306 PROF. A. H. COX AND MR. A. K". WELLS ON THE [vol. lxxvi, 



movements — have contributed to modify the general scheme. The 

 sequence of events has here been : — 



(1) Tilting of the strata and initiation of the transverse -, ,, 



f ij- I Movements 



(2) Initiation of strike-faulting. f . , . 



(3) Intrusion of hypabyssal rocks. - 1 



(4) Main folding. Movements 



(5) Cleavage. I , . , 



(6) Main strike-faulting, with accompanying transverse | . , 



faulting. J 



There can be little doubt that the last three of the phenomena 

 -enumerated above were brought about by the widespread ' Cale- 

 donian ' movement which affected the whole of North Wales. It 

 is quite unnecessary to give here the grounds for this statement. 

 Proof depends upon a consideration of the general structure of 

 North Wales as part of a much more extensive region. But some 

 explanation is needed for placing the first three events in the order 

 given and for ascribing them to the Ordovician Period. Such 

 explanation necessitates a more detailed consideration of the 

 •structures observed and of the relationships between them. 



(1) Earth -movements anterior to intrusion: the re- 

 lations of the intrusive rocks to the tectonic features. — 

 'The proof that tilting, folding, and faulting were initiated during 

 the Ordovician Period depends upon the relations of the intrusive 

 rocks to the various tectonic features. It has already been shown 

 (p. 305) that petro logical and stratigraphical considerations unite 

 in assigning an early Bala date to the intrusions. It has further 

 been demonstrated (p. 301) that the manner in which the intru- 

 sions take part in the folding and in the repetition by faulting 

 proves that they antedated the main folding and faulting move- 

 ments. 



When the question is examined in greater detail, however, some 

 interesting facts with regard to the relations between the igneous 

 rocks and the folding become apparent, and show that a certain 

 amount of movement had taken place prior to the intrusions. 



First, as regards faulting, one of the strike-faults — the Derwas 

 Fault — quite clearly disappears no fewer than three times against 

 intrusive masses without affecting them, yet in each case it reappears 

 on the opposite side of the intrusion precisely in the position where 

 it would be expected, and it affects then, once again, the sedimentary 

 rocks. Details of the separate occurrences are given below (p. 311). 

 In a fourth case, however, an intrusive mass is affected by the 

 fault, though not to the same extent as the adjacent stratified 

 rocks. Thus, the final conclusion is that the intrusions took 

 place after the movements which gave rise to the Derwas Fault, 

 but that these movements were renewed at some subsequent period. 

 It will be shown later (p. 312) that the Derwas Fault is itself 

 Intimately related to the folding. 



