Vol. 68.] GEOLOGICAL STRUCTTTEE OF CENTRAL WALES. 343 



than the complementary convex tracts, thereby giving rise to slow' 

 and persistent movements. 



* Such movements should be admirably adapted to give those gentle, nearly 

 constant subsidences that furnish the nice adjustments of water-depth required 

 for the accumulation of thick strata in shallow water, and those slow upward 

 warpings that renew the feeding-grounds of erosion ' {op. cit. p. 561). 



The concluding portion of this paragraph exactly illustrates the 

 conditions which prevailed along this line during Silurian times. 



The unconformity and overlap in the Llandovery rocks described 

 by Dr. H. Lapworth in the Rhayader district, and the marked 

 overstep at the base of the higher Silurian rocks and Old Red 

 Sandstone near Llandeilo, may probably be attributed to frequently- 

 renewed movements along this line. As shown by Dr. A. Strahan,^ 

 it is probable that this axis shared in the post-Carboniferous move- 

 ments, and may even have been active at a much later date. 



The influence of these structural lines upon the development of 

 the drainage-system of South and Central Wales has probably been 

 considerable, but the available evidence is not sufficient to indicate 

 the exact part played by them. 



It is important to note that, in one instance, these structures have 

 been crossed and apparently displaced by movements of later date 

 than their initiation, namely, the movements that gave rise to the 

 Tstwyth Fault. This line of faulting has exercised a similar trapping 

 influence upon the drainage of that region, as the Yale of Neath, 

 Cribarth, and other disturbances described by Dr. Strahan(op. supra 

 cit.). The faulted belt is followed closely by the Upper Ystwyth, 

 which ultimately leaves it to pursue a north-westerly course, along 

 a broad winding valley of very different character, to the sea at 

 Aberystwyth. The line of drainage is taken up farther west by the 

 River Wyrai, -w'hich occupies a remarkably straight trough-like 

 valley resembling that of the Fpper Ystwyth, and is separated 

 from that valley by a low col situated on the line of fault. This 

 disturbance is known to have cut and displaced certain of the 

 important mineral veins of Cwm Ystwyth. The most important 

 vein was cut out completely, and has never been discovered on the 

 other side of the fault. 



YI. SxTMMARr. 



The tract of Central and South- West Wales consists mainly of 

 rocks of Llandovery age in the centre, and of pre-Silurian rocks at 

 the extreme northern and southern ends. 



This tract is traversed by two anticlinal axes following in part 

 the valleys of the Towy and the Teifi, and an intervening syncline 

 (the Central Wales Syncline) occupying the position of the watershed 

 between those streams. These structures are prolonged northwards 

 towards North Wales and south-westwards into Pembrokeshire; 

 but, whereas the anticlinal axes become enfeebled northwards, the 



1 Q. J. G. S. vol. Iviii (1902) p. 207. 

 Q. J. G. S. No. 271. 2 c 



