the relative Age of Tertiary Deposits. 537 
subject there is one circumstance which should not be passed 
over without consideration: supposing that the disturbing 
forces were acting with equal intensity over the area of chalk 
and coralline crag, the effect produced, so far as regards the 
removal of fossils, would be regulated by their abundance and 
by the nature of the deposit in which they were imbedded. If, 
as is really the fact, we find in the red crag six or eight per 
cent. of fossils belonging to the chalk, we may reasonably infer 
the presence of a much larger number derived from the coral- 
line beds. Were we to discover fossil shells carried down to 
the delta of a river the course of which flowed over an equal 
area of chalk and crag, we should naturally expect that the 
majority of these transported fossils would belong to the latter 
formation. 
The numerical statements drawn up by Mr. Wood have 
been made without any reference to the conditions under 
which a large number of the same fossils have been disco- 
vered in the two deposits. However abundant or naturally 
grouped a shell may occur in the coralline crag, one solitary 
specimen of that species, or even a fragment having been de- 
tected in the upper bed, at once places it on the list of those 
which are spoken of as common to the two formations; under 
these circumstances, and taking intoconsideration the probable 
extent to which the coralline beds have been broken up, I am 
only surprised that there should be so large a number as 200 
species which are only found in them and have not yet been 
observed in the rest of the formation. 
There are however some Mollusca which are either naturally 
localized, or occur in the same abundance in both divisions of 
the crag formation; and setting aside the fallacies which may 
arise from our erroneous identification of species, we are at 
liberty to infer from these the probable approximation of the 
two deposits. It appears, however, that a very large proportion 
of species may be continued through distinct and very remote 
geological epochs, for on referring to the tables of M. Des- 
hayes, we find that there are not less than 40 per cent. of 
species common to the crag and to the formations at this time 
in progress round the British islands. 
Mr. Lyell, when speaking of the newer pliocene formations, 
observes in vol. iii., page 54, °¢ It will be seen that of two hun- 
dred and twenty-six species found in the Sicilian beds only 
ten are of extinct or unknown species, although the antiquity 
of these tertiary deposits as contrasted with our most remote 
historical zeras is immensely great. In the volcanic and sedi- 
mentary strata of the district round Naples, the proportion 
appears to be even still smaller,” 
