222 Messrs. Buckland's and Conybeare's Observations on the 



shire and Monmouthshire, and are there denominated cornstone. The corn- 

 stone is mentioned by Dr. Bright, in the 4th volume of the Geological Trans- 

 actions, page 201, as forming a bed three feet thick towards the middle of the 

 beds of old red sandstone that lie beneath the mountain limestone on the left 

 bank of the Avon. 



The occurrence of vegetable impressions in this formation, on the banks of 

 the Avon near Bristol, is mentioned by Dr. Bright in the page before cited. 

 A similar case may be noticed in the quarries of grey and red flagstone to the 

 west of Leominster in Herefordshire, where the vegetable remains are so 

 abundant, as to render it probable that thin coal-seams may there alternate 

 with the flaijstone. 



4. Carboniferous or mountain limestone. 



The mountain limestone of this basin agrees in mineralogical character, 

 organic remains, and geological position^ with that of Derbyshire and the great 

 central ridge of the north of England. It is in no part of the district so com- 

 pletely displayed, as in the defile of the Avon, which has been described by 

 Dr. Bright and by Mr. Cumberland in the 4th and 5th volumes of the Geolo- 

 gical Transactions. We therefore refer to the papers of these gentlemen, and 

 to our own observations on the same defile, for a detailed account of the entire 

 series of beds composing this formation. 



The divisions of this formation, adopted by Dr. Bright, may be recognised 

 pretty generally, wherever sections are afforded, throughout the whole exte- 

 rior chain of the basin. Of these the black-rock, so called from its dark colour, 

 one of the lowest beds of limestone, abounding in encrinites, presents the most 

 striking features, and is most easily identified, wherever the lower strata are 

 exhibited. Some of the beds of this formation, particularly the lower, are 

 often dolomitic. The calcareous beds are preceded below, and succeeded 

 above, by a set of argillaceous beds, which may be termed the lower and 

 upper limestone-shales. The former of these occupies the interval between 

 the old red sandstone and the calcareous beds of the mountain limestone ; the 

 latter (agreeing with the limestone-shale of Derbyshire) lies between the 

 mountain limestone and the incumbent millstone-grit. 



The lower limestone -shale consists of a soft argillaceous slate, usually of a 

 dark greenish hue inclining to black. It contains a few subordinate calcareous 

 beds, having the same aspect, and containing the same organic remains, as 

 mountain limestone. It rarely exceeds 300 feet in thickness. 



It may be seen upon the Avon in the wood below Cook's Folly, vide Map and 

 Section, PI. XXXIII. fig. 1 and 2. It forms an important feature in the con- 

 stitution of the Mendips, where its extent is usually indicated by a low narrow 



