WILLIAMS ON THE SALLFEROUS MARLS. 151 



posits, there is a natural section close at hand which might help 

 us to the solution of the problem. The effects of exposure have 

 here considerably effaced the evidences which the cutting through 

 the rocks at Bleadon disclosed so admirably ; but I found no diffi- 

 culty in at once recognising them. 



In the memoir already referred to, I mentioned that the cracks 

 or joints and irregular hollows were filled with a variegated marly 

 substance ; and at Weston I noticed the face and joint walls of 

 the limestone bed to be deeply eroded by many of the same deep 

 cavities, in all kinds of positions ; and, in several cases, the joint 

 walls are even now red and discoloured. In two instances, one of 

 an open joint, the other a cavity, they were filled by the same 

 variegated red, buff, and grey laminated marls that I had met 

 with at Bleadon. Several of the cavities show, internally and on 

 their edges, a strikingly rough and irregular outline. They are 

 all of them many feet above the highest spring-tide level, and are 

 often so overhanging or highly inclined that stones could by no 

 possibility be contained in them ; and the other limestone beds 

 exposed along the coast exhibit nothing of the kind. The lower 

 portion of the bed is hidden by an artificial platform of stones ; 

 but evidences enough remain, within the space of a few feet, to 

 indicate the circumstances under which these trappean marl rocks 

 were generated, and of the date when they were erupted. A 

 north and south fissure, which has riven asunder the beds marked 

 2, 3, and 4. on the section, and which is partly filled at the 

 bottom with a hard red marl, was probably the vomitory through 

 which the upper trap was ejected. 



Two instances of trap rocks, of the same mineral composition, 

 one generated in situ, the other erupted, with the effects produced 

 by both on the immediately adjacent rocks, are rarely perhaps to 

 be met with so well disclosed and in so short a distance as in the 

 case between the Bleadon cutting and the cliffs of Weston. 



In using the terms fusion and conversion, I wish it to be dis- 

 tinctly understood that I never supposed that any one elementary 

 earth or substance could be converted into another : such as that 

 lime, for example, could become silex. I contend, only, that the 

 limestone has been converted into trap, volcanic mud, or marl, as 

 the case may be ; and I entertain little doubt that a chemical ana- 

 lysis of these would show that the mineral constituents of the 

 former existed in the latter ; though, no doubt, in different, and, 

 perhaps, very varying proportions. 



I will conclude with one more remark on the immediate subject 

 of this communication. The association of variegated marls, salt, 

 gypsum, and magnesian limestone in various places at different 

 geological levels appears assignable to some one cause which has 

 been common to all. When, therefore, we mark the close analogy, 

 the identity in many respects, of these peculiar aggregates with 

 a variegated, gypseous and apparently magnesian marl, which is in- 

 controvertibly a volcanic product, we may be supposed to have as- 

 certained the common origin of both ; and instead of multiplying 



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