266 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



the "silex" bed and limestone of the Tampa formation. Corals are 

 sufficiently abundant to justify being designated "reefs" at several 

 localities, the most important of which are 18 miles south of Talla- 

 hassee, Florida, in several counties in southern Georgia, and at 

 Tampa, Florida. Coralliferous limestone of the same or nearly the 

 same age is exposed one-half mile south of River Junction, Florida, 

 and at old Jacksonboro, Georgia. Well borings in Tampa show that 

 beneath the coralliferous limestone is a variable thickness of clay 

 which overlies the irregular surface of the Ocala limestone, indicating 

 subaerial erosion, followed by submergence. The coralliferous beds 

 are stratigraphically below the next younger set of deposits grouped 

 under the Alum Bluff formation, indicating the continuation of sub- 

 sidence after the formation of the reefs. The thickness of the reefs 

 and coralliferous beds is not great, perhaps between 10 and 20 feet. 

 The fauna comprises about 20 species of corals. Where not silicified 

 and its character may be studied, the limestone associated with the 

 corals is of complex origin. It is partly organic, probably in part a 

 chemical precipitate, and contains terrigenous impurities. This indi- 

 cates that the reefs and corals of this period grow during subsidence 

 on a previously formed platform, but possess greater value for their 

 aid in stratigraphic correlation than as constructional agents. 



The Alum Bluff formation, which, in my opinion, is of Miocene 

 age, according to the usage adopted by the United States Geological 

 Survey is subdivided into three members, which named from the 

 bottom upward are the Chipola marl, Oak Grove sand, and Shoal 

 River marl. The basal Chipola marl member was known only in 

 an area extending from Alum Bluff on Apalachicola River westward 

 to Chipola River until it was recently identified by Miss Julia Gard- 

 ner from a collection made by Dr. E. H. Sellards at Boynton Landing 

 on Choctawhatchee River, in Washington County. The bed on 

 Chipola River seems conformably to overlie the Chattahoochee forma- 

 tion, it is conformably overlain by higher beds of the typical Alum 

 Bluff formation, and is between 15 and 17 feet thick. Of the four 

 or five species of corals found at this horizon, one is of reef f acies, a 

 massive species of Goniopora. Subsidence was in progress while 

 these coralliferous beds were being deposited. 



Before completing the discussion of the Alum Bluff formation cer- 

 tain events antecedent to its deposition in central peninsular Florida 

 should be stated. Previous to the deposition of Chattahoochee and 

 Tampa sediments, the Ocala limestone was deformed with the pro- 

 duction of a low, elongate dome, the axis of which extends from 

 near Gainesville to near Ocala. On both the east coast and the 

 west coast along an east-west line through Gainesville the surface 



