u 
Baron A. Humboldt on Petrifactions: 
will throw much light on this important subject. Besides, when a 
genus of pelagic shells is supposed to be seen in the midst of a ge- 
nus of fresh water ones, it may be asked, if, in fact, the same ge- 
neric types might not occur in the lakes and in the seas. We 
are already acquainted with the example of a true fluviatile my- 
tilus. Perhaps the ampullarise and corbulse, may be found to 
present analogous mixtures of marine forms and of fresh-water 
ones. (See a Memoir of M. Valenciennes , inserted in my Re - 
cueil cV6bs. de Zoologie et d'Anatomie comparee, v. ii. p. 218). 
It results from these general considerations regarding the 
zoological characters and examination of fossil bodies, that, not- 
withstanding the beautiful and old works of Camper, of Blu- 
menbach and Sommering, the exact specific determination of 
species, and the examination of their relations with very mo- 
dern beds, was only commenced about twenty years ago. I am 
of opinion, that this examination of fossil bodies applied to all 
the other secondary and transition strata by geognosts, who, at 
the same time, consult the relative situation and mineral com- 
position of rocks, far from overturning the system of formations 
already established, would rather serve to confirm it, to perfect 
it, and to complete the vast picture which it presents. The geo- 
gnostical knowledge of formations may, without doubt, be con- 
sidered under very different points of views, according as the 
preference is given to the superposition of mineral masses, to 
their composition (that is to say, their chemical and mechani- 
cal analysis), or to the fossils which are found contained in many 
of those masses. The denominations, geognosy of situation , or 
of superposition , oryctognostic geognosy (analysing the texture 
of masses), geognosy of fossils, designate, I shall not say branches 
of one and the same science, but different classes of relations which 
are attempted to be isolated for their more particular examina- 
tion. This unity of the science , and the vast feld which it com- 
prehends , were well lenown to Werner , the creator cf positive 
geognosy. Although he did not possess the necessary means for 
giving himself up to a rigorous determination of fossil species, 
he did not cease, in his lectures, to fix the attention of his pu- 
pils upon the relations which exist between certain petrifactions 
and the formations of differeni ages. I have been witness of the 
