ZONE OF AMMONITES PLANORBIS. 



61 



LiTHOLOGY. Thickness. 



No. ft- in- 



41. Light-coloured sandstone; micaceous 0 1 



42. Brown clay 0 2 



43. Sandstone ; micaceous 0 2 



44. Dark sbaly clay 0 6 



45. Soft sandstone 0 1 



46. Black clay 0 3 



47. Ferruginous vein, sandy ? 



48. Gray Keuper marls. 



Organic Kemains ; and Local Names of the Beds. 

 PuUastra arenicola, Strickl. 

 Pullastra arenicola, Strickl, 



The beds from No. 37 to No. 48 were found in situ in an escarpment at a short 

 distance from the quarry at Binton. It must be understood that the " Guinea-bed" is 

 the lowest bed seen in situ in the pit, and that No. 37 occupies its natural position 

 relatively to that bed, although it is not exposed in the Binton section. 



Lithology of the Ammonites planorbis beds. — The Am. planorbis beds at Brockeridge 

 (p. 58) consist of dark, laminated shales, with interstratified beds of marl and limestone. 

 The shales split into very thin laminse, between which innumerable shells of Ammonites 

 planorbis lie closely compressed ; the white, decomposed, pulverulent matter of the shell 

 forming a strong contrast to the dark shales enclosing them. In Somersetshire the 

 rock consists, at Uphill, of shales which greatly resemble those at Brockeridge ; at 

 Watchet, of dark clays which are more indurated and have preserved better the shell- 

 structure : here Ammonites planorbis and Am. Johnstoni are found with the iridescent 

 nacreous layer of their shells beautifully preserved. At Street the rock is a light-yellowish 

 clay, with bands of grayish limestone and marl beneath, and in Worcestershire at Stren- 

 sham, and in Warwickshire at Binton, similar lithological conditions prevail. 



The White Lias series of the section at Saltford (see p. 64) represents in part the 

 Am. planorbis beds : here also the relations of that zone to the Saurian beds below, and to 

 the Am. Bucklandi beds above, are well shown. In Dorsetshire the Am. planorbis beds 

 are represented by the upper portion of the White Lias so well exposed in the large 

 quarries at Up-Lyme, and in the coast sections at Charlton and Pinhay Bays. The 

 White Lias is raised at Up-Lyme for caustic lime ; it consists of a fine-grained, cream- 

 coloured limestone, apparently fit to be used as a lithographic stone. The two principal 

 quarries afibrd the necessary details. Mr. Webb's quarry shows — 



In the uppermost portion, thin bands of gray limestone interstratified with shales ; in 

 these are found Ammonites planorbis and A. Johnstoni ; in two thick beds of dark, shaly 

 clay are numerous spines and plates of sea-urchins, as Cidaris Edwardsii, Wright, Fseudo- 

 diadema lobatum, Wright, Hemipedina Bechii, Brod., Hemipedina Bowerbankii, Wright. 

 These same shales are found at low water-mark at Pinhay Bay, and they have yielded 

 nearly all the Echinidee said to be found in the Lower Lias at Lyme Regis. 



Beneath the lower bed of the Cidaris shales are several thin beds of light-coloured 



