﻿FRAAS ON THE JURA FORMATION. 



51 



conspicuous ; and is accompanied by Am. natrix, Am. capricornus, 

 Am. Valdani (4-5 inches in diameter), Am. ibex, Am. centauries, 

 together with a great quantity of Terebratula: rimosce and cinctce, 

 Spiriferi, Plicatula, and Lima. All of these are preserved with 

 their shells, and their appearance and conditions remind us of the 

 celebrated petrifactions at Fontaine Etoupefour, in Calvados. We 

 are indebted to M. Sigmund von Schieder, of Amberg, for the 

 more particular knowledge of this place. Aschach is the only known 

 locality in Franconia where the middle Lias is developed with the Am- 

 monites of the numismalis-ciays. In general we are but indistinctly 

 guided until we reach the blue clays lying at the top of it, with 

 Am. costatus, which is the most important Franconian type, and 

 everywhere serves as a sure guide at the slopes of the Danube and 

 Maine canal, near Altdorf, Bamberg, and Banz. By means of A. costa- 

 tus also a bed of red iron-ore, found at a not less remarkable place 

 in Franconia, near Keilberg, close by Ratisbon, has been recognised 

 as belonging to the middle Lias. As is well known, on the west and 

 east of the Land-Riicken (white Jura) there occur black and 

 brown Jura. On the east side, however, these strata cannot be 

 traced to any extent ; only at some localities the highest points 

 protrude above the drift-sand, which covers everything between the 

 Bavarian forest and the Land-Riicken. The Keilberg, upon the 

 heights behind the Tegernheim Beer -house, near Ratisbon, is such an 

 isolated projection of lias. Some years ago a mine was opened here 

 and a very rich bed of red iron-ore discovered, lying on the great, 

 hard, sandstone strata. This is characterized by Am. costatus, Be- 

 lemnites paxillosus {alveoli), Pecten cequivalvis, Terebratula rimosa, 

 T. acuta, T. vicinalis, Spirifer rostratus, and other species, the 

 specimens being changed into iron-ore ; whilst the sandstone lying 

 beneath must decidedly be considered as belonging to the Keuper. 



In Swabia the boundary between /3. and y. is drawn too sharp by 

 nature to be overlooked by the geologist. The middle Lias com- 

 mences with the light-grey calcareous marls and an abundance of 

 Ammonites of the Capricorni family, together with innumerable spe- 

 cimens of Terebratula numismalis, transformed into quartz. The 

 first section of y. concludes with a bed of Belemnites and a limestone 

 bed with Am. Davcei. Above it dark clays make their appearance with 

 Am. Amaltheus ; light-coloured calcareous marls with Am. costatus, 

 Belemnites paxillosus, and Plicatula sjrinosa, terminating the series. 

 The development of this formation in Swabia, as well as its organic 

 remains, varies in many ways ; often the nitmismalis-cl&ys only are 

 developed ; the others being reduced to a minimum. So it is like- 

 wise in Switzerland, where for the most part the numismalis-bed alone 

 represents the middle Lias ; and on the top of the arietes-heds lighter- 

 coloured, but less sandy, clays make their appearance with Tere- 

 bratula numismalis, T. rimosa, Gryphcea cymbium, Spirifer verru- 

 cosus, and Am. capricorni and lineati ; on the other hand, the amal- 

 tkeus-hed is scarcely anywhere to be seen. 



In Mont Jura there is, as has been already mentioned, no such 

 line of separation as in Swabia. A series of grey clays, alternating 



VOL. VII. PART II. F 



