572 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



MIOCENE. 



ALUM BLUFF FORMATION. 



In the foregoing paragraph and on pages 219-220, as a part of the 

 discussion of the fossil coral-faunas, I have referred the Chipola marl 

 member of the Alum Bluff formation to the basal part of the* Mio- 

 cene — that is, I correlate it with the base of the Burdigalian of 

 European nomenclature. Unfortunately, information on the basal 

 contact of the Chipola is not adequate. According to the description 

 by Matson and Clapp 1 it conformably overlies the Chattahoochee 

 formation. In 1900 I examined the exposure at the type locality, the 

 McClelland farm on the west side of Chipola River, just south of Ten- 

 mile Creek, Calhoun County, Florida, and corroborated the previous 

 observations of Dall and Stanley-Brown that the marl immediately 

 overlies limestone at the top of the Chattahoochee formation, but 

 did not study the nature of the contact in sufficient detail. Although 

 the evidence is not definite, it is probable that the contact is one of 

 erosion unconformity. 



As regards the Mollusca of the Chipola marl, Miss Julia Gardner, 

 who has almost completed a monographic account of them, furnishes 

 me the following statement : ' ' The earlier investigation of the Chipola 

 fauna indicated that ' about 50 per cent of the species in the Chipola 

 beds are peculiar to them; of the others the larger proportion are 

 common to the Tampa Orthaulax bed while in the subsequent Oak 

 Grove sands about 24 per cent of the Chipola species survive.' 2 



"Further investigations have, as is usually the case, materially 

 increased the percentage of peculiar forms and materially diminished 

 the percentage of species common to other horizons. The work 

 upon the Chipola fauna is not yet complete but there is every reason 

 to suppose that at least 75 per cent of the species are restricted to 

 the single horizon. Twenty-three of the Tampa gastropods have 

 been considered identical with those from the Chipola. In 18 out of 

 the 23 the resemblances between the Tampa and Chipola forms are 

 too slight to justify their inclusion under the same specific name. 

 Two other species must be discarded for the present, because it has 

 been impossible to find the Tampa individuals referred to them. 

 Only 3 of the 23 remain; Stromhus cJiipolanus is represented in the 

 Tampa beds by material too imperfect to determine with complete 

 assurance; XenopJiora conchyliopliora is a species which has per- 

 sisted with no perceptible change of character from the Upper 

 Cretaceous to the Recent; Tegula exoleta apparently initiated in the 

 Tampa persisted throughout the Miocene. The relation between the 



i Matson, G. C, and Clapp, F. G., A preliminary report on the geology of Florida, with special refer- 

 ence to the stratigraphy, Florida Geol. Survey 2d Ann. Rept., pp. 102, 103, 1910. 



8 Dall, W. H., A monograph of the molluscan fauna of the Orthaulax pugnax zone of the Oligocene of 

 Tampa, Fla., U. S. Nat. Mus. Bull. 90, p. 8, 1915. 



