Ch. XV.) 
CUAG AND FALUNS COMPARED. 
205 
fresh-water shells, together with sand and pebbles, and occa- 
sionally, perhaps, sweep down the carcasses of land quadrupeds 
into the sea. If the sand and shells, both of the 'crag' and 
the ' faluns ' have each acquired the same ferruginous colour, 
such a coincidence would merely lead us to infer that, at each 
period, there happened to be springs charged with iron, which 
flowed into some part of the sea or basin of the river, by which 
the sediment was carried down into the sea. 
Even had the French and English strata which we are 
comparing shared a greater number of mineral characters in 
common, that identity could not have justified us in inferring 
the synchronous date of the two groups, where the discordance 
of fossil remains is so marked. The argument which infers a 
contemporaneous origin from correspondence of mineral con- 
tents, proceeds on the supposition that the materials were 
either washed down from a common source, or from different 
sources into a common receptacle. If, according to the latter 
hypothesis, the crag and the faluns were thrown down in one 
continuous sea, the testacea could not have been so distinct in 
two very contiguous regions, unless we assume that the laws 
which regulated the geographical distribution of species were 
then distinct from those now prevailing. But if it be said 
that the two basins may have been separated from each other, 
as are those of the Mediterranean and Red Sea, by an isthmus, 
and that distinct assemblages of species may have flourished 
in each, as in the example above-mentioned is actually the 
case % we may reply that such narrow lines of demarcation 
are extremely rare now, and must have been infinitely more so 
in remoter tertiary epochs, because there can be no doubt 
that the proportion of land to sea has been greatly on the 
increase in European latitudes during the more modern geolo- 
gical eras. 
In the faluns, and in certain groups of the same age, which 
occur not far to the west of Orleans, M. Desnoyers has dis- 
covered the following mammiferous quadrupeds. Palceothe- 
* See above, chap. x. 
