EARLIER TERTIARY (EOCENE AND OLIGOCENE). 743 



of lime to form marls. Shell marls with a calcareous or sandy matrix are common and they 

 often occur interbedded with nearly pure sand. In general the beds belonging to this formation 

 are light gray, but occasionally shades of green or yellow prevail. * * * 



"The typical Alum Bluff formation consists of coarse light-greenish gray to white argil- 

 laceous sands, often showing cross-bedding and usually containing more or less interbedded 

 clay and fuller's earth. One of the most characteristic features of the sands is the presence 

 of innumerable flakes of white mica — the 'isinglass' of the well diggers. The change from the 

 shell marls of the Chipola marl member is by & transition zone which contains some of the same 

 species of shells which characterize the marls. This zone also contains nodules of calcium 

 carbonate which often inclose fossils. The upper part of the sand is usually free from shells 

 but occasionally contains impressions of leaves and fragments of plants. Locally the Alum 

 Bluff formation contains some clay, and near Chattahoochee it consists of greenish sticky marl. 



"The fuller's earth has the appearance of a dense, hard fine-grained siliceous clay. It 

 is thinly laminated and commonly light gray to greenish in color. Sand partings sometimes 

 occur, but they are comparatively rare, the material usually being homogeneous. Beds of sand 

 and clay are commonly associated with the fuller's earth, the sections consisting of interbedded 

 sand and clay. 



"The aggregate thickness of the Alum Bluff formation is at least 135 feet, but the maximum 

 thickness of a. single section is scarcely one-half that amount. It is doubtful if the total thick- 

 ness is represented in any single section. The thickness of the sands of the Alum Bluff forma- 

 tion at the type locality is about 20 to 25 feet, but farther north, at Rock Bluff, Dall reports 

 a maximum of 63 feet. The fuller's earth commonly occurs in beds of 2 to 10 feet in thickness 

 associated with several feet of clay and sand or sandstone. In some places two or more beds 

 occur in the same section, separated by beds of sand and clay. The maximum observed thick- 

 ness of fuller's earth in a single section is about 15 feet, and the aggregate thickness of the 

 associated clays and sand which appear to belong to the same horizon is not less than 20 feet." 



Three members of the Alum Bluff formation have been recognized — the Chipola marl 

 member, the Oak Grove sand member, and the Shoal River marl member. 



"The Chipola marl member, named from Chipola River, forms the basal portion of the Alum 

 Bluff formation and rests conformably upon either the Chattahoochee or the Hawthorne forma- 

 tion. At the type locality of the Alum Bluff formation it constitutes the basal division of that 

 formation, but farther north it thins and permits the sandy beds of the Alum Bluff to lap over 

 on the Chattahoochee formation. 



' ' When fresh this member consists of a light-gray to yellow marl, containing many shells 

 and shell fragments. The matrix is composed of calcareous clay containing a small percentage 

 of fine sand. When weathered the marl becomes dark yellow or reddish yellow from the presence 

 of hydrated iron oxide. The character of the deposit indicates comparatively quiet water, with 

 conditions especially favorable for the development of organic life. In some localities this 

 horizon is represented by a very sandy marl." 



This member is not known to have a thickness of more than 15 feet and the average is 

 probably only a few feet. The limestone belonging to it is so imperfectly known that a satis- 

 factory estimate of its thickness is difficult to make. The maximum exposure which has been 

 reported is about 10 feet. 



The Oak Grove sand member, named from Oak Grove, on Yellow River, has been correlated 

 on paleontologic grounds by Dall with the typical sands of the Alum Bluff formation. This 

 member consists of fine-grained light-gray to gieenish sands containing many excellently pre- 

 served shells. As it is not fully exposed at' the type locality, where a thickness of only 2 feet is 

 visible, it may attain a greater thickness. The fauna of the Oak Grove sand member is closely 

 related to that of the Chipola marl member, but it contains large species of both Cardium and 

 Lyropecten, which appear to foreshadow the large species of those genera occurring in the Miocene 

 marls. 



"The Shoal River marl member, which has recently been recognized by Vaughan, lies 

 stratigraphically about 30 feet above the Oak Grove sand member. It thus forms the upper 

 part of the Alum Bluff formation. In the section at Shell Bluff, on Shoal River, the lower sand 



