Geological Society of France for 1847. 157 



heights, the upper and Oxfordian divisions being alone visible, whilst 

 the two lower groups occur more eastward in the lower range of the 

 department of the Ain. The lower part and the great valleys of the 

 Dole and the Keculet are composed of neocomian rocks, and the 

 author infers that when the latter were deposited the mass of these 

 mountains and of Mont-Credoz formed an island bathed by the sea, 

 in which the neocomian deposits were effected. M. Marcou points 

 out, that while the Oxfordian group, surrounding the ancient Hercy- 

 nian and Vosgian islands, is characterised by a considerable develop- 

 niLiit of marls, containing numerous pyritous fossils, the same group 

 of the Jura is formed of a great thickness of greyish-blue limestones, 

 more or less compact or marly. This difference of lithological 

 structure the author attributes to the more littoral character of the 

 Hercynian and Vosgian deposits, and the moi'e pelagic conditions 

 under which the accumulations of the Jura were effected at the same 

 date. After pointing out these changes, M. Marcou states that the 

 pala?ontological character of the deposits corresponds with their litho- 

 logical. Thus, in the littoral regions, the species are numerous, and 

 more especially belong to the cephalopods, the gasteropods, and ace- 

 phala ; in the subpelagic regions, the individuals have much dimi- 

 nished, the cephalopods are stunted, and many species which did not 

 occur in the littoral districts appear, as also many spongy polypifers 

 and large Terebratulse. In the localities occupied by the deep seas, 

 such as those of the Colombier and the Reculet, fossils are extremely 

 rare ; those found are exclusively cephalopods, and hitherto confined 

 to two species of Ammonites and one species of Nautilus. 



The upper group of the Jurassic rocks occupies the highest crests 

 and summits, and is formed of a great mass of compact limestones, 

 without any interstratified marls. Although the mass is not very 

 divisible into sub-groups, that considered equivalent to the coral rag 

 of England, is stated to be distinguishable both from its position 

 above the Oxfordian group, and from its fossils. The author subse- 

 quently enters into a detail of the various dislocations and contor- 

 tions which the Jura may have suffered. 



M. Scheerer discusses the plutonic nature of granite, and of the 

 crystalline silicates associated with it, in a communication translated 

 by M. Frapolli. In the first pai't of this memoir, the author enters 

 upon an investigation of a peculiar kind of isomorphism, an account 

 of wliich it is difficult to present without the formulae and detail em- 

 ployed. Referring to the composition of dichroite and aspasiolite, 

 he states, that while in the proportions of silica and alumina, they 

 are the same, the aspasiolite contains a considerable quantity of wa- 

 ter ; both minerals also possessing, though in different proportions, 

 magnesia and oxide of iron, the former, some lime, the latter, only 

 traces of it ; and he asks if the differences in other characters of 

 these minerals may not be due to the water acting as an isomorphous 

 base as regards the magnesia, oxide of iron, &c. After investigating 



