FAUNA OF OETHAULAX PUGNAX ZONE. 13 



has been identified by means of fossils in Alabama at Roberts, and probably at 

 Wallace. In wells at Mobile, fossils characteristic of the Alum Bluff forma- 

 tion were encountered between depths of 1,250 and 1,550 feet; below these is 

 limestone correlated with the Chattahoochee formation. The marine upper 

 Oligoceue is not known west of Mobile, the sediments becoming estuarine in 

 character as the axis of the Mississippi embayment is approached. The xm- 

 published results of the recent field work of Matson and the parallel paleobo- 

 tanical studies of Berry have shown that the leaf-bearing clays and sand- 

 stones near Chicoria, Wayne County, 5 miles south of Florence, Rankin County, 

 and Raglan, near McCallum, Perry County, Mississippi, are of upper Oligo- 

 cene age. The exposure at Raglan (the Hattiesburg clays of L. C. Johnson) 

 appears to represent the top of the Alum Bluff of Florida, while the one near 

 Florence is stratigraphically lower, and perhaps belongs to the upper part of 

 the Vicksburg Group. The exposure of interbedded sandstone, semiquartzitic 

 sandstone, and clay at Grand Gulf, Mississippi, is, according to the available 

 evidence, to be referred to the upper part of the lower Oligocene, and is the 

 Mississippi representative of the Catahoula formation of Louisiana at the 

 type-locality. These estuarine or fresh-water deposits of clay and sandstone 

 represent the basal portion of the Grand Gulf Group of Hilgard, which, accord- 

 ing to his definition, included the sandstone and clays lying between the Vicks- 

 burg below and the Lafayette above, but which is now known to be a series of 

 formations, including those of lower and upper Oligocene, Miocene, and Plio- 

 cene age, with perhaps some Pleistocene. * * * The detailed tracing of 

 the boundaries between the successive formations is now in progress by G. C. 

 Matson. 



Having summarized briefly the more important publications bear- 

 ing on the subject up to the present time, it remains to sketch the 

 succession indicated by paleontological research as far as it has 

 reached at the time of writing. 



It is generally agreed that in the miscellaneous series of clays, 

 marls, sandstones, limestones, cherts, and gravels of which the 

 Florida Tertiary rocks are composed the only safe and definite 

 guide to their time relations as a whole is furnished by their con- 

 tained fossils. For limited areas the sediments may afford a guide, 

 but, over the region in general, reliance can not be placed on litho- 

 logic characters unsupported by paleontological evidence. 



About 1890, at a conference called by Major Powell, then Director 

 of the United States Geological Survey, to discuss the meaning and 

 use of the term " formation " in a geological sense, after a long dis- 

 cussion in which each geologist of the survey then present took part, 

 the conclusion arrived at was that a " formation " was " a litho- 

 logic unit." However, it may be in older geological epochs, it has 

 long been recognized in Europe that in this sense there are no " for- 

 mations " in the Tertiary, with very rare exceptions ; and that even 

 these exceptions correlated by their faunas form groups which usu- 

 ally are not lithologically identical. The minor divisions are there- 

 fore generally designated by some characteristic fossil as " zones " 

 of such and such a species, and f aunally related aggregations of such 

 divisions are designated as "beds" or "groups." 



