404 Sorace B. Woodward — On the Metamorphism of Strata. 



Cross. Bottom, with a breadth of about a quarter of a mile, is 

 regarded by the former as evidence of the mighty agent which 

 formed the Mendip anticlinal, at " a time not far removed from the 

 deposition of the upper beds of the Trias," or perhaps, we might 

 suggest, in Permian times. 



The trap rock of Bleadon Hill was described by the Eev. D. 

 "Williams, soon after the completion of the railway cutting at Uphill.^ 

 The rock is exposed at the base of the section beneath the Mountain 

 Limestone, which is much altered by it. Mr. Williams points out 

 the change to be one of crystallization, and that the Limestone has 

 altered in colour and become very crisp and brittle.' Mr. Sanders^ 

 subsequently described the section, and stated, without going into 

 particulars, that he differed very materially from Mr. Williams. He 

 regarded the igneous rock to have caused the extensive faulting 

 which is shown in the section, where the Lias is brought abruptly 

 against the Mountain Limestone. And therefore this eruption is 

 probably of the same age as that which, as we believe, affected the 

 Secondary beds at Harptree. 



Siliceous Mountain Limestone. — The Mountain Limestone is in some 

 places very siliceous, particularly in the neighbourhood of Banwell, 

 and also north of Priddy, where I remember using pieces of the 

 rock to strike a light for my pipe. The silicification, I need not say, 

 has nothing to do with the black chert bands which cut across the 

 beds in all directions. 



Elevation and Depression. — We must now look to support from 

 other considerations to account for the metamorphism of the 

 Secondary beds. We know that considerable changes, apart from 

 denudation, have affected the configtiration of the district since the 

 deposition of the Jurassic rocks, for the Lias, Rhsetic, and Dolomitio 

 Conglomerate occur on some of the highest parts of the Mendips, 

 between Charterhouse Warren and Egar Hill, and considerably 

 above the vales and plains on either side of the range where these 

 beds occur in force. Irregularity of sea-bottom, or the higher 

 position of beach deposits over those formed away from the coast, 

 cannot be adduced to account for these ; unequal elevation in Post- 

 Liassic times is clearly the explanation. 



We say unequal elevation, because there is no doubt that 

 portions of the Mendip area were above water during the Triassic 

 and Liassic periods. Indeed, Mr. Moore is of opinion that as an 

 island it constituted a barrier against the incursion of beds imme- 

 diately to the north, v/here the Keuper and Liassic, though not the 

 Bhcetic beds, show a considerable thinning out.* 



We cannot tell the precise period when the higher patches of Lias 

 and Dolomitic Conglomerate, including a portion of the cherty beds, 

 were elevated into the positions they now occupy. 



1 Proc. Geol. Soc, vol. iv., p. 293. 



2 I have an impression that it was rather siliceous. But Dana observes that in the 

 metamorphism of limestone the carbonic acid is not given out if the material is under 

 heavy pressure. — Manual of Geology, 1863, p. 705. 



3 Sep. Brit. Assoc, 1846. " Trans, of Sections, p. 60. 

 * Quart. Journ, Geol. Soc, vol. xxiii., p. 449. 



