130 MANUAL OF THE MOLLUSCA. 
been thirty times submerged, and has formed part of the sea- 
bed during two-thirds of all the past geological time,—there 
will be no difficulty in accounting for the migration of sea-shells, 
or the diffusion of marine genera. 
On the other hand, it may be inferred that every part of the 
present sea has been dry land many different times; on an 
average not less than thirty times,—amounting to one-third 
of the whole interval since the Cambrian epoch. 
The average duration of the marine species has been assumed 
at only one-third the length of a geological period, and this 
harmonises with the fact that so few (either living or extinct) 
haye a world-wide distribution. 
The life of the land-snails and of the fresh-water shells has 
been of longer average extent, enabling them to acquire a wide 
range, notwithstanding their tardy migrations. 
But when we compare the estimated rate of change in physical 
geography with the duration of genera and families of shells, we 
not only find ample time for their diffusion by land or sea over 
large portions of the world, but we may perceive that such 
transferences of the scene of creation must have become in- 
eyltable. 
Method of Geological Investigation.—In whatever way geo- 
logical history is written, its original investigators have only 
one method of proceeding—from the known to the unknown— 
or backwards in the course of time. 
The newest and most superficial deposits contain the remains 
of man and his works, and the animals he has introduced. 
Those of pre-historic date, but still very modern, contain 
shells, &c., of recent species, but in proportions different from 
those which now prevail (pp. 89, 90,93). Some of the species 
may be extinct in the immediate neighbourhood of the deposits, 
but still living at a distance. 
In the harbour of New Bedford are colonies of dead shells of 
the Pholas costata, a species living on the coast of the Southern 
States. At Bracklesham, Sussex, there is a raised sea-bed 
containing 85 species of sea-shells living on the same coast, 
and 2 no longer liying there, yiz.—Pecten polymorphus, a Medi- 
terranean shell; and Lutraria rugosa, still found on the coasts 
of Portugal and Mogador. 
Tertiary Age.—lf any distinction is to be made between 
*«Tertiary” and ‘‘ Post-tertiary”’ strata, the former term should 
be restricted to those deposits which contain some extinct species. 
And the newest of these, in Britain, contain an assemblage of 
Northern shells. Professor Forbes has published a list of 124 
