116 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 astreas which characterise the coralline 

 oolite. 



It is, therefore, possible, by collecting numerous specimens procured 

 from a limited district, to assign to each formation of strata, single stratum, 

 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 ab- 

 sence 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. 32, 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 ananchytes ; and from all those below, by the 



