THE COBALLIAN ROCKS OF ENGLAND. 335 



Generalized Section — Pickering. 



Upper Division (supra-coralline). — Grits, Shales, &c. 



ft. in . 



a. An Upper Calcareous Grit with abundance of tuberous and rami- 



fying forms and many fossils in bad preservation 7 



b. Sandy and marly shales resting upon a thin bed full of Ostrece ... 10 



c. Bed of argillo-calcareous stone, locally termed "Throstler," want- 



ing in places, or represented by a flaggy parting of a few inches... 3 



20 

 Middle Division. — Upper Limestones. 



d. Top-stone — a grey ferruginous limestone, generally false-bedded, 



thicker on the west than on the east side 5 



e. Impure earthy limestones, known as " black posts," poor in fossils, 



but containing Belemnites abbreviates, and towards the base 

 Ammonites varicostatus 10 



/. Chemnitzia-lime&tones, compact and suboolitic. Shell-beds, at in- 

 tervals full of Chemnitzice and Nerincece ; Astarte ovata and other 

 bivalves less numerous 20 



g. Variable limestones and pisolites, with shell-beds and nests of 

 Thamnastrcea arachnoides in the lower part ; indications of Calc- 

 Grit towards the base. Roadstones, walling, &c 13 



48 

 Lower Division. — Shell-beds and Grits (Middle Calc-Grit in part). 



h. The upper part of this division contains the principal Trigonia- 

 beds, full of fossils, alternating with Oalc-Grits. Ammonites 

 plicatilis abundant. Building-stones, walling 17 



i. Calc-Grits begin to predominate, base not seen. Nautilus hexago- 



nus, Ammonites-cordatus group numerous, &c 11 



28 



Subjoined is a detailed description of the general section. 



i. The upper portion of this division shades into the next one, h ; 

 its lower portion is obviously incomplete, as the Middle Calc-Grits 

 are not bottomed in any of the Pickering quarries. There are very 

 few fossils in it beyond those noted in the section, which occur of 

 large size towards the top. 



h. 17 feet. — This is the most interesting and richly fossiliferous of 

 all the groups in the section. It is the horizon whence most of the 

 fossils marked " Pickering " are derived, and is remarkable for three 

 beds charged with Trigonia perlata in great profusion, but always 

 in detached valves (e, £ in fig. 14). Petrologically the group consists 

 of light porous calc-grits and of hard blue beds, locally termed " flint," 

 these latter sometimes passing into shell-beds, which occasionally thin 

 out to a mere string, and then develop into pockets full of the most 

 magnificent valves of Trigonia. The middle bed is perhaps the best. 

 By far the commonest fossils, besides T.perlata, are Gervillia aviculoides, 

 Nerincea visurgis (usually small), Chemnitzia heddingtonensis, Lucina 

 Beanii, Cucullcea corallina-. Besides the principal Trigonia-\>eda 

 there is a very peculiar " small shell-bed " only to be found on the 

 east side, its position on the west side being occupied by a few inches 



