MODERN SHELL-LIMESTONE. 387: 
bank of consolidated shells, chiefly of one species, is in the 
progress of formation, from which blocks may be obtained 
of great firmness and solidity (Zign. 124) ; these, when cut 
and polished (jig. 3), display a variety of markings, produced 
by the sections of the shells. Extensive shoals of loose 
shells, composed almost wholly of the Cardiwm edule, exist 
in several localities, near the embouchure of the Thames ; 
and these are continually shifting with the changes of the 
wind and tide ; it is only in a few places that consolidated 
blocks occur, like that of which a fragment is figured in 
Lign. 124. These examples of shelly limestones and sand- 
stones now in progress of formation will familiarize the 
student with the nature and origin of those ancient deposits 
of a similar character, which contain extinct species and 
genera of mollusca. 
“The vast deposits of fluviatile shells which exist in 
Florida, at Picolata, Volusia, and Enterprize are of great 
geological interest. The two latter places present bluffs and 
hills of from forty to fifty feet in height, extending half 
a mile or more from the river, that are composed of scarcely 
anything but well-preserved shells of Paludina vivipara, 
Ampullaria depressa, some undetermined species of Unio, 
Helix septemvolvis, Melania, and a few others. There is but 
a scanty mixture of earth, and the shells are clean, and 
look as if they had been washed ashore after the death of 
their inhabitants. In some places the beds are sandy, and 
are hardening into a calcareous shelly sandstone. In one 
such bed the superficial stratum furnished a few bones of 
turtles and undetermined fragments, the bones of some large 
vertebrate animal. This is, I believe, the locality where 
Count Pourtalés collected human bones in a recent sand- 
. ee No microscopical forms were detected in 
these beds after the most careful search.” * 
_ * Dr. J. W. Bailey, in Smithsonian Contributions, vol. ii. Article 
Vill. p. 23. 
