252 VAUGHAN AND COOKE: HAWTHORN FORMATION 



1. The material exposed at Rock Island outcrops about 150 feet down 

 stream from the spring exposure. The rock was originally a yel- 

 lowish granular limestone which has been largely silicified. The 

 silicification is frequently or usually as concentric shells. In some 

 instances silicified masses are surrounded by unchanged material. 

 The echinoid genus Cassidulus is common. A number of species of 

 fossil mollusks were collected. Worn specimens of Osirea mauricensia 

 and Siderasirea sp. were also seen on the surface, but these had evi- 

 dently washed out of overlying beds and had been brought second- 

 arily into the position where seen. (The geologic age of this bed 

 will be discussed in a future publication. It is probably not of 

 Vicksburg age, as stated by Professor O'Neal.) Down stream from 

 Rock Island the dip is again down stream and the marls overlying 

 the limestone and chert are again exposed. The exposure at Rock 

 Island is due to a small anticline lying across the river course. 



The silicified corals, Siderastrea, and silicified specimens of 

 Ostrea mauricensis were seen at many places where they overlie 

 the fossiliferous marl bed which carries a fauna very similar to 

 that of the Chipola marl member and the Oak Grove sand 

 member of the Alum Bluff formation.- 



In this locality the Hawthorn formation either overlies or is 

 a part of the Alum Blufi" formation. The lithology, greenish 

 gray sand and clay — the clay frequently resembling fuller's earth 

 — ^is that usual for the typical Alum Bluff. Ostrea inauricensis 

 is a common species in that formation; and as there is no strati- 

 graphic break in the deposition, the Hawthorn formation as here 

 exposed should in our opinion be considered as only a part of 

 the Alum Bluff formation. 



Similar material, in places containing a larger proportion of 

 phosphatic sandstone, usually with silicified oysters and speci- 

 mens of Siderastrea, was found at all the localities cited by 

 Doctor Dall. At Alachua limesink the phosphatic sandstone is 

 interbedded with greenish clays. The material was traced as 

 far southward as Ocala where it immediately overlies the Ocala 

 limestone of the Vicksburg group. 



It seems definitely established that the Hawthorn formation 

 does not represent any part of the Chattahoochee formation but 



" The term Alum Bluff formation as used in this paper, in that it is made to 

 include the Chipola marl as a member, differs from the sense in which Doctor 

 Dall originally applied it. 



