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GEOLOGICAL MEMOIRS. 



is developed in the Upper " Jura." In like manner as the zoological 

 character of the Diceras (according to Ewald) unites the Bivalves of 

 the " Jura " with those of the Chalk, so also, in a geological point of 

 view, this shell, by its occurrence only in the vicinity of the Cre- 

 taceous Formation, is an intermediate link between [vermittelt] these 

 two extensive systems. The Diceras-gvouj) with the Pillar- corals, as 

 a parallel to the more common Star-coral group, constitutes, together 

 with the latter, the important and far-spread Coml-facies of the 

 Upper " White Jura ; " and it forms also, by its being connected 

 with Molluscs of all kinds, the transition to the second great Fades. 



2. The Mollusc-facies. — Where the before-mentioned unstratified 

 rock-masses [plump en Fels-Massen] are not covered by Coral-reefs, 

 they are overlaid with extensive calcareous strata, often in great force, 

 in which Univalves and Bivalves play the principal part. We soon 

 meet with Oyster-banks, Exogyrce, warning us of our approach to 

 Chalk, sometimes with beds of Gasteropoda and Bimya ; here once 

 more with the gigantic Cephalopods, there with little fragile Bivalves. 

 In Germany this condition obtains in the North only, — in Hanover 

 and Brunswick. We find it mostly developed in Switzerland and in 

 Mont Jura, where the stratified Upper or "White Jura" far sur- 

 passes all the other members of this formation, and where whole 

 mountains are formed of hard, whitish-yellow, marble-like limestone- 

 masses. From thence it traverses France, being at a few places only 

 interrupted by coral-banks, enters Calvados, crosses the Channel, and 

 immediately in the south of England forms the peninsula of Portland; 

 thence it extends northwards as far as Yorkshire, where Corals again 

 appear. 



We find three principal names applied to this Fades ; " Sequa- 

 nian," " Kimmeridge," and " Portland," which will serve to point 

 out its most important local modifications. In the Bernese Jura and 

 Mont Jura, for a better knowledge of which science is especially in- 

 debted to Thurmann's researches inBruntrut, immediately above the 

 Coral-rag appear whitish-grey clays, alternating with calcareous 

 shales, the latter especially increasing in force towards the top. This 

 is the Groupe sequanien, characterized by Astarte minima, Apiocri- 

 nites Meriani, and Exogyra Bruntrutana. The alternation of whitish- 

 grey clays and compact limestones of the same colour continues also 

 through the two following groups, the " Kimmeridge" and the 

 " Portland;" no mineralogieal differences can be anywhere observed, 

 the separate groups being characterized only by the different organic 

 remains. In the " Kimmeridge," which is next in order, the Acepha- 

 lous Fauna is represented ; Ostrea solitaria, Ceromya, Pleuromya, 

 Pholadomya Protei, and Ph. truncata occurring in these clays ; 

 whilst in the clays of the " Portland," numbers of Gasteropods, 

 Pteroceras, Natica, and Nerincea, have been principally developed. 

 These three groups, however, continually pass into one another, and 

 their boundary-lines, owing to the many overthrows and dislocations, 

 are either with difficulty, or not at all to be found. Indeed this 

 grouping results rather from the necessity of systematizing these 

 great masses, so similar in external characters, than from the presence 

 of any really existing divisions. 



