Vol. 6$.~] IXL1ER IN THE ElSTERN MEND1PS. 235 



Old Red Sandstone and the Silurian Series must also be borne in 

 mind, especially in view of what has taken place at Ebbor, and 

 perhaps at Yobster and Luckington. In the Kidwelly and Tenby 

 district, too, which in many respects forms a continuation of the 

 Mendip area, Mr. E. E. L. Dixon has recently discovered thrusting 

 on a large scale. 



With regard to the coarse ashy conglomerate, both its nature and 

 its field-relationship are obscure. 



The following possibilities suggest themselves as to its nature : — 



(1) That it may be the basement-conglomerate of the Old Red Sandstone. 



(2) That it may be an aqueous deposit of the same general age, and 



belonging to the same igneous series, as the associated traps and 

 normal tuffs. 



(3) That it may be an old river-gravel, deposited during a terrestrial 



period subsequent to the fossiliferous Silurian and prior to the Old 

 Bed Sandstone of the district. 



(4) That it may represent the necks of the volcanoes from which the trap 



and normal tuff were ejected. 1 



These possibilities may now be considered in order : — 



(1) That the coarse ashy conglomerate is the basement-con- 

 glomerate of the Old Red Sandstone. — This view, although at first 

 sight attractive, is not supported by field-evidence, as the Old Red 

 Sandstone, as already pointed out, almost certainly rests directly 

 at one place upon the fine tuff, at another upon the coarse ashy 

 conglomerate, and at others upon the trap. The character of the 

 matrix too, which is a normal tuff closely resembling some of the 

 undoubtedly Silurian tuffs of Sunnyhill Quarry, is opposed to this 

 view, as it is very unlikely that tuffs of almost identically the same 

 character should have been thrown out in the same district in 

 Llandovery (or Wenlock) and again in Old-Red- Sandstone times ; 

 and there is no suggestion anywhere else in the Old Red of the 

 South of England of the occurrence of any igneous matter. 



(2) That the coarse ashy conglomerate is an aqueous deposit of 

 the same general age, and belonging to the same igneous series, as 

 the associated traps and normal tuffs. — The difficulty, if it be a 

 difficulty, of this view is that it implies that, after the outpouring 

 and complete solidification of the lavas, they became subject to 

 prolonged erosion, during which the resulting blocks (with relatively- 

 few exceptions) were thoroughly rounded and were finally mingled 

 with a large amount of fine ashy material of much the same type as 

 that which had been previously ejected, due to a renewed outburst of 

 volcanic action. This might readily have taken place, if the water in 

 which the fossiliferous tuffs were laid down, originally shallow, had 

 been partly filled up by the outpouring of trap, so as to bring the 

 upper portion of the trap under the influence of breakers. According 

 to this view, the coarse ashy conglomerate is the newest rock of the 

 igneous series; and as the series is found, where dips are obtainable, 

 to dip northward, it should occur to the north of the outcrop of the 

 other rocks. Only at one point — in the area south-west of Tadhill 



1 I submitted specimens and sections of the coarse ashy conglomerate to 

 Mr. A. Harker, F.R.S.. and Dr. J. S. Flett. and am much indebted to them for 

 help in considering the nature of this puzzling deposit. 



