﻿48 



GEOLOGICAL MEMOIRS. 



sandstones, and sometimes have reference to the "Bone-bed." On this 

 are superposed alternating clays and sandstones, in which Am. angu- 

 latus, again accompanied by Thalassites, occurs, and at some favour- 

 able places numerous fine Gasteropoda, also Lima, Nucula, Plagio- 

 stoma, and others are conspicuous. In sequence to these, the first 

 appearance of the fauna, in the blue Arietes-he&s, occurs. The 

 quantity of Ammonites of the Arietes-gvomp, the hosts of Gryphcea 

 arcuata, the beds of Pentacrinites basal tiformis are here always 

 constant. In the second division (ft) of the lower black Jura, the 

 foregoing is succeeded by a strong development of black clays, only 

 sparingly interrupted by insignificant limestone beds. "Whilst in 

 these limestone beds Ammonites of the Arietes-growp, together with 

 Pholadomyce and Terebratulce, make their appearance for the last 

 time, there begins in the clays a new organic series, with Am. Capri- 

 corni and Oxynoti, forming a transition to the rich series of organic 

 remains of the middle Lias. This is, however, by its light grey 

 limestones so sharply separated from the black clays of ft, that we 

 need not here draw an artificial line of demarcation between the 

 formations. 



The principal development of sandstone is upon the four Filder- 

 plains of Wiirtemberg ; westwards it decreases more and more the 

 nearer we come to the Wutach and Switzerland. In this decrease 

 Am. psilonotus also participates ; this form, however, according to 

 M. Merian, has been found in Switzerland, but is there no longer the 

 type of a geological horizon ; because generally in Switzerland A. 

 Bucklandi occurs immediately above the Keuper. The clays with 

 A. Turneri, coated with quartz, are developed near Pratteln. 



In the case of Mont-Jura, some quarries near Salins are exceed- 

 ingly instructive. Here the Keuper supports a yellowish grey lime- 

 stone bed, lj foot thick, with Thalassites (Cardinia) concinnus, 

 Th. securiformis, and Plagiostoma ; upon which rests the Arietes-bed 

 with Pentacrinites ; the lower black Jura terminating here with the 

 dark blue Gryphite limestones. Above these are found greyish black 

 clays, interstratified with limestone bands, clays full of Ammonites 

 oxynotus and A. bifer, calcareous marls with Pholadomyce, Mactro- 

 myce, and Ar corny ce (Corbula cardioides and Thalassites Listeri), 

 with Ammon. raricostatus and Gryphcea cymbium, but without form- 

 ing a line of demarcation with regard to the middle Lias. Those 

 greyish black clays which towards their base contain the Oxynoti, 

 have still lower down A. Taylori, A. Jamesoni, &c. ; in short they 

 become our numismalis-maxh. It is clear that with the French geo- 

 logists the middle Lias begins with our ft, as they take Gryphcea 

 cymbium as the typical shell, which is found from ft. throughout to 

 y. Respecting the geognostic conditions of Mont Jura, my friend 

 Jules Marcou has published a valuable work in his ' Recherches Geo- 

 logiques sur le Jura salinois*' ; our ft. forms the first section of 

 his "Lias moyen," which he formerly called Marnes a Gryphcea 

 cymbium, but now, after having seen its strong development at 

 Balingen, he has named it Marnes de Balingen. 



* Mem. de la Soc. Geol. de France, iii. 



