SECOND ANNUAL REPORT—STRATIGRAPHIC GEOEOGY. 
87 • 
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 which might be useful in the construction of 
foundations for buildings. Fossils are abundant in some parts of the 
limestone, but they are largely represented 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 partially replaced 
by chalcedony. Resting upon the “silex” at some localities 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 beautifully preserved pseudomorps 
and casts of shells. These fossils are commonly composed of chal¬ 
cedony which frequently exhibit the characteristic markings of the 
original shells. (See Fig. 2, Plate IV.) 
Beneath the limestone beds is a greenish clay which commonly con¬ 
tains a considerable admixture of sand. This clay is very plastic and 
resembles the clay which overlies the limestone. Judging from well 
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. 
Thickness: — The information concerning the thickness 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 for¬ 
mation has a known thickness of fifteen feet. The limestone between 
the “silex bed” and the upper clay is about forty feet thick. The 
thickness of the “silex bed” varies considerably, ranging from about 
four feet to more than ten feet. Beneath the “silex bed” is a lime¬ 
stone which has a known thickness of six 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 forty-one feet to 
sixty-four feet. 
Physiographic Expression: — The area underlain by the Tampa 
formation is so near sea level that no marked physiographic features 
can be discerned. The’influence of the limestone of this formation is 
seen in the rapids of the Hillsboro River; and it may be the solution 
of these limestones which gives rise to some of the depressions north¬ 
east of Tampa. Aside from these minor features, the surface of the 
formation is not very diversified. 
Paleontologic Characters:—In addition to the characteristic Or- 
thaulax pugnax, the “silex bed” of the Tampa formation has fur¬ 
nished a very large number of species, including some corals, many 
species of gastropods, pelecypods and a few specimens of Orbitolites 
floridanus which becomes abundant in the overlying limestone. At 
