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



tain limestone is to be seen in situ, which is covered in the immediate neigh- 

 bourhood by overlying dolomitic conglomerate. 



In the valley of the Severn from Tewksbury dov^^nwards to Pyrton passage, 

 an extensive and almost continuous sheet of lias reposes on the newer red 

 sandstone, the sinuous course of the river repeatedly crossing their junction, 

 and thus affording numerous sections, of which those of Westbury cliff and 

 the cUffs on either side of Aust passage are the most remarkable. 



The most important appearance at Aust is the display of 5 nearly vertical 

 faults, which occur in the face of a cliff not exceeding 5 of a mile in length nor 

 60 feet in height. The beds exposed in the cliff are three varieties of lias, 

 which repose on two varieties of the marl belonging to the newer red sand- 

 stone, viz. the green marl and the red marl. The distinct character and 

 colours which belong to each of these strata, and the clear display of them 

 which is afforded along the whole line of section by the vertical state of the 

 cliff, render this the most eligible spot that we have ever seen for observing the 

 phenomena of faults. The drawings of PI. XXXVII. will supersede the 

 necessity of a description. 



Each of the faults has acted as a dam to the rain-water, which, percolating 

 downwards from the surface, drains over the inclined planes of the strata, and 

 has occasioned the discharge of a spring, which, by covering with a slight sta- 

 lactitic incrustation the soft materials over which it runs, has protected tbem 

 from equally rapid destruction with the naked surface of the marl, and given 

 to them a bastion-like projection* beyond the face of the cliff. The waves of 

 the Severn are continually undermining the cliff, and washing out of it large 

 and valuable quantities of gypsum. 



Examples of fractured beds of lias, similar to those at Aust, have been 

 already noticed as occurring at Bitton and at other places in the Bristol coal- 

 basin. Another instance may be noticed on the west side of the valley between 

 Keynsham and Compton Dando, where portions of lias are thrown down 

 below an escarpment of red marl ; another may be seen on the road-side a 

 little to the south of Cannards grave near Shepton Mallet, and others at Barry 

 island, and thence along the coast of Glamorganshire to Dunraven. Of how 

 rare occurrence are such fractures in strata more recent than the coal-measures 

 in comparison of those which affect the older and inclined beds, may be learnt 

 from the few irregularities of stratification in the lias of Glamorganshire, con- 

 trasted with the perpetual disturbances which have heaved the more ancient 



* Similar projections are common, under similar circumstances, on the coast of Dorsetshire and 

 Devonshire. 



