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GEOLOGICAL MEMOIRS. 



moreover, of fragments of wood and groups of Algce is evidence of 

 the former existence of an extensive shore-formation in Swabia. In 

 Switzerland and in Mont Jura, T. impressa is found towards the top 

 in the clays containing Am. Lamherti and Am. ornatus. This bed, 

 therefore, so extensive with us, is reduced to a minimum in the 

 French "Jura," and is comprehended under the name of Marnes 

 oxfordiennes. 



Beyond Burgundy a trace only is to be found either of this or of 

 the succeeding formation. 



II. Middle White Jura. 

 Scyphia-limestones and Lacunosa-beds. 

 Terrain argovien. Terrain a chailles. 



From out of the regularly stratified limestone-beds in Swabia spring 

 up a great many coral-reefs, upon which again, at some places, repose 

 regular limestone-strata. This Division of the " White Jura" is 

 the natural continuation of that below, for it retains Am. planidatus, 

 Am. fiexuosus, and Belem. hastatus. In the coral-banks of the 

 " Lower White Jura" lived numbers of small Mollusca and Radiata, 

 such as are always found on coral-reefs. As regards the German 

 " Jura," this bed is one of the most important, for the development of 

 these coral-reefs contributed, without doubt, largely to the whole 

 formation of the Alpine margins. The Spongites-beds are chiefly 

 developed in Swabia and Franconia ; from thence they may be traced 

 through Switzerland to Mont Jura as far as Burgundy, with a 

 gradual loss of force. At many places in Switzerland, as near An- 

 delot, both beds lie directly over the ornati-cl&ys, within a distance 

 of 8 — 10 feet, the latter containing Am. Lamherti and T. impressa, 

 calcified Am, plamdati, Scyphice, CnemidicB, and T. lacunosa. The 

 last place I met with T. lacunosa and the Spongites was at Chatel 

 Censoir (Dep. Yonne), where the celebrated collection of M. Cotteau 

 is. Here the ornati-cl&ys, as well as most of the argillaceous 

 beds, have disappeared, and on the top of the Forest-marble and the 

 Great Oolite lies the fissured, marble-like limestone with Spongites, 

 and immediately upon that the bed with Cidaris Blumenbachii, 

 Apiocrinites Milleri, &c. 



Further from hence we find no trace of this bed, either in Nor- 

 mandy or in England. Mr. J. Toulmin Smith* is at present engaged 

 in more particularly examining the sponges in the British Museum from 

 the Randen ; and I understand that he has never seen similar forms 

 from the Jurassic rocks of England, but that in the Greensand he finds 

 many species that are related to the Spongites of our " White Jura." 



The " Middle White Jura," closely connected with the " Lower," 

 is to be particularly looked upon as a German formation, generally cha- 

 racterized by Am.planulati, Ter. lacunosa, and Sponge-corals. Corals, 

 the indication of a shallow sea, would only first spring up from the 

 sea-bottom after the "Lower White Jura" deposits had approached the 

 sea-level. The accumulation of clay was greatest in Franconia, Swa- 



* [Compare Mr. J. Toulmin Smith's 4 Ventriculidse of the Chalk,' pp. 42, 43. 

 — Transl.] 



