214 JM. Alen. Brongnart sur les caracteres Zoologiyues, &c. 
formations which M. B. refers to chalk, are found in very 
distant localities and with very different characters of con- 
sistence, stratification, colour, super-position, &c.3 yet 
from similarity or identity of the fossils imbedded in them, 
he refers them all to the general head of the Chalk 
Formation. He divides this formation into 3 sub-forma- 
tions—the superior or White Chalk—the middle or Chalk - 
Tufa—and the inferior or Green-sand, (Glauconie crayeuse 
Br.), i.e. chalk mixed with green grains ascertained by 
M. Berthier, to be silicated hydrate of iron, and not chlo- 
rite, as had beea supposed. ‘These 3 sub-formations inclose 
fossil species which are in part different in each, and in 
part common. 
He first considers the value of Zoological characters tr 
geology. 
The species of shells, zoophytes, &c. found in forma- 
tions of different periods, differ from each other in propor- 
iion to the difference of time between the deposition of 
the formations, asin proportion to their perpendicular dis- 
tance. This law is now well established, and its apparent 
exceptions have been in most cases accounted for by the 
particular circumstances attending them, and have thus 
been reduced to the general rule. It follows from this 
that the different formations, which overlie each other on 
the earth’s surface, were formed at different and distant 
periods for they are marked by a succession of generations 
possessing distinct and peculiar characters, which could 
have arisen only from the slow and almost imperceptible 
change in organized beings during the course of ages. This 
difference in the character of the imbedded fossils is 
usually accompanied avith other mineralogical differences, 
suchas chemical compositions, structure in mass, order of 
superposition, accompanying minerals, &c. ; but some- 
times these differences are in apparent opposition to the 
geological differences derived from the imbedded fossils.— 
‘The question to be answered then, is the following : 
‘‘ when, in two distant formations, the rocks are different 
in mineralogical characters, whilst their organic remains 
are analogous, ought we to regard them as of distinct for- 
mation, or, on account of the general well ascertained re- 
semblance of the fossils imbedded, to consider them the 
same period of formation, when the order of superposition 
