SECOND ANNUM, REPORT—STRATIGRAPHIC GEOEOGY. 53 
is usually the chert which forms a nearly water-tight cap to the ar¬ 
tesian water beds in these formations. Certain horizons are abund¬ 
antly fossiliferous, containing innumerable specimens of Orbitoides 
and shells of mollusks such as Pecten poulsoni, etc. At several local¬ 
ities the rock is so soft that it can be cut into blocks with a saw; and 
upon exposure to the weather these blocks harden rapidly, making a 
very fair quality of building stone. Beds of sand, sometimes ten feet 
or more in thickness, are reported in some of the wells which pene¬ 
trate this formation. In general, these sands appear to be most 
numerous in the northwestern part of the State. 
Thickness:—The thickness of the “Peninsular'’ limestone and the 
Marianna limestone appears to be exceedingly variable. The thickness 
given by Foerste, 1 from his investigations of the Vicksburgian lime¬ 
stones in southwestern Georgia and the adjacent part of Florida, is 
220 feet, and probably this may be regarded as the approximate 
measure of the thickness of the Marianna. At Salt Mountain, Ala¬ 
bama, the rocks of the Vicksburg group are reported by Dr. Dali 2 to 
have a thickness of 140 feet, and on the basis of well borings the same 
writer estimates the thickness to be over 350 feet at Gainesville, 212 
feet at Fake Worth, and 1,068 feet at St. Augustine. From recent 
examinations of well borings by Drs. Vaughan and Bassler, limestone 
of Vicksburg age is known to have a thickness of over 225 feet at 
Quincy, 250 feet at Alachua, and 325" feet at Bartow. Apparently 
there is a marked thickening of these limestones from the exposures 
of Georgia and Alabama southward. It is hard to tell just how much 
reliance can be placed on well records, because the drill may penetrate 
some distance into a formation before characteristic fossils are ob¬ 
tained ; and it is possible for fossils to drop from the side of the bore 
and thus continue to appear in the drillings far below the base of the 
formation to which they belong. Of all the estimates given above the 
one at Gainesville is probably the most reliable because the well is 
cased to the bottom. 
Physiographic Expression: — The “Peninsular” and Marianna 
limestones are characterized by a topography produced by solution 
with numerous underground streams, natural bridges, sink-holes and 
large irregular depressions. The underground streams of these form¬ 
ations attain considerable size as is shown by a number of large 
springs which emerge, apparently from definite channels. The most 
noted natural bridge of the Marianna limestone is on the Chipola 
River near Marianna; but there are many of smaller size, both in 
1 Foerste, Aug. F., Am. Jour. Sci., 3rd ser., vol xlviii, 1890, p. 46. 
2 Dali, Wm. H., Neocene of North America, U. S. Geol. Survey, Bull. 84, 
1892, p. 103. 
