200 
OLDER PLIOCENE PERIOD. 
[Ch. XIV. 
The other organic remains of the brown coal are principally 
fishes ; they are found in a bituminous shale, called paper-coal, 
from being divisible into extremely thin leaves. The indi- 
viduals are extremely numerous, but they appear to belong 
to about five species, which M. Agassiz informs me are all 
extinct, and hitherto peculiar to the brown coal. They 
belong to the fresh-water genera Leuciscus, Aspius, and Perca. 
The remains of frogs also, of an extinct species, have been dis- 
covered in the paper coal, and a perfect series may be seen in 
the museum at Bonn, from the most imperfect state of the 
tadpole to that of the full-grown animal. With these a sala- 
mander, scarcely distinguishable from the recent species, has 
been found. 
All the distinguishable remains of plants in the lignite and 
associated beds are' said to belong to dicotyledonous trees and 
shrubs, bearing a close resemblance to those now existing in 
the country. The same is declared to be the case with the 
remains found in the trachytic tuffs and in the trass ; but the 
absolute identification of species on which some geologists have 
insisted must be received with great caution. 
As trachytic tuff has been observed at several places inter- 
stratified with the clay beds of the brown coal formation, and 
containing the same impressions of plants, there can be no 
doubt that the oldest eruptions began when [the fresh-water 
deposits were still in progress, and when the geographical 
features of the country must have been extremely different 
from those which it has now assumed. 
We have stated that the volcanic ejections of the Roderberg 
repose upon a bed of gravel. This gravel forms part of an 
ancient alluvium which is quite distinct in character from that 
now found in the plains of the valley of the Rhine. It consists 
chiefly of quartz pebbles, and is found at considerable eleva- 
tions both on the graywacke and brown coal beds. It forms 
indeed a general capping to the latter, varying from ten to 
thirty-five feet in thickness, and was probably an alluvium 
formed at that period when the ancient lake, in which the 
