222 DR. WILLS AIs^D MR. SMITH OX THE LOWER [vol. Ixxviu, 



The cleavage movements have produced a variety of minor 

 structures of interest, Avhich space forbids us to describe here. It 

 may, however, be pointed out that these structures usually occur 

 where hard and soft bands are interbedded. It is characteristic 

 of this district that the effect of the pressure has usually been to 

 break up the harder bands into phacoidal fragments, rather than 

 to produce contortions of these bands as a whole. It is by the 

 shuffling of the phacoidal fragments that the necessary adjustment 

 appears to have been brought about. 



V. The Pre-Caeboi^ferous Floor. 



The folding, faulting, and cleavage so far described formed part 

 of the Devonian mountain-building. Vigorous erosion accompanied 

 and followed the elevation, and eventually produced a peneplain. 

 But we can still see in the form of the floor upon which the 

 Carboniferous rests, the influence of the major tectonic features on 

 tlie then-surface relief ; for the anticlinal areas tended to stand out 

 as hills, and the synclinal areas as depressions. On the line of the 

 Llangollen Synclinorium the red Basement Beds occur, followed by 

 the maximum thickness of Carboniferous Limestone, susrorestinsr 

 that here was a bay opening eastwards. The successive overlap of 

 higher and higher beds against the anticlinal area of C3a'n-3^-Brain 

 can be clearly seen on the north (see p. 210), while southwards 

 towards the Berwjai Dome the thinning of the Limestone is even 

 more rapid. The steeper submarine contours on this side of the 

 ancient bay may have been related to the Llangollen Fault. 



It has already been pointed out, in passing, that much of the 

 movement along the Llanelidan Fault took place in pre-Carboni- 

 ferous times. This may be argued from the fact that the Lime- 

 stone lapping round the northern slope of the Cyrn-y-Brain and 

 Cricor Domes transgresses rapidl}^ from the Ordovician on to the 

 higher part of the Lower Ludlow rocks. 



In a similar way the assumption of movement along the Bryn- 

 eglwys Fault in pre -Carboniferous times helps to explain the 

 occurrence of the Carboniferous Limestone at C.orwen. Were it 

 not for this possibility, the amount of movement required to 

 introduce it there, by faulting, would be stupendous. 



VI. Post-Devoxiax Modieicatioxs. 



The discussion of the post-Devonian changes is difficult, because 

 miost of the evidence lies beyond the limits of the Lower Palaeozoic 

 outcrops, in the tectonics of the newer rocks on the east. 



All the post-Devonian movements appear to be explicable on the 

 assumption of the continuous application of stresses, of varying 

 intensity, which found expression in {a) widespread warping, in- 

 fluenced by the major folds in the older series; and (5) renewed 

 mov^ement along already-established fault-lines. 



Although the folding or warping had little effect on the Lower 

 Palseozoic substratum, the faulting that accompanied it produced 



