142 
ELEMENTS OE GEOLOGY. 
It was soon, therefore, suspected that this fauna might be¬ 
long to a period intermediate between that of the Parisian 
and Subapennine strata, and it was not long before the evi¬ 
dence of superposition was brought to bear in support of 
this opinion ; for other strata, contemporaneous with those 
of Bordeaux, were observed in one district (the Valley of the 
Loire), to overlie the Parisian formation, and in another (in 
Piedmont) to underlie the Subapennine beds. The first ex¬ 
ample of these was pointed out in 1829 by M. Desnoyers, 
who ascertained that the sand and marl of marine origin call¬ 
ed Faluns, near Tours, in the basin of the Loire, full of sea- 
shells and corals, rested upon a lacustrine formation, which 
constitutes the uppermost subdivision of the Parisian group, 
extending continuously throughout a great table-land inter¬ 
vening between the basin of the Seine and that of the Loire. 
The other example occurs in Italy, where strata containing 
many fossils similar to those of Bordeaux were observed by 
Bonelli and others in the'environs of Turin, subjacent to 
strata belonging to the Subapennine group of Brocchi. 
Value of Testacean Fossils in Classification. —It will be ob¬ 
served that in the foregoing allusions to organic remains, 
the testacea or the shell-bearing mollusca are selected as 
the most useful and convenient class for the purposes of gen¬ 
eral classification. In the first place, they are more univer¬ 
sally distributed through strata of every age than any other 
organic bodies. Those families of fossils which are of rare 
and casual occurrence are absolutely of no avail in estab¬ 
lishing a chronological arrangement. If we have plants 
alone in one group of strata and the bones of mammalia in 
another, we can draw no conclusion respecting the affinity 
or discordance of the organic beings of the two epochs com¬ 
pared ; and the same may be said if we have plants and ver- 
tebrated animals in one series and only shells in another. 
Although corals are more abundant, in a fossil state, than 
plants, reptiles, or fish, they are still rare when contrasted 
with shells, because they are more dependent for their well¬ 
being on the constant clearness of the water, and are, there¬ 
fore, less likely to be included in rocks which endure in con¬ 
sequence of their thickness and the copiousness of sediment 
which prevailed when they originated. The utility of the 
testacea is, moreover, enhanced by the circumstance that 
some forms are proper to the sea, others to the land, and 
others to fresh water. Rivers scarcely ever fail to carry 
down into their deltas some land-shells, together with spe¬ 
cies which are at once fluviatile and lacustrine. By this 
means we learn what terrestrial, fresh-water, and marine spe- 
