Correspondence — Mr. G. H. Ilorfon. 231 



directly on marmoiized Carboniferous Limestone in part of the 

 quarry ; elsewhere it is separated from it hy vesicular ' toadstone ' 

 or by a bed of clay which is indurated and rendered columnar for 

 a considerable depth. Above the dolerite comes another vesicular 

 ' toadstone.' Thus the dolerite does not always rest on rocks of the 

 same horizon ; while the amount of alteration effected by it is far 

 greater than is usually associated with lava-flows. Taking one bed 

 in the limestone as a datum, the author establishes the following 

 conclusions : — The dolerite does not cut across the beds of lime- 

 stone ; the clay varies in thickness, and is sometimes absent ; the 

 clay is rendered columnar and the limestone marmorized to a con- 

 siderable depth, unless vesicular ' toadstone ' of sufficient thickness 

 intervenes, when the ' toadstone' itself is indurated ; the base of the 

 compact dolerite is approximately parallel to that of the meta- 

 morphosed rock. The dolerite itself is ophitic at its centre, granular 

 above and below, and fine-grained at its margin ; it is different in 

 microscopic aspect from the vesicular 'toadstone.' Descriptions of 

 the limestone and marble, the lavas, tuffs, and clay, are also given, 

 and the positions of the faults bounding the inlier are defined. 



oos,i?-:BSi=oisrnD:E3srGE. 



THE AGE OF THE VALE OF CLWYD. 



Sir, — In the article on this subject in your March issue, Mr, Strahan 

 refers to a recent paper of mine on "The Carboniferous Limestone 

 of the Vale of Clwyd," ^ and to a difi"erence of opinion between us 

 as to the period during which the faulting of the strata occurred. 

 He states that " We differ only on the theoretical question whether 

 the faulting is in part pre-Triassic " ; but it seems to me that we also 

 differ as to whether the Trias anywhere overlaps the older rocks. 

 In my paper it is stated that " I failed to find a satisfactory section 

 showing the Trias overlapping any portion of the Carboniferous 

 Limestone or the Purple Sandstone in the Vale of Clwyd," and 

 I might have added, " or the Wenlock Shale." 



Mr. Strahan gives several instances where he supposes the Trias 

 to overlap the Limestone, but particularly along the east of the 

 ridge on the west of Llanfair, Dyffryn Clwyd, and refers to " the 

 Trias creeping across the edges of zone after zone of the limestone, 

 until near Euthin half that formation has been overlapped." In that 

 locality I could not find any evidence of overlap, but that it was the 

 gradually increasing denudation of the Limestone from south to 

 north that caused it to become several hundred feet thinner. Of other 

 examples he gives Bodfari, about which he remarks that " The 

 evidence, however, is more suggestive than conclusive," and he 

 qualifies his references in like manner as to other places. I have 

 searched in vain for some evidence of an overlap by the Trias, but 

 could not find any. 



Mr. Strahan, in support of the pre-Triassic faulting, seems to rely 

 on the N.N.W. faults so general along the west of the British Isles, 

 1 Proc. Liverpool Geol. Soc, vol. viii (1897-8), p. 32. 



