332 PEOF. 0. T. joxEs ON THE [Sept. 191 2,. 



closely the beds underlying the Gwastaden Grits of Rhayader, 

 this may be regarded as confirming the Eala age suggested for 

 the latter by Dr. H. Lapworth. 



By a fortunate chance, the position of the Plyulimon area 

 proved to be exceptionally favourable for demonstrating the 

 internal stnictnre of the surrounding tract of Central Wales ; and, 

 as I have been able to recognize over a considerable stretch of 

 country the rock-groups there established, it is possible approxi- 

 mately to indicate their distribution. These observations, with 

 others made in various parts of the district between the Paver 

 Dovev and Pembrokeshire, are embodied in the map accompanving 

 this paper (PI. XXXIV). 



In 1910 appeared the 'Geology of Xorth & Central A\'ales'* by 

 Mr. W. G. Fearnsides,^ where the structure of the district herein 

 described is indicated. The region was compared with the 

 Southern Uplands of Scotland, and a prophetic account is given 

 (p. 816) of the partition of the area among the Ordovician and 

 Silurian rocks. The structure of the area is, however, in several 

 respects simpler than that of the Southern Uplands of Scotland, 

 and the formations occupy larger and more continuous tracts than 

 is suggested in the above-mentioned paper and the accompanying 

 map. 



III. The DisTEiBriiox of the Eock-Geoups. 



For the purpose of indicating the major structures, it is only 

 necessary to consider a broad grouping of the strata. It is possible,, 

 as a rule, to distinguish the rocks belonging to the larger groups 

 by inspection of their lithological characters, although in many 

 instances the discovery of graptolites enables the precise horizon 

 to be determined. 



In most cases a subdivision into Ordovician and Silurian rocks 

 suffices ; but it is convenient to consider in addition the distribution 

 of the rocks called by earlier observers (Sedgwick, Keeping, etc.) 

 the Aberystwyth Grits. In Pembrokeshire, more particularly,, 

 certain additional lines have been added, as they have a bearing 

 npon the structure. In fixing the boundary between the Ordovician- 

 and the Silurian the peculiar characters and mode of weathering of 

 the Lower Birkhill rocks have been found useful. 



I have refrained, as far as possible, from introducing theoretical 

 considerations as to the structure in drawing the lines on the map. 

 Consequently, they may be regarded as defining the general course 

 of the boundaries. The actual boundaries are almost certainly 

 more complicated than here appears ; these, however, are matters^ 

 of minor detail which are unessential in the presei>t connexion. 



(a) The Ordovician Rocks. 



On the published maps an enormous area of the western and 

 central portions of Wales is coloured as Lower Silurian, which may 



1 ' Geology in the Field ' (Jubilee vol. Geol. Assoc.) p. 786. 



