South-western Coal District of England. 245 



data we shall obtain ^d of a mile as the aggregate thickness of the beds of 

 mountain limestone in this quarter^ a thickness far exceeding that which is 

 deducible from the ordinary breadth of the calcareous zone, unless we admit 

 the existence of a fault, and a partial repetition of the upper beds of the series. 



The lower limestone-shale consists of beds of shale alternating in the upper 

 part with beds of limestone, and in the lower part with beds of sandstone. 

 The aggregate thickness of these beds is here 500 feet. The beds of lime- 

 stone are Nos. 292, 293 and 294 of Mr. C, the lowest of which is a well- 

 characterised red encrinal limestone, forming a conspicuous band on the ascent 

 of the woody bank to the south of Cook's Polly, and one equally distinct on the 

 left bank of the river. 



The old red sandstone not being exposed to any extent in the face of the 

 cliffs, cannot be so well examined as the limestone. It may best be studied on 

 the left bank of the river, where the towing-path ascends over a steep rise. 

 We have there observed the outcrop from beneath the shale of the following 

 beds. 1. Red sandstone and marl; 2. grayish marly sandstone; 3. red 

 marly sand and variegated sandstone ; 4. thick strata of quartzose conglome- 

 rate ; 5. red gritty sandstone. Dr. Bright mentions vegetable impressions and 

 traces of coal as occurring near the top of the series, and a bed of calcareous 

 concretions, of the nature of the Herefordshire cornstone, near its middle. 



As the defile opens, at Sneyd park on the right bank of the river, and in the 

 corresponding hill on the left, the old red sandstone is overlaid by horizontal 

 strata of dolomitic conglomeraie. The junction of the two formations on the 

 left bank is represented in Plate XXXV. fig. 1. The sandstone rises again 

 to the surface, and is displayed in the face of a precipitous bank near the Pow- 

 der Magazine below Lord de Clifford's park ; but, with this exception, the 

 conglomerate occupies both sides of the river, until it sinks below the marshes 

 that border on the Severn. 



We shall now pursue the calcareous chain on the right bank of the Avon, 

 and trace its return on the opposite side of the nucleus of old red sandstone. 

 The district occupied by this nucleus, and overlaid for the most part by the 

 conglomerate, comprehends a triangular space inclosed between the Avon from 

 Cook's Folly to the Powder Magazine, and lines drawn from these two extre- 

 mities of the base of the triangle to its apex at the village of Westbury. The 

 fundamental rock is laid open by denudation along the brook to the north of 

 Sneyd park, and in the bed of the Trim rivulet, where it issues from the ravine 

 of Blaze Castle close to the water-mill : but near to Westbury the conglome- 

 rate comes in contact with the mountain limestone. The calcareous zone 

 ranges round this area, presenting towards it a steep escarpment. From the 



