and a beautifully oolitic specimen from Tampa had but a trace. 

 A very similar-looking rock at Enterprise, on the east side of the St. John's 

 River basin, also showed but little. This rock, however, seemed to have been 

 of aeolian origin, as it contains large numbers of land shells of recent species, 

 and no other fossils. It occurs on the east side of the St. John's River in 

 many places, covered with from two to six feet of sand and humus, and, not- 

 withstanding its similarity of appearance and structure, cannot be synchronous 

 with the phosphatic rock of the west side of the peninsula.* The latter 

 wherever observed by me was the uppermost rock of the country, and may 

 have been formed within salt-water lagoons, bordered by coral reefs ; at least 

 all the circumstances would fit into such an hypothesis. It is of upper Oligo- 

 cene age, and should be distinguished from the lower Oligocene or Vicksburg 

 series. A notable feature of the rock is, that where it is the substratum there 

 are springs and running streams ; as soon as one comes eastward from it, and 



* It was in this rock that I'ourtales found the human bones which gave rise to so much discus- 

 sion and wide-spread error. 



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