Geology and Geography. 387 



the mountain limestone. Specimens of the plants were exhi- 

 bited, and also some interesting lias fossil fishes. 



10. M. Agassiz, in placing before the Section the fifth part of 

 his researches on fossil fishes, and also 112 plates of Enghsh 

 ichthyolites newly figured, and which will appear in the suc- 

 ceeding numbers of his work, made some observations on the 

 general results he had obtained from the inspection of the 

 greater part of the collections of Great Britain. One circum- 

 stance, which is certainly remarkable, and which gives great 

 consistence to these researches, is, that the discovery of nearly 

 400 new species of fossil fishes, has in no degree modified the 

 conclusions which the author had deduced from the examination 

 of 500 species, with which he was acquainted, when he com- 

 menced the publication of his work. The most important of 

 these results for the geologist is, that very intimate relations in 

 organization exist among fishes of the same epoch, and that pro- 

 gressive changes may be remarked in the fossils of different for- 

 mations ; so that it appears to the author possible to determine 

 the relative age of a deposit, even by the examination of un- 

 known species, and solely by the peculiarities of their organiza- 

 tion. M. Agassiz then offered some remarks on the mode of 

 deposition of the beds which contain the fossils, and on the dif- 

 ficulty of reconciling the mineralogical phenomena of stratifica- 

 tion, and the conditions of existence in which the fossils con- 

 tained in these beds must necessarily have been placed. It ap- 

 pears to him probable, that all the fossils contained in a forma- 

 tion have lived together, and perished in consequence of one ca- 

 tastrophe ; that the organic remains which have been preserved 

 in these beds, are rarely the debris of animals which have suc- 

 cessively perished during epochs of comparative repose, but ra- 

 ther the remains of those which have been suddenly buried, in 

 consequence of revolutions which produced great changes in the 

 ensemble of organic beings. It seems to M. Agassiz impossible, 

 that the animals and plants, which appear in different layers of 

 the same geological formation, could have lived successively on 

 the inferior, middle, and superior beds of that formation, since, 

 after the conditions of their existence, and the conditions under 

 which the beds must have been deposited, can neither be recon- 

 ciled, nor have alternated, as it would be necessary to admit, in 



