Vol. 68.] THE GEOLOGICAL STRTTCTTTRE OF CENTRAL WALES. 339" 



of Tregaron, a keen amateur geologist, who, having noticed the"^ 

 conglomerates of Craig Twrch and the peculiar red rocks, was struck 

 by the unusual characters of the latter, and conducted me to the 

 place. 



It is possible that other undiscovered outliers of these peculiar 

 rocks may exist among these lonely hills, but to the east of 

 Draenllwyndu occur lower beds descending towards the Lower 

 Birkhill rocks near Ystradffin. From the evidence of the section 

 just described, it may be inferred that the area between the Teifi. 

 and Towy Valleys is synclinal in form. 



IV. The Structfre of the Area. 



From the manner in which the map has been compiled, it is 

 evident that it can only yield information as to the large-scale- 

 structure of this extensive region. Certain broad features can, 

 however, be distinguished at once, namely : — 



(1) An anticline, or rather anticlinorium, on the west separating 

 the two outcrops of the Aberystwyth Grits. Its axis follows in 

 part the Teifi Valley, and the structure may accordingly be called 

 the Teifi Anticline. This axis appears to be a southward con- 

 tinuation of the principal axis of uplift in the Plynlimon district, 

 which ranges along the east side of the Rheidol Valley. 



(2) A syncline, or synclinorium, the axis of which ranges almost 

 parallel with the above, and coincides in a general way with the> 

 watershed dividing the Teifi from the Towy drainage-system. The 

 direction of the axis is clearly indicated by the form of the outcrop 

 of the Aberystwyth Grits ; it is assumed also that it passes through 

 the outcrop of green and purple rocks south-east of Tregaron. This- 

 may be referred to as the Central Wales Syncline. 



(3) Another anticline or anticlinorium, which ranges along the 

 valley of the Towy between Carmarthen and Llandovery. This 

 structure has been referred to by previous observers as the Towy 

 Anticline ; it is probably accompanied, unlike the above-named 

 structures, by considerable strike-faulting. The existence of the 

 anticline is clearly shown on the map by the occurrence of the 

 Silurian rocks on both sides of the Towy Valley: for, although 

 the existence of a powerful anticlinal axis along the Towy Valley 

 has long been known, it was diflScult to recognize from a study of 

 the geological map, as the equivalents of the Llandovery rocks which 

 occur on its south-east side had not been definitely recognized on 

 the west side (see p. 335). 



It will be observed that in the northern area the boundary-lines 

 converge southwards in the limbs of the anticline, and diverge in that 

 direction in the synclines. This indicates that the major structures 

 in that region have all a pitch to the south, and therefore are in 

 accordance with the structures observed in a limited area south of 

 Plynlimon. 



Farther south, however, the boundaries tend to approach paral- 

 lelism, while in South Cardiganshire they (particularly the base of 



