A. Strahan — The Age of the Vale of Clwyd. 113 



a reluctance to supersede it by an entirely wq'n explanation of the 

 structure of the Vale." I would point out, however, the map of 

 1850 was in fact superseded by an entirely new explanation, the 

 same which is now adopted almost in its entirety by Mr. Morton. 

 Of this he had full opportunity of judging both from the publications 

 on the district and from periodical inspections of my field-maps, 

 while he was completing his field-work. We differ only on the 

 theoretical question whether the faulting is in part pre-Triassic. That 

 my reasons for believing it to be so have not carried conviction, 

 makes me fear that I have not stated them with sufficient clearness. 



The displacement of the Vale of Clw^^d Fault, which may be 

 anything over 1.500 feet, is usually effected by two or three parallel 

 fractures, I'arely more than 100 yards apart. The fault runs somewhat 

 west of north on the whole, but becomes nearly north and south 

 at its southern termination. Where there are three fractures, as 

 often happens, the first throws limestone against Silurian, the second 

 throws the purple beds against the limestone, and the third Trias 

 against the purple beds, in other cases the Trias is faulted against 

 the limestone or even directly against Silurian rocks. On the west 

 side of the Vale the strike of the Carboniferous rocks is not along 

 but across it. Thus they would close it in at Denbigh were they 

 not thrown down by a large north-and-south fault, which shifts 

 their ontcrop a mile and a half southwards, and it is by several 

 repetitions of this structure that the Vale has been carried its total 

 length of 20 miles into the heart of the Silurian uplands. These 

 faults, having a general north-and-south direction, presumably run 

 into the Vale of Clwyd Fault northwards. That they were con- 

 temporaneous with it is suggested by the fact that they also 

 obviously break through the Trias. 



Although I do not know of any case where this arrangement of 

 main fault and branches can be exactly matched, the Vale of Clwyd 

 Fault belongs to one of the most consistently developed systems in 

 the British Isles. The system is characterized everywhere by its 

 N.N.W. direction. In West Cumberland the red rocks are repeatedly 

 shifted by such faults, while they pass quite undisturbed over some 

 huge fractures ranging S.E. to N.W. in the Carboniferous strata. 

 The Pennine Fault, to which the Vale of Eden owes its existence, 

 has the N.N.W. direction, and is also partly, at least, of post-Triassic 

 age, though it follows the line of a far larger pi'e-Triassic dislocation. 

 The Dent Fault is presumably of the same age. In South Lancashire 

 the Triassic outcrop is repeatedly shifted by faults of this direction, 

 while in North Wales there is a well-marked system of N.N.W. 

 faults or 'cross-courses,' which are believed to be of later date than 

 the east -and -west faults or ' lodes.' ^ The same relation holds 

 good in Cornwall also between the cross-courses and lodes.^ In 

 Worcestershire and North and South Staffordshire the N.N.W. post- 

 Triassic system is well exhibited, and includes one fracture injected 



1 " Geology of Flint, Mold, and Ruthin." 



' De la Beclie, " Report on the Geology of Cornwall, Devon, and Somerset," 

 1839, pp. 366-7. 



DECADE IV. VOL. YI. NO. III. 8 



