214 Proceedings of Philosophical Societies. [March, 



red sand throughout this chain is characterized by a degree of 

 sterihty as remarkable as that of its fertihty in the countries of 

 Monmoutli and Hereford : this arises from its being composed 

 in Mendip of a hungry, sihceous grit, alternating with beds of 

 brown tenacious clay ; and being deficient in those beds of marl 

 coloured by red oxide of iron, which give their extraordinary 

 fertility to the last two named counties, and which extend their 

 influence to the similarly constituted strata of the young red 

 sandstone and red rock marl formation. 



Jan. 7, 1820. — The continuation of a paper was read, " On 

 the Coal Fields adjacent to the Severn," by Prof. Buckland and 

 Rev. W, D. Conybeare. 



The southern boundary of the triangular area of the Somerset 

 and South Gloucester coal field, having been described as con- 

 stituted by the entire chain of the Mendip hills, it remains to 

 examine its west and eastern border, and afterwards to proceed 

 to the history of the coal field itself. 



On the west border, the old red sandstone and mountain lime- 

 stone do not, as in the Mendip chain, form a continuous unbroken 

 frontier, but occur in the detached groups of Brondfield Down, 

 Leigh Down, Derdham Down, Kings Weston Down, and the 

 ridgway near Almondsbury, extending thence to Tortworth, 

 which is situated on the north apex of the area under consider- 

 ation. 



The strata of mountain lime composing Brondfield Down dip 

 in every direction from the centre to the circumference, as is 

 expressed on the map by arrows and figures ; its south hangings 

 are covered by beds of calcareous conglomerate, some of which 

 are crowned with caps of lias. 



The west side of Brondfield Down, being also the steepest, is 

 intersected by three magnificent gorges, resembling Chedder 

 Cliffs; of these Brockley Comb and Gobble Comb are the most 

 important; the latter has a strong resemblance to Dovedale, in 

 Derbyshire. 



In a trough of limestone between Brondfield Down and Leigh 

 Dovvn is situated the small coal field of Nailsea, which is a kind 

 of satellite exterior to, and attendant on, the great coal basin, to 

 the edge of which it is externally adjacent. 



The cliain of Leigh Down extends from Clevedon on the Severn 

 to the gorge of the Avon, near Clifton; it dips south, and has 

 beds of old red sandstone forming the base of the escarpment of 

 its east portion ; nearly parallel to this chain on its north-west 

 side is a similarly constructed ridge called Weston Down, having 

 a. similar dip ; and in the valley between them are the shattered 

 strata of the coal field of Clapton reposingr on the limestone of 

 Weston Dovvn, and touching the base of the escarpment of the 

 west portion of Leigh Down ; an extensive fault occurs along the 

 base of this portion of Leigh Down, and brings the coal measures 

 ii;to contact with the basset edges of the limestone. ■ 



