﻿FRAAS ON THE JURA FORMATION. 



67 



the Ammonites above referred to, Belemnites with almost perfectly 

 preserved shells, impressions of Insects with other rarities; the 

 line and unctuous mud having particularly favoured their pre- 

 servation. Lastly, in Yorkshire, as in Swabia, we again meet 

 with silicified shells ; and here are also found teeth of Squalus, 

 Crustacea, Astacus (Clytkia Mandelslohi) in the concretions of 

 this bed. 



With this the " Brown Jura" of Leopold von Buch closes. Here 

 also the " Brown Jura" in Germany has its natural boundary ; the 

 lighter-coloured clays and limestones, which are now developed, an- 

 nouncing a new period, — that of the " White Jura." 



In Switzerland and in France there is no absolute boundary between 

 the "Brown" and the "White Jura," as was also the case in the 

 transition of the "Black Jura" into the "Brown." Hence, that 

 which constitutes the " Lower White Jura " in Swabia is so closely 

 connected with the ornati-cl&ys, that this name is used for the "Lower 

 White Jura" in the non-German countries. 



C. The White Jura. 

 A powerful development of this formation is peculiar to the Ger- 

 man "Jura." Here it attains a thickness of above 1000 feet, whilst 

 in England and France it is often scarcely 100 feet thick. The loss suf- 

 fered by the Jurassic system in Germany by the absence of the Great 

 Oolite and Forest-marble, is counterbalanced by the great develop- 

 ment of the " White Jura." What the " Brown Jura " is for France 

 and England, the "White Jura" is for Germany, — each having in 

 the respective countries surpassed in development the other members 

 of, the Jurassic series. We here see how difficult it is to parallel all 

 the strata of different countries. In particulars they differ entirely, 

 and there only remain general similarities ; — geognostically, the pre- 

 dominance of limestones ; — palseontologically, the development of 

 Corals and Echinoderms ; the Cephalopods playing a less and less 

 important part, and ultimately almost disappearing. 



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



Marnes oxfordiennes saperieures. 



On the ornati-clajs in Swabia are superimposed alternations of 

 clay and limestone to a thickness of 600 feet, — very regular and thin 

 layers of white limestone traversing grey clays. In the lower clay- 

 beds (a) Terebratula impressa is a very characteristic shell ; there are 

 also found small silicified Am.plamdati, with Rostellaria and Astarte; 

 whilst the limestones towards the top are characterized by Am. poly- 

 gyratus and Am. fiexuosus. I do not know a more instructive locality 

 for the "Lower White Jura" than the Hunds-Kiicken, a mountain 

 2800 feet high, eastward of Balingen, on the borders of Prussia and 

 Wurtemberg . At the foot of the mountain, near the village of Streichen, 

 are the oma^'-clays, above which lie the light-grey limestones with 

 T. impressa ; the alternating limestones and clay -beds rise to the 

 summit of the mountain, where we find in the white limestone-beds of 

 a great steep precipice, the finest Am. planulati and fiexuosi, Belem. 

 hastatus with large alveoli } teeth of Squalus, &c. The presence, 



VOL. VII. PART II. G 



