Fossil Organic Remains. 29 
number of productions, whether this land were contiguous to 
New Holland, or to one of the Molucca Islands, or to New 
Guinea. To compare formations with relation to petrifactions, 
is to compare Floras and Faunas of different countries and of 
different epochs ; it is to resolve a problem so much the more 
complicated, that it is modified at the same time by space and 
by time. 
Among the zoological characters applied to geognosy, the 
absence of certain fossils often afford a better characteristic than 
their presence. This applies particularly to the transition rocks ; 
in general there only occur in them, madrepores, encrinites, trilo- 
bites, orthoceratites, and shells of the family of terebratules, 
that is to say shells of which some species, not identical but 
analogous, are met with in very modern secondary beds ; but 
these transition rocks are destitute of many other organic re- 
mains, which appear abundantly above the red sandstone. The 
judgment which we form regarding the absence of certain 
species, or the total absence of fossil organic bodies, may, how- 
ever, be founded upon an error which it will be of importance 
to explain here. On examining the formations which contain 
imbedded shells in a general manner, we observe that the or- 
ganic bodies are not always equally distributed in the mass ; 
but, That strata entirely destitute of shells alternate with 
others which are provided with them ; Qdly, That, in the same 
formation, particular associations of fossils characterize certain 
strata, which alternate with other strata having distinct fossils. 
This phenomenon, which has been long since observed, occurs in 
the shell limestone (muschelkalk,) and in the alpine limestone 
(zechstein), which are often separated by a bed of trochites from 
the coal sandstone, (Buch, Beob. v. i. p. 135, 146, 171) ; it is 
found also in the Jura limestone, and in several tertiary forma- 
tions. On examining the chalk of the neighbourhood of Paris 
only, it might almost be thought that the univalve shells are 
entirely awanting in this formation. The polythalamous uni- 
valves, however, the ammonites, as we have already mentioned, 
are very common in England, in the oldest beds of the chalk. 
Even in France, (coast of St Catherine near Caen), the tuface- 
ous and chloritic chalk, contains many fossils which are not met 
with in the white chalk. (Brongniart, Caracteres Zook p. 12.) 
