8o FLORIDA GEOLOGICAL SUR\EY SIXTH ANNUAL REPORT. 



resting upon the Ordovician phosphatic limestones, much as the 

 brown phosphates of the present time are formed. After this 

 residual material accumulated the area was depressed, allowing the 

 sea to cover the limestone. By the action of the waves in shallow 

 water the residual mass was thoroughly washed, the soil and clay 

 material being sifted out and carried away, while the phosphatic 

 material was left to form the phosphate rock as found at present. 

 The sea subsequently deepened, so that shales and other forma- 

 tions were* deposited upon the phosphate. 



The richest of the blue rock deposits is believed to have been 

 formed from the Ordovician limestone, known as the Leipers for- 

 mation. This formation is full of the same minute spiral and other 

 shells that occur abundantly in the immediately overlying phosphate 

 rock. Phosphates were formed from Ordovician limestone other 

 than the Leipers formation, but they are of a lower grade and at 

 present not workable. 



It would thus seem that the blue rock of Tennessee was formed 

 during the interval between the Ordovician and the Devonian, 

 v.'ashed during the Devonian, and in this condition was preserved 

 for modern mining operations. 



PHOSPHATES OF THE WESTERN UNITED STATES. • 



The extensive phosphate deposits of the western United States 

 are interbedded w'ith sedimentary formations, and to this extent 

 resemble the blue rock phosphates of Tennessee. The rock in these 

 deposits is described as prevailingly oolitic, although an exceptional 

 occurrence is recorded by Richards and Mansfield* in which high 

 grade rock was found to consist of shell fragments regarded by 

 Girty as broken shells of pelecypods. The source of the phosphoric 

 acid and the history of its accumulation in the form in which it 

 is now found in these deposits, if at present obscure, will perhaps 

 upon further investigation become apparent. 



PHOSPHATE DEPOSITS FROM GUANO. 



The phosphate deposits of Navassa, a small island in the West 

 Indies, may be mentioned as an illustration of those which are be- 

 lieved to have been formed from guano. In the case of phosphate 



*BuIl. 470, U. S. Geol. Surv., p. 376, 191 1. 



