TRANSACTIONS OF THE SECTIONS. 51 



fied and noted on the spot by Mr. Etheridge, the Palaeontologist to the Geological 

 Survey. The Rhtetic beds, although not always exposed, or at times only imper- 

 fectly visible, after careful searching, were found to be of invariable occurrence 

 over the entire area examined, between the Red Marls of the Keuper and the base 

 of the Lias ; and they were mapped by Mr. Bristow during the present summer, iii 

 the country round Bristol, as well as at Penai'th and other places in the West of 

 England. 



The general section of the Rhsetic beds was described as consisting of a central 

 mass of black, thinly laminated shales (weathering into paper-shale), with thin 

 occasional bands of hard and tough, blue-grey, coarsely-fissile limestone, very 

 imlike those of the Lias in appearance, and containing great numbers of a highly 

 characteristic shell (Pecten valoniensis), as also do the shales of the other emi- 

 nently distinctive shells, Aviada contorta, Cardimn rka-ticion, &c. 



At Aust, Patch way, and other p'aces north of Bristol, and likewise at Penarth, 

 a thin layer of sandstone is met with near the base of those black shales, which is 

 often very pyritiferous, of a brecciated structure, and frequently crowded with 

 bones, teeth, coprolites, and other remains of lishes, to which the name of " Aust 

 Bone-bed " has been given by collectors, in consequence of its well-known occur- 

 rence in the cliffs of the localitj' in question. 



The central mass of black shales rests upon a series of hard and soft greenish 

 marls, which pass by an almost insensible downward passage into the red and 

 variegated marls of the Keuper, so that it is scarcely possible to adopt any other 

 line of demarcation on a map than the top of the great mass of the red beds of the 

 latter series. 



The uppermost division of the Rhsetic beds, lying at the base of the Ijias, con- 

 sists of beds of marl and marly (argillaceous) limestone, composed in the upper 

 part of beds of cream-coloured or nearlj- white argillaceous limestone, breaking with 

 a smooth conchoidal fracture, and with sharp-cutting splintery edges (and closely 

 resembling in appearance the lithographic limestone of Solenhofen), to which the 

 name of " White Lias " is given by certain quaiTymen in the West of England. 



The curious stone called Gotham Marble, to which the name " Landscape Stone' 

 has been given on account of the fanciful resemblance which the darker delinea- 

 tions sho^%Ti on its fractured surface bear to a landscape, with trees, water, &c., is 

 of almost invariable occurrence at the base of the White Lias series, and was found 

 of much use in indicating the position of the upper boundary of the Rhsetic beds, 

 especially when (as is the case over a large part of the area north of Bristol and 

 elsewhere) the " WTiite Lias " beds of the quaiTymen are altogether wanting, or 

 only very attenuated and imperfectly represented. 



Although the passage from the lowermost Rhaetic beds into the Keuper Marls is 

 very gi-adual, there are clear indications of a pause or break in the deposition of the 

 beds forming the two overlying subdi\ isions, in the signs of erosion sometimes 

 shown in the upper sm-faces of the hard bands of limestones containing Pecten 

 valoniensis, and commonly in that of the Gotham marble. The proofs are still 

 stronger in the beds of " White Lias," which not onlj^ aSbrd unmistakeable evidence 

 of having suffered erosion since their deposition and prior to that of the superim- 

 posed Lias, but also of having been penetrated by boring mollusca, the cavities 

 made by which are in many cases still remaining. 



The palseontological evidence on this point tends equally to show that there is 

 nothing in common between the fossils of the uppermost Rhsetic beds and those of 

 the overlying liassic strata; tlie former consisting of Modiola minima, Ptdlastra 

 arenicola, Axinns, &c., the latter of Ammonites planorhis, at Watchett, and at 

 Penarth of that shell together with immense numbers of Ostrea liassica in a re- 

 markable state of preservation. 



In conclusion, Mr. Bristow stated that, it being desirable that the Rhsetic beds 

 of this country should be distingiiished in the maps of the Geological Survey by a 

 synonym derived from a British locality where these beds are well displayed and 

 fully developed, he was induced, at the suggestion of the Director-General Sii- Ro- 

 derick Mm-chison, to propose Penarth as a good typical name, in preference to many 

 others which had been recommended, but which were for several reasons objec^ 

 tionable. 



4* 



