Vol. 6 3. J INLIKR IN THE EASTERN MEN DIPS. 237 



their whole diameter and the absence of any vesicular centre make 

 it impossible to regard them as bombs ; and, according to the view 

 that the coarse ashy conglomerate represents the agglomerate of 

 necks, the blocks would be considered to have been ejected by the 

 volcano in an already solidified state, and to have gained their present 

 character by friction against one another in the throat of the volcano, 

 or perhaps partly in the air. In their well-rounded and non- 

 vesicular character, the blocks closely resemble those met with at the 

 base of a thick mass of tuff of Llandeilo age in Tan-y-graig Quarry, 

 near Built h. 



The occurrence of the coarse ashy conglomerate in two well- 

 marked regions, one at the extreme western end of the area and the 

 other in the gap between the two large trap-masses of Downhead- 

 Tadhill and Moon's Hill is, so far as it goes, in favour of regardiug 

 the rock as forming two necks. The chief objection to this view is 

 found, as already stated, in the remarkably well-rounded character 

 of the great majority of the blocks, large and small, which is 

 strongly suggestive of water-action. 



The microscopical character of the matrix agrees well with the 

 ' neck theory,' as does the complete absence of stratification. 



On the whole, although there is little to choose between theories 

 (2) and (4), the latter or the ' neck theory ' appears to agree best 

 with the observed facts. 



The series of fossils found is, in Mr. Cowper Reed's opinion, 

 sufficient to prove that the rocks are definitely older than the Ludlow, 

 and in all probability of Llandovery age. The only other Silurian 

 volcanic series known in England, that of the not very far-distant 

 Tortworth district, is also of Llandovery age. 1 



In the absence of a conformable sequence between the Silurian 

 and the Old Red Sandstone, the Eastern Mendips are in accord with 

 Gower, 2 where the Old Red, which is much attenuated, especially in 

 its lowest subdivision, rests with probable- unconformity upon the 

 Silurian. 



At Tortworth, too. where the Old Red is reduced to a thickness 

 of some 200 or 300 feet, it is probable that the Ludlow Beds, 

 although shown in the Survey-map, are not represented. 3 and that 

 the Old Red rocks rest directly upon the AVenlock. The only 

 locality in Gloucestershire or Somerset where there seems to be a 

 possibility of a conformable sequence between the Silurian and the 

 Old Red Sandstone is at Tite's Point on the Severn ; but the section 

 here does not seem to have been restudied since Phillips 4 described 

 it, and it is much to be regretted that the thick deposits of Severn 

 mud render an adequate examination of the exposures almost 

 impossible. 



1 See Quart. Journ. Geol. Soc. vol. lvii (1901) p. 271. The upper ash-band 

 is here considered to be probably of Wenlock age, but the finding of a further 

 series of fossils, including StricJelandinia lirata, proves the horizon to be 

 Llandovery. 



2 ' Summary of Progress of the Geological Survey for 1901 ' (1902) pp. 38-40. 

 ■' C. Lloyd Morgan, Brit. Assoc. Handbook to Bristol district (1898) p. 12. 



4 Mem. Geol. Surv. vol. ii, pt. i (1848) p. 192. 



