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



small insulated crag- at the mouth of the Wye, belonging- to the same formation, 

 called Chapel-rock ; and thus the mountain limestone of the basin of Bristol 

 becomes united to the chains of mountain limestone in Monmouthshire. 



It is probable that the axis of old red sandstone, which we traced from 

 Portishead to Westbury, continues to range northwards along the foot of Al- 

 mondsbury Down, though concealed by overlying superstrata and a tract of 

 marshland. To the west of this axis, beneath the marsh-lands of the Severn, 

 the coal-measures seen at Portishead-point may be expected to be prolonged. 

 The exterior limestone chains of this unexplored coal-basin appear to be 

 Portishead -hill, Kingsweston Down, Olveston Down, Aust Cliff, Chapel Rock, 

 the Monmouthshire chain from thence to the village of Undy, and the Denny 

 Rock opposite to Portishead. 



From Olveston Down the frontier of limestone and old red sandstone sweeps 

 near Thornbury to Tortworth, where it reaches its northern apex. On the 

 edge of the Coal-basin at Cromehall near Tortworth, at the junction of the 

 mountain limestone with the incumbent millstone grit, we find an alternation 

 of the beds of grit and limestone, very similar to that which at the defile of 

 the Avon we have described as the upper limestone-shale. The grit, however, 

 in this instance assumes the form of a quartzose conglomerate, agreeing in 

 character with the rock which on the north of Merthyr Tydvil, in South 

 Wales, supports the coal-measures. The strata dip at an angle of 35° to the 

 south. 



We had formerly devoted a chapter expressly to the district immediately 

 north of Tortworth ; but finding that a detailed account of the northern angle 

 of the basin has since been presented to the Society by a geologist, who to 

 talent of a high order adds the advantages of local residence, we determined 

 to suppress our own more rapid sketch. We wish only to express one opinion 

 respecting the rocks of that district. 



Two masses of amygdaloidal trap, about 2 miles long, are there found tra- 

 versing the transition limestone and the old red sandstone, which from their 

 general parallelism to one another and to the strata which bound them, might 

 appear at first sight to be regular beds. We are of opinion, however, after a 

 careful examination of their course, that they really are portions of di/kes ir- 

 regularly traversing the other rocks. The thinness of these masses at their 

 eastern extremity contrasted with their thickness on the west towards Wood- 

 ford-green, and more especially the mode in which at the latter place they 

 penetrate among and entangle fragments of the contiguous coralliferous lime- 

 stone, which have been altered by the contact ; — these are the circumstances 

 which have induced us to form this opinion concerning their nature. 



