South-western Coal District of England. 223 



tract of stiff wet clay-land^ immediately encircling the base of the old red 

 sandstone^ and extending from that base to the dry surface of the limestone. 

 Its exterior flank is often overhung by a precipitous scar, occasioned by the 

 outcrop of the superincumbent limestone ; which from its greater powers of 

 resistance has been less cut away by denudation than the shale. [Vide PI. 

 XXXII. fig. 3. and 3. A & B. 



The contact of the shale with the limestone is marked in many places by the 

 sudden engulfment of streams, which rising in the old red sandstone, main- 

 tain their course above ground, while flowing on that rock or on the retentive 

 beds of shale ; but no sooner come into contact with the encircling zones of 

 limestone, than they suddenly disappear in its numerous fissures and caverns *. 



The upper limestone-shale may be recognised at Clifton in those alterna- 

 tions of shale with sandstone, impure limestone and thin seams of coal, which 

 form a link between the subjacent beds of pure limestone and the incumbent 

 beds of millstone grit. We comprehend under this title the beds from No. 1 

 to No. 46 inclusive of Mr. Cumberland's section. This shale diff'ers from the 

 analogous limestone-shale of Derbyshire in the oolitic structure of some of its 

 subordinate beds of limestone, and in the abundance of red oxide of iron 

 which pervades the whole formation. This red oxide or ruddle prevails also 

 in the millstone and coal grits of the district, the red taking place of the grey 

 by which the similar grits in Derbyshire are tinged. 



The aggregate thickness of the beds belonging to the formation of moun- 

 tain limestone, as determined by comparing their horizontal breadth with 

 their inclination on either side of the Mendips, is from 500 to 700 yards. 

 This is considerably less than the thickness ascribed to these beds in the defile 

 of the Avon by Mr. Cumberland ; but we shall assign the reasons for this 

 want of agreement in our measurements, when we come to treat of that defile. 



Respecting the organic remains of the mountain limestone in this basin, we 



* The openings thus engulfing the streams are known provincially in this district, as well 

 as in Derbyshire and Yorkshire, by the name of swallet-holes, being abundant in those 

 counties, as v.ell as in every other where mountain limestone prevails. Examples of these swallet- 

 holes occur a' Downhead Mill, near the south-eastern extremity of the Mendip chain ; at the 

 village of Prtdu ; and at Charterhouse, on the south-east flank of Blackdown. The exit of the 

 streams swallowed up may be traced with much probability to the following points : that of the 

 Downhead stream to the limestone chasm about a mile east of the village of Downhead ; that of 

 the Priddy rivulet to Wokey hole, and that of th© Charterhouse brook to the foot of Cheddar 

 clifiFs. The streams which emerge are all greater, indeed, than those engulfed ; but this arises 

 probably from the tributary waters which they have received during their subterranectus course. 

 The streams, if excessively flooded, might, after suff'ering partial engulfment continue to flow 

 above ground through the valleys of denudation in which the swallet-holes are situated, 

 \0L. VI. 2 G 



