﻿FRAAS ON THE JURA FORMATION. 



73 



Iii Burgundy it is chiefly at Auxerre and the neighbourhood that 

 there lies above the unstratified rock-masses a system of regular cal- 

 careous beds ; these are of a whitish-yellow colour, and are used for 

 paving purposes. Small molluscs are here entirely wanting ; but the 

 following are characteristic : Perna plana (Mytilus amplus), Am. 

 gigas, Sow. (a gigantic planulatus, forming a passage to Am. corona- 

 tus), and Nautilus giganteus. These limestones are called " Kimme- 

 ridgian;" and a hard white limestone with Exogyra virgula, lying 

 above it and bearing the Neocomian clays, is termed " Portlandian." 

 In Calvados, above the Coralline Oolite, we sometimes again find 

 oolitic limestone, sometimes black clays, in neither case extensively 

 developed, — and indeed this is generally the case with the " Jura " 

 in r this country. The oolitic calcareous marls commence near Port- 

 l'Eveque, where the Tronques passes through the country, and extend 

 inland over Lisieux ; there they are mostly used for the manufacture 

 of cement. The yellow marls are towards the top filled with casts of 

 My a, Venus, Lucina, Cardium, 'Pinna, Modiola lithophaga, Ptero- 

 ceras, Natica, and Nerincea, and contain also Cidarites and Echi- 

 nites ; whilst in the Oolite beneath, Perna, Pinna, Trigonia clavel- 

 lata, Astarte, and Venus are abundant. As numerous Corals are 

 also present, this latter bed ought rather to be associated with the 

 Coral-rag. Parallel to this bed appear the clays of the Tronques 

 Valley, which constitute a passage to the English formations, where 

 above the Coral-rag lie dark Kimmeridge-clays, and above these the 

 light-coloured Portland-limestones and Oolite. Near the village of 

 Mault, or between Honfleur and Trouville, we find the last Jurassic 

 beds of this locality, — black clays, Argiles de Honfleur, full of Bi- 

 valves, as MycE, Lucince, &c, having white shells, but mostly crushed 

 and fragile. In like manner the Kimmeridge-clay is represented in 

 the south of England by black or bluish clays ; for instance, near 

 Oxford, where at the large quarries in the Coralline-oolite, which 

 supply building material for Oxford, the dark Kimmeridge-clays 

 have to be first removed. Am. planulati, Terebratulce pugnacece, 

 Trigonice with concentric rings, and especially Ostrea deltoidea, 

 are found here in great quantities. It is also to be remarked, that 

 here, as well as in Calvados, these dark clays are full of small clear 

 crystals of Gypsum, which are found nowhere else, and which may 

 be regarded as characteristic. 



Lastly, in the south of England the "Jura" Formation concludes 

 with the Portland-stone ; a light-coloured limestone and oolite, oc- 

 curring in much force at Portland on the top of the dark Kimme- 

 ridge-clays. Ammonites planulati {Am. gigas, Am. biplex), Bucci- 

 num naticoides, Terebra Portlandica, Nerita angulata, Trigonia 

 incurva, Tr. gibbosa, Perna ampla, Pecten lamellosus, Ostrea fal- 

 cata, O. expansa, Astarte cuneata, Cardium dissimile, and Colum- 

 naria oblonga are regarded as characteristic by the English geologists. 

 Near Oxford I found the Portland-stone as a bed of white calcareous 

 marl, of a few feet only in thickness, with Am. planulatus and frag- 

 ments of some Bivalves. The limit between the Portland-beds and 

 the Kimmeridge-clay is sharply defined. 



Thus we find the Portland-stone as a peculiar local formation, ab- 



