GEOLOGY OF THE BRISTOL COAL-FIELD. 49 



The Rhaetic series may be conveniently divided into two- distinct 

 and easily distinguishable groups, viz, — 



I. — ^The White Lias. 



II. — The Avicula Contorta Beds. 



Altogether they form a thickness of about fifty feet, and excellent 

 sections may be seen at Aust, Keynsham, Saltford, Willsbridge, 

 Horfield, Gotham, and Bedminster. 



Immediately above the Keuper Marls are fifteen feet of thin 

 black shales, very pyritous, and containing a tolerably large 

 quantity of fibrous carbonate of lime. Towards the top are two 

 fish beds, one thin arid only containing the scale and teeth of 

 fishes, and the other, about twelve inches thick, containing the 

 scales, teeth and coprolites of fishes, and bones and coprolites of 

 saurians. The last is the one so [celebrated as the ^'Aust bone 

 bed." In the black shales are several beds of limestone, with a 

 plentiful supply of Peden Valoniensis (Debr.) and Pullastra 

 arenicola (Strickl.) 



The shales themselves in many places have very fine specimens 

 of Avicula contorta (Portl.), Cardium Rhceticum (Mer.), Natica 

 Oppelii (Moore), &c. 



An excellent section for the study of the Rhaetic beds with their 

 junction with the Trias, occurs in the cliff at Aust, capped at their 

 top with the Planorbis beds of the Lias. 



Fig. 28. — Section of Aust Cliff. 





(Keuper Marls at Base.) 

 rt— Lima beds. J— Planorbis beds, c— Fish beds, rf— Pecten beds. 



