﻿FRAAS ON THE JURA FORMATION. 



57 



extension in width, so that it can scarcely be put down upon small 

 maps. It is quite different in France, where the limestone beds of 

 the Inferior Oolite, and especially of the Great Oolite, form large 

 plains, and often have six times the development of the Lias. In the 

 south of England the same condition exists, whilst in the northern 

 part there is an approach to the Swabian proportion. These very 

 opposite proportions of clay and limestone in the "brown Jura" are 

 shown by comparison of the district near the Wutach with Burgundy. 



Near Blumberg the whole "brown Jura" consists of a steep slope 

 300' high; upon the summit of the mountain is "white Jura," at 

 the foot of the rent is Lias. Almost the same happens on the upper 

 Neckar, near Spaichingen, Aldingen, Schomberg, and at many other 

 places in Swabia, — a rivulet, rushing down from the "white Jura," 

 within half an hour traverses the "brown Jura." The contrary 

 exist in Burgundy, where the clays are represented by great lime- 

 stone masses extending over several miles, and forming sometimes 

 plains, sometimes bold rocky valleys. In Lorraine, in Calvados, and 

 in the north of England, it is Oolite that constitutes, by its wide ex- 

 tension, the principal part of the Jura. These different geognostic 

 formations point back to the different local conditions of the old 

 Jura-sea; extensive calcareous deposits being evidence of pelagic 

 conditions ; whilst clays and sandstones testify to the neighbourhood 

 of a shore. Hence the comparison of individual strata becomes still 

 more difficult, and as it becomes impossible to find the same bed in 

 all countries, we must confine ourselves to the synchronism of the 

 strata ; a task the more difficult as the members of the Jura-series 

 attain wider and wider extension. 



It is moreover interesting to trace a parallel between the Lias and 

 the "brown Jura" in the above-mentioned countries where the sand- 

 stone formation again appears in the lower "brown Jura," as well as 

 in the lower "black Jura," in Germany and in the north of England ; 

 whilst in France and in the south of England these formations are 

 very insignificant, if not sometimes entirely wanting. This leads us 

 to the first division of the " brown Jura." 



I. Lower Brown Jura, a. and (3. 



Inferior Oolite. Northampton Sandstone. Cheltenham Freestone. 



(Ferruginous beds.) 

 Gres superliasique. Oolite ferrugineuse. 

 Etage bajocien (D'Orb.). Oolite inferieure, in part. 



In this bed, especially in a, the opalinus-clscys, we have almost ex- 

 clusively a German local formation. The Vosges constitute its bound- 

 ary towards the west ; Alsace, Swabia, and Franconia form its centre ; 

 Switzerland likewise comprises a small portion of it. These are 

 unctuous, black clays, in which the mollusks are very well preserved, 

 usually with the shell and a natural nacreous lustre. In general 

 these extensive clays, sometimes above 100 feet thick, are very poor 

 in fossils ; it is only in occasional beds that an abundance of the finest 

 shells is presented. Leopold v. Buch calls these peculiar fossils the 

 real German mollusks. The first place, as a typical form, is occu- 



