742 INDEX TO THE STRATIGRAPHY OF NORTH AMERICA. 



over a large part of the peninsula, where the sole representative of the Hawtliorne formation 

 is the phosphatic or siliceous rock, the thickness is but a few feet. 



"The Chattahoochee formation consists of light-colored limestones and marls, containing 

 some thin beds of chert, clay, and sand. The colors vary from creamy white to light gray or 

 green on recently exposed surfaces to light yellow, brown, or more rarely pink on weathered 

 outcrops. Lithologically there is a gradation from nearly pure limestone to sands and clays, 

 but in general the argillateous and siliceous limestones predominate, forming impure limestones 

 or marls. The formation is in part composed of semicrystalline limestone, but soft, loosely 

 coherent rock resembling an impure chalk is more common. "Wbile chert beds occur at various 

 horizons, they are much thinner and less persistent than those of the underlying group. At 

 times organic life appears to have been abundant and hence some layers are very fossiliferous, 

 though the fossils are usually preserved in the form of imperfect casts and molds which have been 

 left by the solution of the shells. 



"Vaughan's observations alon^ the Apalachicola River show that the Chattahoochee 

 attains a considerable thickness near the type locality. 



"Well borings from Quincy indicate that the thickness of the Chattahoochee formation 

 at that locality is slightly greater than 100 feet, but here, as elsewhere, it is difficult to determine 

 the exact thickness of formations from well samples. The maximum thickness of the formation 

 is probably double the figure given above and it may be even as great as 250 feet." 



The Tampa formation, named from the city of Tampa, has as its upper member "a well- 

 stratified greenish clay containing some calcareous nodules and thin beds of limestone near the 

 base. Scattered throughout the clay are many silicified corals, some of them having a diameter of 

 2 or 3 feet. The clay is very plastic and hence is valuable for the manufacture of brick. Beneath 

 this clay is the fight-gray to yellow limestone which was formerly called the 'Tampa limestone.' 

 The 'silex bed' represents a silicified zone in this limestone and is therefore a zone of replace- 

 ment. This is well shown by some of the fossils, which have been only partially sificified, and 

 by the presence of more or less unaltered carbonate of fime in the original rock. Small nodules 

 of chert occur at other horizons in the limestone, and outcrops of the rock are often denser and 

 harder than exposures in quarries. Locally the limestone is hard enough to make a durable 

 building stone. Fossils are abundant in some parts of the limestone, but they are largely repre- 

 sented by casts and molds, which have been left by the solution of the original shells. The 

 'silex bed' contains numerous fossils which have been wholly or partly replaced by chalcedony. 

 Resting upon the 'silex' at some locafities is a siliceous residual material which was formerly 

 thought to be infusorial earth but is now known to be weathered material derived from the 

 underlying rock. In such cases the action of the percolating water has removed the matrix, 

 leaving many beautiftiUy preserved pseudomorphs and casts of shells. These fossils are com- 

 monly composed of chalcedony which frequently exhibits the characteristic markings of the 

 original shells. 



"Beneath the limestone beds is a greenish clay which commonly contains a considerable 

 admixture of sand. This clay is very plastic and resembles the clay which overlies the limestone. 

 Judging from weU records, the deposit is homogeneous, but there is a possibility that the sand ■ 

 contained in the well samples may be derived from thin sand partings in the clay bed. 



"The information concerning the tliickness of the Tampa formation is meager, but it is 

 sufficient to fix the maximum thickness at more than 130 feet. The clay bed at the top of the 

 formation has a known thickness of 15 feet. The limestone between the 'silex bed' and the 

 upper clay is about 40 feet thick. The thickness of the 'silex bed' varies considerably, ranging 

 from about 4 feet to more than 10 feet. Beneath the 'silex bed' is a limestone which has a 

 known thickness of 6 feet. The clay bed at the base of the formation has been penetrated by 

 two wells within 200 feet of each other, and the thickness varied from 41 feet to 64 feet. 



"The Alum Bluff formation consists of marl, sand, and clay, which are sometimes fairly 

 distinct but more often interbedded. Limestones also occur in the formation, but they are not 

 extensively developed and usually contain enough earthy material mixed with the carbonate 



