158 
FLORIDA STATE GEOLOGICAL SURVEY. 
Both flood plains and terraces are composed of coarse white sand; or, 
locally, of yellow and red sands derived from the erosion of the 
Lafayette formation. Silt and clay are comparatively rare, though, in 
a few instances the smaller swamps are underlain by these materials. 
^Shells of land and fresh water mollusks occur in the flood plain de¬ 
posits but they are seldom of importance. The fluviatile deposits of 
this age often contain more or less gravel, with phosphatic pebbles 
derived from the erosion of the Bone Valley and other phosphate¬ 
bearing formations. It was this material which was formerly dredged 
from the beds of the streams and was known as river pebble phosphate . 
Lacustrine Deposits:—The deposits here classed as lacustrine in¬ 
clude those now being formed in the numerous lakes and swamps. 
They consist of white sand derived from the beds of Pleistocene age, 
and animal and vegetable remains supplied by the growth and decay 
of aquatic organisms. The sand does not differ greatly from the gray 
sand of Pleistocene age which covers such a large part of the surface 
of the State. The organic deposits sometimes form shell marls where 
they consist of molluscan remains mixed with sand; and, in some 
localities, where animat remains predominate, they form beds of soft 
marly limestone. On the bank of the St. Johns River at the mouth 
of Blue Spring outlet in Volusia County, there is a semi-crystalline 
limestone, having a thickness of over 4 feet, overlain by 3 feet of sand. 
The limestone contains many shells of a fresh-water mollusk (Unio) 
and the sands contain shells of the common land snail (Helix albil- 
abrum ). From the position of these beds, the sand is evidently of 
Recent age and the limestone probably belongs to the Pleistocene. 
The vegetable remains form beds of peat or muck which frequently 
reach a considerable depth. They sometimes contain thin beds of sand 
which were evidently washed into the swamp and there is locally 
considerable organic silica in the form of tests of diatoms. These 
deposits which are known as diatomaceous or infusorial earth are 
sometimes mined and sold for silver polish. Such a mine was former¬ 
ly operated near Eustis; and, judging from the samples received, the 
material is of good quality. 
“Vermetus Rock”:—Under the head of Pleistocene, mention was 
made of the f Vermetus rock” which is formed of the tube-shaped 
calcareous skeletons of a gastropod. While the formation of this rock 
probably began in the Pleistocene, its deposition has continued down 
to the present time, and it forms one of the most interesting of the 
Recent deposits. 
Beach Deposits:—The Recent beach deposits consist largely of 
sand, though a friable sand rock is not uncommon; and the formation 
of coquina which began as early as the Pleistocene, has continued 
down to the present time. The gray sand derived from the Pleisto- 
