314 PROF. A. H. COX AXD MR. A. K. WELLS OX THE [vol. lxxvi, 



(iv) thinning of volcanic rocks, are evidently interconnected. The 

 connexion between (i) and (ii) is readily explained. The north- 

 east and south-west, or ' Caledonian ' strike, may be regarded as 

 that typical in North Wales; wherever deviations from that strike 

 occur, faulting at once sets in. The connexion between (ii) and 

 (iv) is also clear. Further, the position of the intrusions was 

 doubtless determined b}>- the rigid cover of volcanic rocks ; where 

 the cover becomes less massive, and consequently less rigid, the 

 intrusions decrease in number and size. Accordingly, it appears 

 reasonable to assert that (a) the position of the faults was deter- 

 mined by the change of strike, or in other words, by the direction 

 of the earth-movements, and (b) the position of the intrusions 

 was determined bj r the volcanic rocks. The question then arises, 

 did the position of the volcanic rocks (with their associated 

 intrusions) determine the direction of subsequent earth-movements, 

 or does the reverse hold, that early earth-movements controlled the 

 disposition of the igneous rocks ? The available evidence all points 

 to the latter being the case. It has already been shown that the 

 tilting movements which were ultimately to produce the Harlech 

 Dome were already in progress before the pre-Bala intrusions 

 arose, and that the faulting had likewise been initiated at this 

 early date. It seems, therefore, a reasonable supposition that the 

 •course of these Ordovician movements determined the position of 

 the volcanic outbreaks and the lines which the later movements 

 were to follow. 1 



The relationships between the various structures, between the 

 tilting and the intrusions, the intrusions and the Derwas Fault, 

 the Derwas Fault and the Harlech Dome, and other faults, go to 

 show that the Harlech Dome must have had a very long period of 

 growth, beginning at least as early as Ordovician time. They 

 further show that the so-called post-Silurian or ' Caledonian ' 

 movements were, in reality, spread over a long period. In fact, it 

 is quite possible that movement was continuous, or almost so, from 

 Cambrian to Devonian time. The cumulative effect we speak of 

 as due to 'post- Silurian' movement. But, in speaking thus, we are 

 apt to lose sight of the long drawn-out character of the movement. 

 In fact, to assign certain of the effects to a ' post-Silurian ' 

 movement is equivalent to referring to, say, Cretaceous time, when 

 we really mean Mesozoic time. 



The fault-system and the Mawddach Estuary. — A 

 point of some interest is the relation of the Wnion Valley and its 

 •continuation, the Mawddach Estuary, to the fault-system. Above, 

 or north-east of Dolgelley, the Wnion Valley follows a straight 

 course for some miles. This line is evidently a continuation of 

 the Ceunant repeating fault, and it meets and coalesces with the 



1 See W. W. Watts, ' The Ig-neous Eocks of the Welsh Border ' Proc. Geol. 

 Assoc, vol. xix (1905) p. 174 ; and W. G. Fearnsides, ' North & Central Wales ' 

 Geology in the Field, Geol. Assoc. Jubilee Vol. (1910) p. 810. 



