340 PROF. 0, T. JOXES OX THE [Sept. 19 1 2, 



the Aber3'stwyth Grits, aud probably also the base of the Silurian) 

 converge or diverge in the opposite direction. This may be taken 

 to indicate that the direction of pitch has changed to a north- 

 easterly direction. A region of no pitch should occur about the 

 centre of the county, and is indeed indicated by the parallelism of 

 the boundary-lines, also by the fact that the synclinal outcrops 

 reach here their greatest width while the anticlinal outcrops have 

 here their minimum width ; this region should also mark the deepest 

 part of the syncline. As the outcrops had been drawn, as stated 

 above, without reference to the possible or probable structure, it 

 is satisfactory to find that the highest strata which have been 

 observed in the district occur exactly where this interpretation of 

 the structure would lead one to expect them. 



Again, north of Plynlimon the direction of pitch appears to change 

 towards the north ; this can be easily verified in the district about 

 Glandovey and Machynlleth, and is probably universal over the 

 extreme north of the county. The region of change of pitch in the 

 anticline marks locally the greatest elevation of the strata; again, 

 it is in this neighbourhood that the lowest beds in the northern 

 district have been detected (namely, the DiceTlograptus Eeds of 

 jS'ant-y-Moch, west of Plynlimon). Wherever, therefore, the 

 deductions drawn from a study of the map can be put to the test 

 of observation they are found to be verified. 



A brief account of the above-mentioned structures and their con- 

 tinuation into Xorth Wales on the one hand, aud south-westwards 

 into Pembrokeshire on the other, may be of some interest, as it will 

 serve to connect up the structure of the former region, so ably 

 discussed by Mr. Fearnsides, with that of other districts of South 

 "Wales fully discussed in various memoirs already published, and 

 in some others which are shortly to appear. 



It may be noted here that the two axes of uplift had not escaped 

 the attention of Eamsay and others engaged in constructing the 

 horizontal sections across the district, for in Sheet 4 it is stated that 



' from Castell Craig Gwyddon. to Cardigan Pay the country is composed of 

 repetitions of the same series of rocks, under various aspects, carried to the 

 north-west bj- two principal anticlinal axes, the first [" the north-east con- 

 tinuation of the great anticlinal axis of the Vale of Towy"]; the second 

 spreading wide across the rales of the Teifi and the Aeron.' 



On this section, too, the highest beds between the Towy and the 

 Teifi are shown as a synclinorium, but the fold is really much less 

 shallow than is there indicated. 



The Teifi Anticline. — It has not been found possible, so far, 

 to follow this axis of uplift as a separate structure much beyond 

 the Plynlimon district ; but its south-westerly continuation appears 

 to be an important element in the structure of Iv^orth Pembroke- 

 shire. The manner in which the igneous rocks of that county 

 terminate eastwards has been referred to above ; there is some evi- 

 dence that this is due to the eflfects of severe folding upon igneous 

 masses which have a certain parallelism with the strata, that is, 



