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



sloping planes and escarpments of the mountain chains. Mr. Warburton 

 has thus accounted for the appearance of those insulated masses of conglo- 

 merate which had been observed by Mr. Tennant on the southern hangings 

 of the Mendips. In the narrow valleys, having a base of old red sandstone, 

 which lie inclosed within ridges of mountain limestone, the intervals between 

 the ridges are generally occupied by the conglomerate, which conceals with 

 its horizontal beds the fundamental rock. Striking examples of this maybe 

 seen in the valleys of Shipham on Mendip, and of Westbury near Bristol. 



Associated with the conglomerate, there often occurs a sandstone of so fine 

 and compact a grain, as to assume the character of chert. It occurs inter- 

 stratified with the conglomerate on the road between Pitcot and Holcombe, 

 at the new colliery between Chilcompton and Stratton, and in a quarry by the 

 side of the lane leading from that colliery to Stratton, at the entrance of the 

 village. Nearly adjoining the places here enumerated, on the south-west, and 

 to the south of Emborrow mere, is an insulated tract * f of a mile long, and 

 5 broad, in which a similar chert is found. It is bounded by conglomerate 

 and mountain limestone : but there are no sections which distinctly show its 

 relations to those rocks. In all these places the chert is destitute of shells, 

 except at Emborrow, where it contains fragments of encrinites ; but those 

 are probably derived from the debris of mountain limestone. 



The chert found on the elevated platform above East Harptree, and on the 

 neighbouring summits on either side of the Tar Hall valley, is of so peculiar 

 a character, that it is with some hesitation that we refer it to the dolomitic 

 conglomerate. 



The situation and extent of this platform, which is traversed by the road 

 from the Castle of Comfort to East Harptree, have been already described in 

 the 2nd chapter. The rock which crowns the platform is a compact cherty 

 sandstone, which occasionally passes into a conglomerate, consisting of frag- 

 ments of old red sandstone imbedded in a cherty cement. This cherty sand- 

 stone abounds with heavy-spar, and is sometimes very calcareous. It here 

 contains shells belonging to the genera Ammonites, Modiola, Pecten, Tellina, 

 and Plagiostoma (2 species), which in general are reduced to the state of 

 hollow casts, partially filled in some instances with heavy-spar ; the shell it- 

 self is sometimes chalcedonized. The summit of the platform is covered 

 with blocks of the chert, which appears also in numerous excavations forming 

 a thick horizontal bed f , reposing on ochreous sand. The whole surface is 



* There is a remarkable swallet-hole in this tract. 



+ The Druidical circles of Stanton Drew, about 9 miles distant to the north, are principally 

 constructed of this cherty sandstone. 



