240 Messrs. Bucrland's and Conybeare's Observations on the 



hills of old red sandstone ; the sandstone however is occasionally discovered sup- 

 porting the conglomerate in the indentations of the coast. The northern side 

 of Portishead-bay belonging-, not to the subsided mass of strata, but to the return 

 of the principal chain on the northern side of the nucleus of old red sandstone, 

 will be described when we come to treat of that part of the calcareous border. 



We now resume our account of the principal chain at that point where the 

 celebrated defile of the Avon divides Leigh Down from Durdham Down. This 

 natural section would require a full description, had not an excellent paper on 

 the subject by Dr. Bright, and a detailed account of its strata by Mr. Cumber- 

 land, been already published in the Transactions of the Society. We shall 

 therefore content ourselves Avith a few additional observations, which with the 

 help of the Map and Section.s, PI. XXXIII. fig. 1 and 3, and PI. XXXIV. 

 fig. I, we hope to render intelligible. Our remarks are chiefly directed to the 

 right bank of the river, as being the best calculated for geological examina- 

 tion, the left bank being much less precipitous^ and covered in great part with 

 uxuriant woods. 



The inclined strata, that crop out successively in this defile, are the millstone- 

 grit, the upper hmestone-shale, the mountain limestone, the lower limestone- 

 shale, and the old red sandstone. The millstone- grit passes about 200 yards 

 west of Rownham ferry, beneath the south-w estern extremity of Windsor ter- 

 race at its foundation, and immediately behind its gardens at the north-east end. 

 Hence it may be traced to Clifton church, Brandon-hill, Park-street, St. Mi- 

 chael's hill, and the contiguous parts of Bristol. The lowest beds of this grit 

 here consist of a red argillaceous sandstone. 



The upper limestone-shale, consisting of alternating beds of shale, grit, and 

 limestone, includes the first 5 strata of Dr. B. and the first 46 of Mr. C. It ad- 

 mits of the following 4 subdivisions. 



1. Upper limestone, having the usual characters of mountain 



limestone. Thickness about 



It is the upper part of No. 1. of Dr. B., and its lower part 



is No. 1. of Mr. C. . .20 feet. 



2. Middle grit, consisting of shale and grit alternating, and 



having a thin coal-seam. 

 It is included in No. 1 of Dr. B., and includes from No. 

 2 to No. 8 of Mr. C. It may be traced up the guUey 

 beneath the wall of Windsor terrace gardens to the 

 middle of Prince's Buildings. 60 



3. Middle limestone, consisting of several beds having the 



character of mountain limestone. 



