﻿FRAAS ON THE JURA FORMATION. 



45 



by its application becomes false 1 I am certain that there exists no 

 Bradford clay either in Germany or in France, — that grey clay of the 

 S. of England at the top of the Great Oolite with its abundance of 

 fine Apiocrinites intermedins, elongatus, and Parkinsoni, and with its 

 herds of well-preserved Terebratulce, Aviculce, Myce, &c. In Swabia 

 we have nothing similar ; the Bradford clays are a peculiar local for- 

 mation of the south of England. So it is with " Kimmeridge " and 

 " Portland," — they are and remain local names, which cannot be ap- 

 plied to the deposits of other localities. 



The names of the strata according to their fossils are of greater 

 value. In so far as the typical shells in the different strata are the 

 same, by comparison we soon arrive at some degree of clearness : but 

 here also we meet with peculiar difficulties ; because those orga- 

 nisms which are leading in one country, and which give the name to 

 the stratum, disappear from this stratum in other countries, appear- 

 ing again as typical characters in a higher or a lower stratum. This is, 

 for instance, the case with Ammonites Parkinsoni : in Swabia it is the 

 type for the stratum above A. coronatus and beneath A. macroce- 

 phalus ; in France and England it is the typical shell for the lower 

 Oolite, and is followed by A. Murchisonte, A. Humyhriesianus, and A. 

 coronatus. In the same manner, we can neither use for France or 

 England such appellations, so well adapted to the German Jura-rocks, 

 as Amaltheus-hed, opalinus-cl&ys, Jurensis-m&rh, Scyphice-lime- 

 stones, and others, because the fossils referred to appear there either 

 not at all, or very rarely, and other kinds of fossils are found which 

 characterize the stratum better. A synonymic examination of the 

 different stratum-names is therefore necessary for the comparison of 

 the Jurassic rocks. 



A. Black Jura. Lias. 



The foundation of the Lias is not always the Keuper. Almost as 

 frequently it is an older rock, of the secondary or primary age. 

 But sandstone formations in Lias are always connected with the deve- 

 lopment of the Keuper series. The close connection of the Lias 

 sandstone and that of the whole black- Jura-rocks with the Keuper 

 cannot at all be denied. Hence some French geologists* regard 

 the Keuper sandstones as liassic beds inferior to the " Jura," be- 

 cause the Keuper in that part appears much too insignificant to be 

 regarded as a formation of itself. In Swabia, where the Keuper and 

 "Jura" Formations are so distinctly separated, this is regarded as an 

 important error. But the slight development of the French Keuper 

 is an excuse for those geologists who, from its insignificance, added 

 it to the " Jura." Even the whole Trias to the west of Lorraine be- 

 gins to disappear. 



The boundary of the Lias consequently varies considerably. In 

 Germany it is everywhere the Keuper, — sometimes the upper sand- 

 stones, sometimes the red marls. Likewise so in Switzerland and in 

 the Jura, although nothing certain can be observed here on account 

 of its confusion at most places. Towards Burgundy, however, the 



* Sec Thirria, Notice sur le -Jura de la Haute-Saone. 



