200 OLDER PLIOCENE PERIOD. [Ch. XIV. 



The other organic remains of die brown coal arc 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 k 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 quart/ 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 



