M. Brongniart on Fossil Organic Remains 
constantly found in agreement with their difference in the oppo- 
site case. 
However, there are cases where these two classes of charac- 
ters, without being in manifest opposition, do not perfectly 
agree. Two of these cases present themselves in the two for- 
mations which I have referred to the Chlorite Chalk. We must' 
therefore know to which of the two characters the preference 
should be given, ir> order to determine the period of formation 
of the deposit which does not present them associated ; that is 
to say, to reply to the following question : 
When, in two deposits separated from each other, the rocks 
are different in nature, while the organic remains are similar, 
should these deposits, from this difference, be regarded as of 
different formation, or should they rather, from the general and 
well determined resemblance of their fossil organic bodies, be 
regarded as of the same period of formation, when, at the same 
time, no circumstance of superposition is evidently in opposi- 
tion to it ? 
It must be kept in view, that one of the principal objects of 
geognosy, is to distinguish the different periods which succeed 
each other in the formation of the globe, and to determine what 
are the deposits which have been formed nearly at the same pe- 
riod. 
Now, it will be found, that rocks of very different natures 
may be formed at the same time, — almost at the same mo- 
ment,— not only in different parts of the globe, but also at the 
same place. 
We cannot refuse our assent to a consequence derived from 
facts, which we have before our eyes ; for all that at present oc- 
curs at the surface of the earth, belongs to the same geognosti- 
cal epoch, which commenced at the moment when our conti- 
nents assumed their present form : and although this epoch has 
a character of stability, and even of repose, which permits the 
formation of new rocks only, in a very limited number of cir- 
cumstances ; it still, however, produces enough to let us see 
that the trap-rocks formed by Vesuvius, and by the greater 
number of our volcanoes, the calcareous rocks formed by many 
of our springs, and the siliceous rocks formed by some others 
(those of Iceland, &c.), are assuredly very different from one 
