﻿FRAAS ON THE JURA FORMATION. 



In the neighbourhood of Caen, where no rocky land-marks of the for- 

 mations are visible, like those in Swabia where every one can perceive 

 them, all the members of the Jurassic series are of trifling thickness, 

 and lie side by side, covered, moreover, mostly with alluvium. Close 

 to Caen lies the Great Oolite (Oolite de Caen) . If we go a few thou- 

 sand steps further upon the road to Alencon, we come to the red 

 Transition rocks with Trilobites ; if we turn from thence half a league 

 towards the north, we are at Fontaine Etoupefour, the celebrated 

 Lias quarries ; the uniform Great Oolite then again appears, under 

 which, in the sections made by narrow valleys, the inferior Oolite or 

 the Lias is seen exposed. In short, it is very difficult, and almost 

 impossible, for one who is not acquainted with the normal Lias, to 

 find his way in the " black Jura" of France. At the same time, the 

 relative altitudes cause confusion, because in France the reverse con- 

 dition exists in comparison with Swabia, the elevation of the " white 

 Jura" being less than that of the "black;" the Marne, Seine, 

 Yonne, Loire, Cher, Indre, &c, all of which flow westward, descend 

 from the Lias to the " brown Jura," and thence to the " white ;" 

 whilst in the German " Jura " the rivers flow from the heights of 

 the "white Jura" through the "brown" and the "black." 



As before said, there is no comitry in which the members of the 

 "black Jura" are so distinct, and where the demarcations are so 

 sharp, as in Swabia. I place the Swabian before the French and En- 

 glish " Jura," in order to see at once how different are the local 

 arrangements of the Swabian strata. I use Quenstedt's classification*, 

 as being the best for the Jurassic system of Swabia. 



■ 



I. Lower Black Jura, a. and /3. (Quenstedt.) 

 Limestone of the Lower Lias Shale. Lias. 

 Lias inferieur ; Calcaire a Gryphee arquee. 

 Etage sinemurien (D'Orb.). 



In the German Jura a difference of structure already appears in 

 the lower Lias. We meet there with two principal formations ; both 

 sandstone and clay being developed. Sometimes the two appear 

 together, sometimes the one, or the other, or even both are wanting. 

 The latter is the case in Franconia : in the valleys of the Maine, Reg- 

 nitz, Wiesent, and Pegnitz the lower Lias nowhere attains to any 

 significance, appearing only as coarse-grained, hard sandstone of a 

 few feet in thickness. Occasionally it contains, as a sign that we are 

 no longer in the Keuper, a Gryphcea, or Ammonites Bucklandi, or a 

 Thalassites. It appears as if the Keuper, which in that district is 

 so strongly developed, had left no room for the Lias. The blue 

 clays with Am. costatus, constituting the middle Lias, often seem 

 to lie directly upon the Keuper. In this form the Franconian Lias 

 extends as far as the Ries, where the Swabian clay-formations com- 

 mence, which often rise to 100 feet and more. If these are fully 

 developed, then dark blue limestone beds with Am. psilonotus and 

 Thalassites form the undermost stratum above the yellow Keuper 



* Das Flozgebirge Wiirtembergs. Fr. Aug. Quenstedt. 8vo. Tubingen, 1843. 



