464 Mr. Horner on the Geology of the Environs of Bonn, 



Of the relative Ages of the Sedimentary Rocks. 



The oldest of these is the grauwacke, and, as I have ah-eady said, it has 

 every appearance, in the more northern part of the district at least, of belong- 

 ing to the latest periods of that formation. The whole series of the secondary 

 rocks, from the grauwacke to the chalk, inclusive, is wanting ; not a trace of 

 them is to be found in situ in any part of the district *. 



This brown-coal formation has been hitherto held by those who believe in 

 the universality of tertiary formations, according to the types in the Paris ba- 

 sin, as belonging to the plastic clay ; but on what grounds I have not seen 

 stated. The clays, sands, sandstones, clay ironstone, pyrites f , gypsum, and 

 the lignite itself, when without a recognisable organic structure, prove nothing 

 as to age, for all these are found in formations much earlier and much later 

 than the plastic clay. We can found no safe reasoning upon any other ground 

 than upon a comparison of the organic remains with those in other deposits. 



The Flora of the several brown-coal or lignite deposits has not yet been 

 explored by a botanist of authority, with the exception of the short notices we 

 have received from M. Adolphe Brongniart. It is to be hoped that in the 

 future progress of his valuable general work he will direct his special attention 

 to this branch of his subject. In his Prodrome d'une Histoire des Vegetaux 

 Fossiles, published in 1828, he says, '^'From all I have yet seen I do not be- 

 " lieve that a single distinctly dicotyledonous leafh-A?, ever been found in any 

 " formation older than the chalk. Future researches may prove the existence 

 " of such vegetable productions in the secondary periods, but it is at least cer- 

 " tain that they are extremely rare, and that they did not begin to be very nu- 

 "merous until the period which succeeded the deposition of the chalk." (p. 148.) 

 In speaking of the lignite formation, including those of the molasse and plastic 

 clay, he says, "This formation is one of the richest in vegetable remains, but 

 ''it is however very rare to find them in such a state of preservation that they 

 " can be determined with any degree of precision ; or they belong to classes of 



* M. Alex. Brongniart has been much misinformed when he says, " Les masses immenses de 

 " hgnite terreux de Bruhl et de Liblar, ne peuvent etre placees que sur la crate tufau, qui est la roche 

 " fondamentale de ce pays." — Descr. Min. des Environs de Paris, 345 ; and Diet, des Sciences Natu- 

 " relies, art. Lignite, p. 386. 



t M. Alex. Brongniart says, " Les terrains thalassiques ne contiennent meme an dessus des ar- 

 " giles plastiques aucun sulfure metallique en quantite notable." — Tableau des Terrains, p. 129. Were 

 there no exceptions to this rule, the existence of pyrites would become a valuable document for 

 the determination of the ages of deposits ; but the London clay abounds in pyrites. 



