RH^ETIC BEDS. 245 



form the most characteristic and persistent portion of the Rhcetic 

 Series. They consist of paper-shales and laminated clays, much 

 impregnated with iron-pyrites, and containing also selenite and 

 fibrous carbonate of lime ('Beef'). At or near the base of the 

 shales, there may usually be found a layer, and sometimes two or 

 more bands, of pyritic, micaceous, and calcareous sandstone, 

 containing coprolites, and bones, scales, and teeth of Fishes. 

 These bands constitute the well-known ' Bone-bed,' which was 

 formerly termed the Bristol or Lias Bone-bed. The Fish-remains 

 include Gyrohpis Albert i, Ceratodus alt us, Acrodus ininimtis, Hybodus 

 reticulatus, Saiirichthys apicalis, and other species ; bones of Ichthyo- 

 saurus and Plesiosa ui'us also occur. The Mollusca found in the 

 Black Shales are generally much compressed. Most characteristic 

 is the Avicula {Cassia7iella) contorta, originally described by Major- 

 General J. E. Portlock ; ' and the Rhsetic Beds, as a whole, are 

 sometimes termed the Zone of A. contorta. Cardium Rhaiticiim is 

 also abundant, and has sometimes, although unnecessarily, been 

 taken to indicate a zone. Pccte7i Valoniensis, another characteristic 

 form, is best preserved in thin layers of shelly limestone, which are 

 met with here and there in the higher portions of the shales. 

 Sandy beds occur in places, and are sometimes crowded with casts 

 of Pullastra arenkola. Remains also of Insects, Echinoderms, 

 and Plants, are occasionally met with in the Rhaetic Beds. A 

 Brittle-star, Ophiolepis Damcsii has been obtained from the Shales 

 at Garden Cliff. One Brachiopod, Discina, has also been recorded. 



Grey Marls. — Below the Black Shales there is generally found a 

 series of grey, tea-green, and cream-coloured marls, which pass 

 down, often imperceptibly, into the red, green, and variegated 

 Keuper Marls. The mere difference in colour between the de- 

 posits is likely to be subject to so much variation, that although 

 the red disappears in the higher part of the series, yet a boundary- 

 line drawn solely in reference to colour cannot be regarded as a 

 definite horizon. On the Maps of the Geological Survey it has, 

 however, been found most practicable to draw the boundary 

 between the Rhaetic and Keuper Beds at the base of the Grey 

 Marls. In some localities the more marked boundary would be 

 taken at the base of the Black Shales, as the Grey Marls appear 

 more closely linked with the Red Marls;- but in other localities 

 the Grey Marls present features markedly different from the Keuper 

 Marls, and appear more closely connected with the Black Shales 

 and White Lias. Near Axmouth and Watchet, bands of very dark, 

 if not black, marl, alternate with pale grey and buff marls, above 

 the Red Marls of the Keuper ; so that we have a transition from 

 the Red Marls into the Black Shales. The Grey Marls also con- 

 tain bands of hard white and cream-coloured marl, which resemble 

 the softer beds of White Lias. 



Nevertheless, whether the Grey Marls be taken as the top of 

 the Keuper, or the base of the Rhaetic Beds, is a matter of little 



' Rep. Geol. Londonderry, 1843, p. 126. 



^ E. Wilson, G. Mag. 1881, p. 464 ; Q. J. xxxviii. 451. 



