1845.] LYELL ON LAVA CURRENTS IN AUVERGNE. 79 



Gergovia is composed. I consider the section however as per- 

 fectly clear, and the Melania and other fossils are evidently im- 

 bedded in a stratum, M^hich crops out from beneath the capping of 

 basalt which constitutes the flat summit of the mountain. It will be 

 unnecessary to give a separate section, because the position of the 

 beds is fully indicated by the point A, which I have placed in the 

 upper freshwater strata below the basalt on the steep north-west 

 slope of Mont Perrier. Many palaeontologists who had been in- 

 clined to regard the strata of the Liraagne as considerably newer 

 than the tertiary beds of any part of the Paris basin, had been sur- 

 prised that a fossil eminently characteristic of the lower part of that 

 basin, and found also under similar circumstances in the plastic 

 clay of the London basin, should occur in the highest beds at Ger- 

 govia. I find however that all these Auvergne shells belong, if not 

 to a distinct species, at least to a perfectly different variety from 

 that found in the neighbourhood of Paris and London ; and what 

 renders the fact still more interesting, it is a variety which can 

 hardly be distinguished from the recent Melania of the Philippine 

 Islands, which M. Deshayes first identified with the Parisian fossil. 

 Mr. G. B. Sowerby, in the Malacological Magazine, part 1, 1838, 

 pointed out the differences existing between all the varieties of the 

 Eocene Melania inquinata then known, and the varieties of the 

 recent shell, which he proposed to name Mela7iia Philippinarum ; 

 but he now admits that the most marked of these points of differ- 

 ence cannot be detected in the specimens 1 brought from Gergovia, 

 which, although they may not be perfectly identical with any one 

 variety of the living species, would yet be considered by most con- 

 chologists as merely another variety of the same. The most obvious 

 distinction between the ordinary Paris or London basin fossil, from 

 the living Melania of the Philippines, consists in the absence in the 

 former of those spiral ridges or raised striae which in the living shell 

 intervene between the principal row of tubercles in each whorl and 

 the suture. In all the fossil individuals from England and the Paris 

 basin these are wanting, although I possess one specimen given me by 

 M. Graves from the department of the Oise in which the striae are 

 visible, though obsolete, a fact to which Prof. E. Forbes called my 

 attention, and which may lead us to suspect that a series of inter- 

 mediate gradations may one day be detected between the Parisian 

 (Epernay), the Gergovian and the recent shell. In the meantime, 

 however, we may regard the Auvergne and Philippine Island species, 

 both in the form of the volutions, the prominent striae above men- 

 tioned and other characters, as belonging to one type, while the fos- 

 sils of the Paris and London basins are referable to another. If all 

 could be regarded as one species, its cha!)ges may be compared to 

 those which the Cardiiim jyorulosum undergoes, as M. Deshayes has 

 shown, as \i appears successively in the sands of the Soissonois, in 

 the calcaire grossior, and in the upper marine formation near Paris. 

 No argument can therefore be founded in favour of the identify in 

 ago of the Parisian and (Jergovian beds Irom the occurrence in both 

 of this Melania, as the Auvergne variety is so distinct. This modi- 



