Mr. Horner on the Geology of the Environs of Bonn. 465 



" plants in which the power of determination becomes much more difficult." 

 (p. 205.) He only alludes to the brown-coal of this district, by naming among- 

 the Palms " Cocos Faujasii," and under Monocotyledons of uncertain families 

 " Endogenites "; both from Liblar. M. Alex. Brongniart, who has entered 

 more fully into the subject of lignite than any other systematic writer I have 

 met with, both in a special article on lignite in the Dictionnaire des Sciences 

 Naturelles, and in his Tableau des Terrains, assigns this deposit to the plastic 

 clay ; but it is obvious that he has not visited the spot himself, and that what 

 he says about it, as well as the above notice of it by his son, is taken in great 

 part from the memoir of Faujas St. Pond, above quoted, who thirty years ago 

 visited the Liblar mine only, and in whose memoir everything that relates to 

 organic remains is far from satisfactory, in so far as regards observations on 

 the spot by the author himself. 



Now although the leaves of dicotyledonous trees may settle the question of 

 this lignite deposit being superior to the chalk, they do not fix it to any parti- 

 cular epoch of the tertiary period, unless it can be shown that they have an 

 equally limited range ; and it has not yet been ascertained what species of di- 

 cotyledonous leaves are peculiar to the earlier, what to the later beds of the 

 tertiary series, and whether there be any common to all. So far, therefore, 

 the present state of our knowledge of the fossil botany of the deposits above the 

 chalk does not afford any sure criterion for determining the age of a bed. 



The usual and most exact medium of identification, by the remains of mol- 

 luscous animals, entirely fails in this case ; for, with the exception of the sili- 

 ceous mass or blocks at Marienforst, not a single shell of any description has 

 been found in any part of this brown-coal formation*. Now these are abun- 

 dant in the lignite deposits of the Paris basin, of Soissons, of those in the mo- 

 lasse of Switzerland, and in the great deposit in the valley of the Inn at Ha- 

 ring, which may be taken as types for comparison ; and, moreover, in all of 

 these, marine shells are more or less mixed with those of fresh water. So far, 

 therefore, all proofs of similarity fail ; nor do we derive any aid from superior 

 beds, as these are entirely wanting in this district. We are thus limited to 

 the evidence which can be derived from the remains of fish, insects, reptiles, 

 and the Marienforst shells. 



Although found in considerable abundance, one species of fish only has 

 been met with in this brown-coal formation, the Leuciscus papyraceus of 

 Agassiz, and that belongs to fresh water. Now in none of the great lignite 

 deposits above mentioned have the remains of fish of any kind been yet 



* See Appendix V. p. 473. 



