Ch. XXIII.] 
SECONDARY FOSSILS. 
327 
great period ; and, consequently, the coal, which alternates in 
some districts with mountain limestone, and the old red sand- 
stone which intervenes between the mountain and transition 
limestones, will be considered as belonging to the same period. 
The coal-bearing strata are characterized by several hundred 
species of plants, which serve very distinctly to mark the vege- 
tation of part of this era. Some of the rocks, termed graywacke 
in Germany, are connected by their fossils with the mountain 
limestone. 
With this group we shall conclude our enumeration for the 
present ; for although other divisions may hereafter be requi- 
site,, we are not aware that any antecedent periods can yet be 
established on the evidence of a distinct assemblage of fossil 
remains. Traces of organization undoubtedly occur in rocks 
more ancient than the transition limestone, and its associated 
sandstones, called graywacke; but we cannot refer them to a 
distinct geological period, according to the principles laid down 
in this work, until we have obtained data for determining the 
specific characters of a considerable number of fossil remains. 
In reviewing the above groups we may first call the reader's 
attention to the important fact stated on the authority of M. 
Deshayes, that no species of fossil shells has yet been found 
common to the secondary and tertiary formations*. This 
marked discordance in the organic remains of the two series 
is not confined to the testacea, but extends, so far as a careful 
comparison has yet been instituted, to all the other departments 
of the animal kingdom, and to the fossil plants. I am in- 
formed by M. Agassiz, whose great work on fossil fish is 
anxiously looked for by geologists, that after examining about 
500 species of that class, in formations of all ages, he could dis- 
cover no one common to the secondary and tertiary rocks ; 
nay, all the secondary species hitherto known to him, belong to 
* M. Deshayes assures me that he has seen no tertiary shells in the Gosau beds, 
supposed by some geologists to be intermediate between the secondary and tertiary 
formations ; but that some of the most characteristic species of Gosau occur in the 
green-sand beneath the chalk, at Mons in Belgium, 
