﻿FRAAS ON THE JURA FORMATION. 



49 



The inferior Lias of Burgundy is again differently modified from 

 that of the Jura. It has, above all, more extensive limestone beds, 

 especially the stratum of the Thalassites. At Semur and Beauregard 

 the strata contain iron ; and the numerous Thalassites are replaced 

 by [oxide of] iron and excellently preserved. This stratum is invested 

 with such importance in this district, that D'Orbigny denotes the 

 whole of this division of the lower Lias as etage sinemurien. Besides 

 the Thalassite beds, the clays and limestones with Ammonites an- 

 gulatus {A. Moreanus, D'Orb. and A. Boucoltianus, D'Orb.), are 

 also well-developed, upon which the Arietes-heds, having a thickness 

 of from 10 to 12 feet, repose. Above these is an argillaceous limestone 

 bed, of a few feet thickness, with Am. BrooJcii and " cincta" Tere- 

 bratulce, and with Am. oxynotus, which however is calcified and with 

 a diameter of about 6-8 inches ; this represents /3. in Swabia. We 

 find Am. oxynotus converted into quartz, with A. bifer and A. rari- 

 costatus, in the Cher-Department, near St. Amand, where Swabian 

 conditions exist. On the other hand, things appear different in the 

 south of France, in the Rhone district, where, according to M. 

 Victor Thiolliere*, black limestones, without the specified Arietes and 

 Gryphcece, are found in much greater force than anywhere else, in- 

 stead of the Arietes-heds existing in England, in the north of France, 

 and in Germany. This formation he calls the " type mediterranean," 

 as in general the whole Jurassic series in the south of France presents 

 a peculiar aspect. 



Whilst the lower Lias gains in development in the south of France, 

 it decreases towards the north. The whole section in Calvados is 

 reduced to 1 or 2 feet, and a Gryphcea arcuata or an Ammonite of 

 the Arietes-gmvtp becomes very rare, until in England (Lyme Regis) 

 the full development of the strata recommences. Here again also, 

 as in Swabia, we find the Lias sandstones (Linksfield and Brora) 

 and Ammonites psilonotus. 



In Swabia and England, therefore, there is a perfect development 

 of the lower black Jurassic rocks, to which those of France are very 

 inferior. The same relative proportions are found in the appearances 

 of the strata : Swabia is in this respect the richest country ; richer 

 than England, which only has the preference in regard of the pre- 

 servation of specimens ; such as Ammonites obtusus, Sow., with its 

 interior chambers elegantly filled with white calcareous spar, and the 

 external chamber occupied with the blue limestone of the matrix. 

 France, however, may be called poor, in spite of the many species that 

 D'Orbigny mentions. Each country has also its peculiar organic 

 remains ; what is found in one country is either wanting entirely in 

 another, or at least is not so abundant. If we only consider how 

 the different families of the Arietes offer at different localities differ- 

 ent species and varieties, we shall see that the genuine Ammonites 

 obtusus, Sow., is peculiar to England and the Aisne-Department ; Am. 

 Turneri, Sow., is only found in England and Swabia ; and, accord- 

 ing to my view, the calcified Am. Turneri occurs but rarely in the 

 east of France on the top of the Arietes-heds (compare, in D'Or- 

 * Bulletin de la Societe Geologique : Seance du 4 Nov. 1847. 



