88 
ORGANIC REMAINS. 
Having thus obtained the general principle, let us endeavour to as- 
certain the extent of its applicability, and the precautions necessary to 
ensure accurate results. 
So unequally are the different species of fossils distributed in the 
earth, that, whilst some are dispersed through several neighbouring strata, 
as clypeus clunicularis among the oolitic rocks, others are confined to one 
stratum, as ammonites calloviensis to the Kelloways rock, and some to 
a particular bed of stone, as the astreae which characteiise the coralline 
oolite. 
It is, therefore, possible, by collecting numerous specimens procured 
from a limited district, to assign to each formation of strata, single stra- 
tum, or even characteristic bed of stone, all the fossils which have ever 
been discovered in it. Such catalogues being compared, formations, 
strata, and beds, may be found to differ from one another by the presence 
or absence of particular species. A given formation may possess species 
never found in any of those above, and it may be deficient in others 
which do occur above ; and in like manner it may differ from those 
below. Hence it may be concluded, 
1. That a formation or stratum may differ from all those above it, 
by the presence or absence of certain species, and from all those below 
it, by the presence or absence of other species : 
2. That it may contain some particular species, unknown either above 
or below. We may add, that formations and strata may differ by the 
relative abundance or paucity of their imbedded fossils. 
EXAMPLES. 
1. The coralline oolite formation, as defined p. 1, appears to me to 
differ from all the formations above, by the presence of ammonites per- 
armatus, mya literata, and clypeus clunicularis, and by the absence of 
ostrea delta, hamites, and anancliytes ; and from all those below, by the 
