8G 
FLORIDA STATE GEOLOGICAL SURVEY. 
and the existence of clay beds at both the base and top of the forma¬ 
tion. The limestone below the “silex bed” is similar to what Dali 1 has 
called “Cerithium rock,” and in this connection it is interesting to note 
what he has said concerning its existence: 
From these observations it appears that, while the existence of a Cerithium 
rock under the Orthaulax bed is a priori probable, sufficient evidence of its 
existence is still to be collected, and the rock identified as such by Heilprin 
may very possibly have been a portion of the Tampa limestone. 
Since the publication of the report from which this quotation is 
taken, a series of wells have been drilled, and the samples which 
were preserved show the presence of the limestone below the silex. 
Stratigraphic Position:—Evidence of an unconformity at the base 
of the Tampa formation was obtained in drilling wells for the city of 
Tampa. The log of one of these wells is given on page 89, and it 
will be noted that after passing through thirty feet of limestone and 
chert the drill encountered a blue clay forty-one feet in thickness. 
The limestone and chert represent the limestone and “silex bed” of 
the Tampa formation and the clay appears to belong at the base of 
that formation. At a distance of 200 feet from the well mentioned 
above, the clay was encountered at about the same depth and was said 
to have a thickness of sixty-four feet. This variation shows that the 
underlying limestones of the Vicksburg group have an irregular sur¬ 
face which was doubtless produced by erosion. 
The relation of the Tampa formation to the Hawthorne and Chat¬ 
tahoochee formations has already been discussed, and its stratigraphic 
relation to the Alum Bluff formation is probably similar to that of the 
other two formations mentioned. The post-Oligocene formations, 
which occur in the area where the Tampa formation is known, rest 
unconformably upon it. 
Lithologic Character:—The upper member of the Tampa forma¬ 
tion comprises a well stratified greenish clay containing some calcare¬ 
ous nodules and thin beds of limestone near the base. Scattered 
throughout the clay are many silicified corals, some of them having 
a diameter of two or three feet. The clay is very plastic and hence 
is valuable for the manufacture of brick. Beneath this clay is the 
light-gray to yellow limestone which was formerly called the “Tampa 
limestone.” The “silex bed” represents a silicified zone in this lime¬ 
stone and is, therefore, a zone of replacement. This is well shown 
by some of the fossils which have been only partially silicified; and 
by the presence of more or less unaltered carbonate of lime in the 
original rock. Small nodules of chert occur at other horizons in the 
1 Dali, Wm. H., Neocene of North America, U. S. Geol. Survey, Bull. 84, 
1892, p. 119. 
