308 PROF. A. H, COX AND ME. A. K. WELLS OX THE [vol. lxxvi, 



already been suggested by Prof. W. Gr. Fearnsides, 1 and it may well 

 be that the remarkable thinness of the Bifidus Beds of Arenig 

 Mountain compared with those in the Dolgelley district is due to 

 such an overlap. 



With regard to the possibilit} r of the Lower Ordovician rocks 

 having been tilted and faulted as early as Bala time, it must be 

 remembered that by the time the Archis Beds (low down in the 

 Hartfell Group) were deposited, a space of time sufficient for the 

 accumulation of at least 5000 feet of bedded rocks had elapsed since 

 the beginning of the Ordovician Period, and further, that during 

 that time there had been four quite distinct volcanic episodes. The 

 mere existence of such pronounced volcanic activity suggests that 

 earth-movements were in progress. 



It may be noted that the possibility of inter-Ordovician folding 

 proceeding in the same general direction as a later and more 

 powerful folding has already been indicated by Mr. J. F. N. Green 

 in the case of the Ordovician rocks of the Lake District. 3 The 

 same author has further adduced evidence that the Ordovician 

 movements in the Lake District were on such a scale that con- 

 siderable denudation of the volcanic rocks had taken place before 

 an early Bala date. 3 



That elsewhere movements were taking place during Bala time 

 is suggested by the overlaps that occur among the Bala rocks of 

 South Wales/ 



(2) Movements posterior to the intrusions: (i) The 

 main folding. — Since the district is situated on the southern 

 flanks of the Harlech Dome, the general trend of the strata is 

 determined by that dominating structure. Accordingly, the strike 

 is almost due east and west in the eastern part of the area, gradually 

 swinging round to a north-east-south-west direction in the western 

 part. 



The direction of the dip is invariabty away from the dome, and 

 remains at about 40° with remarkable uniformity throughout the 

 district, except in the belt of country extending through Bryn 

 Brith and Pared-y-Cefn Hir, where the dip rises to 60° or even 70°. 



The Harlech Dome itself is not, however, a simple structure, but 

 is crossed by various minor folds, some of which range into the 

 area now under consideration. The minor folds within this area 

 have a trend that departs but little from due north. The most 

 marked of them are the anticline of the Arthog Valley and the 

 complementary syncline. Both are quite shallow folds, but they 

 are nevertheless sufficiently powerful to cause some oscillation in 



1 Q. J. G. S. vol. lxi (1905) p. 618. 



2 Proc. Geol. Assoc, vol. xxvi (1915) p. 219. 



3 Ibid. vol. xxviii (1917) p. 25. 



4 T. C. Cantrill, H. H. Thomas, & O. T. Jones, ' The Country round Haver- 

 fordwest : Geology of the South Wales Coalfield, pt. xi ' Mem. Geol. Surv. 

 1914, p. 55 ; and 'The Country round Milford: op. cii.pt. xii ' ibid. 1916, 

 p. 46. 



