﻿FRAAS ON THE JURA FORMATION. 



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bia, and Switzerland, traces of which continue even over Mont Jura 

 into Burgundy ; the sponge-beds, however, always keep pace with 

 the argillaceous accumulations. The Mont Jura geologists call the 

 formation Terrain argovien in consequence of its extensive develop- 

 ment in Aargau, and comprehend in this the Lower and Middle 

 " White Jura," considering the coral-reefs to be local and unimport- 

 ant accompaniments of the clay- and limestone-beds. 



In this category likewise falls a local formation peculiar to the 

 Swiss " Jura," the terrain a chailles, a bed so rich in quartz that 

 most of the fossils are silicified. These shells partly remind us of 

 the Oxford-clays ; they are, Am. cordatus, Am. convolutus, Gryphcea 

 dilatata, Trigonia clavellata, Terebr. biplicata, and T. lagenalis, 

 with Apiocrinites and Pentacrinites. This formation is clearly local, 

 and finds no parallel in other countries ; here, however, it occupies 

 the place where in the German " Jura " an indication of the same 

 fauna appears. The greatest similarity of the terrain a chailles 

 exists palseontologically with the Lower Calcareous Grit of Yorkshire, 

 a local formation between the oolitic Coral-rag and the Oxford-clays, 

 and in which also Am. cordatus, Am. convolutus, and Gryphcea dila- 

 tata are typical shells. 



III. Upper White Jura. 



Massive limestones [plumpen Fels-kalke] . Coral-rag. Groupe coral- 

 lien. Saccharoid limestone. Marble. Dolomite. 

 The unstratified masses of the Upper "White Jura" throughout 

 Germany, and as far as Burgundy, constitute a good geological hori- 

 zon. They form the picturesque rocky valleys of the Swabian and 

 Franconian Alps, and their fissures give origin to the celebrated 

 caves. These rocks are almost destitute of fossils ; in the upper por- 

 tion, however, there appear a few Corals, Radiaria, and Terebratulce. 

 The English Coral-rag has been compared to this formation, so 

 characteristic of the German "Jura;" and indeed these unstra- 

 tified rocks have had this name applied to them ; but quite erro- 

 neously, for in England and the north of France the Upper " White 

 Jura" is always bedded, and nowhere resembles the bold, massive 

 rock-ridges of the German " Jura." In the south of England the 

 Coral-rag consists of a hard, bluish-grey limestone, of a few feet in 

 thickness, full of Ostrece, Cidarites, Corals, and fragments of shells, 

 passing upwards into the Coralline Oolite, which occurs in some- 

 what greater force. In the neighbourhood of Oxford it lies im- 

 mediately upon the dark Oxford-clays with Am. perarmatus and 

 Am. ornatus. In the north of England also this formation is strati- 

 fied, and we find that the calcareous beds of the Coralline Oolite 

 are separable into three divisions. The lowest bed is the Lower 

 Calcareous Grit, distinguished by a great many small Bivalves, Gry- 

 phcea dilatata, and Ammonites, which already in the Oxford-clays 

 are found silicified. There are not yet any Corals ; these first appear 

 in the Coralline Oolite, lying above it, and, together with the accom- 

 panying fossils, remarkably agree with those of the German "Jura." 

 We here find Anthophyllum obconicum (Turbinolia dispar, Phill.), 

 Manon capitatum (Spongia Jloriceps), Lithodendron (Caryophyllia) 



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