Yol. 68.11 GEOLOGICAL STRUCTURE OF CENTRAL WALES. 341 



either contemporaneous flows or intrusions of sill-like habit. Pop 

 the outcrop of the Dicranograptus Shales and the base of the Blue 

 Mudstone Group which succeeds them appear to pursue between 

 Llanfirnach and Newport a course approximately parallel to the 

 limit of the igneous rocks. Hence we may probably attribute the 

 wedge-like termination of that area to folding along a broad anti- 

 clinal axis with an easterly pitch. This does not, of course, preclude 

 the possibility of strike-faults influencing the outcrops in addition. 



A similar structure is indicated independently by the wedge -like 

 eastern extremity of the Cambrian and pre-Cambrian rocks of Hays- 

 castle and St. Davids. In this case it can be verified by a detailed 

 study of the structures at the eastern end of the Hayscastle 

 district.^ 



It can hardly be an accidental circumstance, therefore, that the 

 Lower Cambrian and older rocks, the igneous rocks of jS'orth 

 Pembrokeshire, and the Dicranograptus Shaies all terminate in 

 a wedge-like form, and therefore project farthest eastwards along a 

 line which lies in the continuation of the Teifi Anticline. 



The Central Wales Syncline. — It is diflicult to recognize 

 from the behaviour of the outcrops the course of this structure 

 westwards through Pembrokeshire, and it is probable that the 

 structure flattens out in that direction and ceases to be recognizable 

 as a separate structure. Northwards, however, it seems clearly to 

 fall into the line of the Tarannon Syncline, which is thence pro- 

 longed in the great trough that divides the Bala from the Berwyn 

 region. This syncline seems to have been generally overlooked in 

 descriptions of the southern portions of Central Wales. 



The Towy Anticline. — This structure appears to have been 

 detected and its importance recognized by all observers except pos- 

 sibly Murchison, though anticlinal dips are indicated in some of the 

 sections across it.^ Its course northward from Llandovery is indi- 

 cated by the small outcrop of black Dicranograptus Shales forming 

 the Sugar Loaf on the watershed between Llandovery and Llanwrtyd. 

 They are marked by ¥ on the Geological Survey map, are in associa- 

 tion with blue or grey mudstones of the Upper Bala type, and appear 

 to be faulted at least on one side. Along the same line farther 

 north are the igneous rocks of Llanwrtyd described by Murchison. 

 and the small masses coloured as ' Greenstone ' east of Rhayader. 

 These igneous masses are probably all of early Ordovician age, like 

 those of the neighbouring Builth area, and lie near the axis of up- 

 lift. The structure is clearly indicated in each of the horizontal 

 sections which cross it (Sheets 4, 5, and 6). 



From the comparatively even course pursued by the Wenlock 

 and higher Silurian rocks north-east of Rhayader it may be 

 assumed that the anticline fades away in that direction, as does 



Q. J. G. S. vol. Ixviii (1912) pp. 374 et seqg. 



' Silurian System ' 1839, pi. xxxiii, figs. 6-7, & pi. 



xxsiv, figs. 5, 9. 



