342 PROF. 0. T. JONES ON THE [Sept. I9T2, 



tlie parallel Teifi Anticlme. Possibly, however, it may be in part 

 of pre-Wenlock age : this suggestion requires investigation. 



To the south-west, its course is clearly marked ; along it occur 

 the Upper Cambrian rocks of the Caimarthen district, and the 

 Lower Arenig rocks associated with igneous rocks near Llangynog, 

 5 miles south-west of Carmarthen. It is probable that hereabouts 

 the main axis of uplift passes beneath later rocks; but, before doing 

 so, it has given oif several important branches which range westwards 

 through Pembrokeshire. 



From a comparison of these structures it would appear that both 

 the anticlinal axes tend to diminish in importance northwards, 

 while the intervening syncline becomes more pronounced in that 

 direction. 



y. EeLATION of SlETJCXrEE TO ToPOGEAPHK! FEATURES. 



The influence of these structures upon the present topography is 

 too obvious to be ignored. It will be observed that the axes of 

 folding all pursue gently curved lines which show a remarkable 

 correspondence with the form of the coastline of Cardigan Bay. 



The two anticlinal axes coincide for a great part of their course 

 with the valleys of the principal rivers of the district, the Teifi, the 

 Upper Eheidol,and the Towy ; while the intervening syncline appears- 

 to have determined in great part the position of the principal water- 

 shed of Central Wales. There is thus a fundamental connexion 

 between the great structural lines and the more important topo- 

 graphic features, but exactly how, and at what period, this corre- 

 spondence came into existence requires much further investigation. 



It is clear, however, that at least one of these lines has been 

 marked out from times of great antiquity,^ and in all probability 

 has shared in more than one period of earth-movement. On its 

 eastern or southern limb the shelly facies of the Llandovery rocks 

 is typically developed (Llandovery, south of Newbridge, and 

 Haverfordwest) ; while on its other limb rocks of similar age belong 

 almost exclusively to the graptolitic facies (Rhayader, Llansawel, 

 etc.). At this period, therefore, this Jine ajjpears to have separated 

 the shallow-water areas from the deeper-water tract in which the 

 graptolitic deposits of Central Wales were laid down. As the 

 change of facies takes place with extraordinary ra])idity across this 

 line, it is not impossible that the line of the Towy Valley repre- 

 sented the edge of a coastal platform of that period. It has been 

 shown very clearly by Prof. T. C. Chamberlin & Prof. R. D. Salis- 

 bury^ that the outer limits of the continental platforms of the 

 present day form ' concave tracts ' from 100 to 300 miles in width, 

 as opposed to the convex outline on a large scale of the land-surface 

 and ocean-bottom ; and they consider that under conditions of strain 

 set up in the earth's crust these tracts would yield more readily 



' See W. G. Fearnsides, ' Geology of North & Central Wales ' (Jubilee vol. 

 Geol. Assoc.) 1910-11, pp. 796, 802. 

 '^ ' Geology ' vol. i (1904) chap. ix. 



