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



will only notice that, besides those which are most common in this formation, 

 it contains the palates, teeth, fin-bones, and vertebrae of fishes ; which serve to 

 refute a prevailing^ opinion, that vertebral animals do not occur in formations 

 more ancient than the coal-measures. 



The mountain limestone, like other compact limestones, is very liable 

 to the occurrence of caverns, of which many remarkable instances are found 

 within our district*. They consist of a series of vaulted chambers connected 

 by narrow passages. They are frequently traversed by streams derived from 

 swallet-holes, but are more frequently destitute of any water, but such as per- 

 colates through the sides and roof. The strata in which they occur are in 

 general almost wholly concealed by incrustations of stalactite ; so that it is not 

 easy to determine by observations on the strata how far the caves owe their 

 origin to mechanical disturbance. In several instances, however, when the 

 mouths of caverns occur in the faces of cliffs, affording distinct sections, the 

 strata do not appear to have been dislocated, but are continuous. 



Another remarkable feature in the mountain limestone is its intersection by 

 narrow precipitous mural chasms, which lie transversely to the calcareous 

 chains which they intersect, and to the principal lowland valleys contiguous. 

 These often form the gorges by which the upland valleys descend into the 

 neighbouring plains ; they are then simple excavations in the limestone, unat- 

 tended by any disturbance in the strata, and are the result, apparently, of the 

 furrowing action of some violent cause, acting from without. At other times 

 these ravines completely intersect the calcareous ridges to a depth beneath the 

 level of the contiguous longitudinal valleys, and they then turn the rivers aside 

 from pursuing those valleys in their course. In the latter case they are often 

 due to internal derangements of the highest antiquity, though probably con- 

 siderably modified in their forms by subsequent denuding forces. Cheddar 

 cliff is an example of the former kind ; and the defile of the Avon, which turns 



* A notice of the caverns near Loxton and Banwell, in the western part of the Mendip chain, 

 will be found in the Gentleman's Magazine for 1794. That of Hutton Hill, containing antedilu. 

 vian bones, is described in the Reliquice Diluvianw, page 57, from Mr. Catcott's notes ; that of 

 Burringdon, near the entrance of Burringdon Combe, which has served for a place of sepulture, and 

 contains human bones incrusted by stalactite, in Collinson's History of Somersetshire. Several 

 small caves may be seen in the face of Cheddar cliffs ; as may that called the Giant's Cave in the 

 face of St. Vincent's rocks. Penpark hole, not far from Westbury, near the outer edge of the 

 northern extremity of Durdham Down, has been described and represented in the Philosophical 

 Transactions for 1682. It has sometimes been considered as a deserted lead-mine; but the di- 

 mensions of its principal chambers are inconsistent with this notion, though its shaft and lateral 

 galleries miay be due to mining operations, which may have cut into this natural chamber, as in the 

 instance of the Speedwell level in Derbyshire. 



