282 On the Tertiary Strata of the Atlantic Coast. 
low lagoon was soon filled up by transported gravel and sand. The 
temperature of this region then appears to have been equivalent to 
that of West Florida at the present day, as the Rangia and Mytilus 
hamatus are common associates of the fossil, just as they now are of 
the recent oyster shells in Florida and South Alabama. 
In regard to the species varying greatly in different localities of 
the same geological age, it is easily explained by the facts which may 
be adduced in relation to the habits of recent shells. If our coast 
were now suddenly elevated, we should find spots where the shells 
would consist chiefly of an immense number of Modiola demissa mixed 
with Littorina littorea and Melampas bidentatus ; these are found on 
the margin of the lagoons at high water mark, the Modiola imbedded 
in a tenacious soil. At a little distance would be found Venus mer- 
cenaria, Mya arenarea, Solen ensis, Solecurtus caribeus ; among 
these would be Ostrea Virginiana, Fusus cinereus, and a few of Pecten 
concentricus. Such is the group existing on the sandy shore of the 
estuaries. Hard by, would be a vast deposit of oyster shellswith 
Echinus, and immense masses of Serpula. These live on the bottom 
of the lagoons, which is composed of a mixture of sand and mud. 
Then would be found another group of shells which live only in 
deep water, the Astarte lanulata, Nucula limatula, N. proxima, Car- 
dita borealis, Pholas costata, in company with great numbers of My- 
tili. This deposit we should recognize as having been formed in 
harbors, like those of Newport and Charleston, and the imbedding sub- 
stance would be clay, and the finer materials washed by feeble cur- 
rents from the rivers, which discharge their waters into these large 
estuaries. Then we should find the same shells mixed with a variety 
of other species imbedded in sand, abundantly intermixed with frag- 
ments of the same species. This would represent the bed of the 
ocean in the shallower parts along the coast ; and then perhaps farther 
to the east, other species again would be discovered, which now live 
only in the deeper parts of the sea. 
In adopting the terms given by Lyell to the European tertiary 
formations, we by no means imply that the American equivalents are 
of precisely the same age, or that they were influenced by the same 
convulsions. We only mean that they occupy the same relative po- 
sitions, and they may have all originated through the agency of con- 
vulsions not operating beyond the limits of the continent of America 
and the adjacent islands, 
