2 



THE GEOLOGIST. 



formation has been found and named ; its rocks and fossils have been 

 described ; and the Purbeck beds have returned to their former 

 associates — the Jurassic rocks. 



During this period of time several geologists have tried to find the 

 marine equivalents of the Purbeck and Wealden rocks, without, 

 however, arriving at results satisfactory to all observers. The ques- 

 tion is still very far from a solution ; but I have lately learned several 

 new facts on the subject which perhaps may be of some assistance in 

 facilitating a recognition of the marine deposits coeval with the 

 Wealden. 



In order that my references and data may be clearly understood, I 

 will first give a short description, or resume, of the strata in the Jura 

 Mountains comprised between the Portland stone and the Lower 

 Greensand. This ixsume is also graphically presented in the Abstract 

 section of the series of strata comprised between the Banne Limestone and 

 the Rhodanian group, in the Jura Mountains (PI. I., Fig. 1) — under 

 the form of a tabular and proportional view . 



The Jurassic divisions known under the names of Banne Limestone 

 {Calcaires du Banne) and Salins Marls (Marnes de Salins)* contain a 

 fauna identical with that of the Portland beds, with the addition of 

 some new species peculiar to the Jura, and a few fossil shells common 

 to the Kimmeridge clay. The Salins Marls are succeeded by a series 

 of compact limestone strata, very thick (100 feet at least), of a 

 whitish-grey, and sometimes clear yellow colour, containing beds of 

 lithographic stone a little above the middle of the division, and 

 always capped by a sort of magnesian limestone (Dolomite). This 

 series, called Salins Limestone {Calcaires de Salins), contains numerous 

 fossil remains, all of marine animals, especially Corals, Echinodermata, 

 Nerinea, and Natica ; two hundred different species at least. The Leit- 

 muscheln (guide-shells) are : — Hemicidaris Purbecheiisis, Forb. ; Pygurus 

 Jurensis, Agass.; Pinna Barrensis, Bav. ; Trigonia gibhosa, Sow. ; Natica 

 Marcousana, D'Orb. ; N. AtJileta, D'Orb.j Postellaria Barrensis, Buy. ; 

 Nerinea Salinensis, D'Orb. ; iV^. Elea, D'Orb. ; iY. subpyramidalis, 

 D'Orb. j N. grandis, Volt. ; N. trinodosa, D'Orb. ; N. cylindrica, 



* See, for explanation of these terms, LeUres snr les Roches du Jura et leur 

 distribution geographique^ dam les deux Jlcinispheres, par Jules Marcou ; 

 Paris, 1857. 



