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I. Introduction and Historical Survey During the survey of the country between Elan Valley (Radnorshire) and Pumpsaint (Carmarthenshire) (Davies, 1926 & 1928), a collection was made of the grits and of the pebbles from the conglomerates which occur at many horizons in the district. To these have been added further specimens from the country to the north-east, so that the arenaceous deposits between Camlo Hill (Radnorshire) and Llansawel (Carmarthenshire) are described in the following paper. The whole of the sediments dealt with occur in rocks of the graptolitic facies and lie on the north-western side of the Towy anticlinorium, and they range from the upper part of the Bala stage to the lowest part of the Gala stage. Although the largest accumulations of coarse rocks are confined principally to the Upper Bala and the Lower Birkhill stages, arenaceous material occurs locally throughout the whole succession, and it is only the Monograptus convolutus zone that lacks a noteworthy amount of coarse material; even at this horizon, however, grits up to 6 inches in thickness are found in the country near Ystradffin. As the rocks of the area consist predominantly of fine-grained shales and mud-stones, the widespread occurrence in them of arenaceous and conglomeratic deposits gives to these sediments an additional importance. Some of the arenaceous deposits appear at first sight to occur at constant horizons, and therefore to afford data for correlation purposes throughout Central Wales. Closer investigation, however, proves that they cannot be so applied, since they are only local

Several published studies suggest that a large part of the south-west Highlands, of which the area described forms a small part, represents an inverted stratigraphic succession (e.g. Bailey 1922; Shackleton 1958), and that the garnet isograd in the area is inverted (Tilley 1925; Elles & Tilley 1930). The present study was undertaken in order to determine, as far as possible, the structural and crystallization history of these rocks. Three deformation phases have been recognized: the first resulted in the development of folds with north–south axes and axial planes lying statistically parallel to the regional foliation-surfaces; the second formed folds having axes with a 120° azimuth and axial planes dipping to the north-east; the third formed more open folds with Caledonoid (60°) axes. It is concluded that if large-scale regional inversion has occurred the rocks have possibly been overturned before the earliest distinguishable deformation phase. Microfabric study shows that recrystallization accompanied the three deformation phases. An intense and complex series of crystal growths, including growth of biotite, garnet, and albite, and regional diaphthoresis, occurred after development of the third-phase structures. This post-tectonic recrystallization formed the most common microfabric of the area, partly or totally obliterating earlier microfabrics; it is characterized by a low degree of preferred orientation of quartz c -axes. Detailed examination of the garnet isograd leads to the conclusion that there is no evidence of its inversion in this area.

I. Introduction In 1926, I described the area between Abergwesyn and Drygam, and subsequently the district on the north-east, between Abergwesyn and Rhayader, formed the subject of another paper (Davies, 1928). The present work is a continuation of the previous surveys, and deals with the country between Abergwesyn and Pumpsaint in Carmarthenshire (see fig. 1). The southern boundary is the main road from Lampeter to Llanwrda, while the northern and western limits are defined by the Towy–Teifi watershed. The area forms part of the western flank of the Towy anticline, and has been mapped on the scale of 6 inches to the mile. It is about 130 square miles in extent, and is included within the Geological Survey maps, Old Series, 1-inch sheets 41 and 56 S.W. It is essentially that part of the Central Wales plateau which is drained by the upper Towy and its tributaries. The country is given over almost entirely to sheep-farming, and on the north-west it is desolate and somewhat inaccessible. II. Historical Summary Sir R. I. Murchison (1839, p. 342) mentioned the ‘slaty rocks and their associated conglomerates’ which are found on the north-west of the better-known areas with shelly fossils. He placed these slaty rocks within the ‘Cambrian’ system, and referred especially to Rhandirmwyn, where lead-mining was being carried on even at that time. The Geological Survey Horizontal Sections, published in 1844 and 1845, throw some light on the nature of the folding, for No. 3 passes through Ogofau, near Pumpsaint, and No.

The region with which this paper deals lies in the north-western corner of Breconshire, and is situated some 5 miles north of Llanwrtyd Wells, the nearest place of note. The area surveyed is one bordered on the south-west by the rivers G-wesyn and Culant, both tributaries of the Irfon, and stretches roughly 4 miles to the north of this line; its western boundary is marked by the Irfon—Towy watershed. Abergwesyn, a small village, stands at the junction of the Irfon and the G-wesyn. The district comprises part of the plateau of Central Wales, and is drained by the Towy and tributaries of the Wye, of which the Irfon, roughly bisecting the area, is the most important. The Irfon and its two main tributaries, the Gwesyn and the Cammarch, cut through the rocks generally at right angles to the strike, so that the streams (at least on the south side, where the valleys are deeply incised) afford good sections through the greater part of the succession. The country on the north, however, closes in to form a desolate moorland at a height of about 1600 feet, rising to 2115 feet in Drygarn, a prominent mountain-top which can be seen from most parts of Central Wales. This northern region is thickly covered with peat, so that the rock-exposures are much fewer and of by no means so satisfactory a character as on the southern side of the area. The district has, apparently, never been described in detail, and there are no difinite

A study of the composition and crystal size distribution of garnet from four metamorphic aureoles in the west of Ireland has been carried out in an attempt to derive information about the external factors controlling garnet formation. Garnet compositions varied between And 10 Gro 90 and And 30 Gro 70 . The distribution curves all fit the same equation (Galwey & Jones 1963). Possible explanations of variations in the distribution functions for different aureoles are considered and it is concluded that these were controlled by rate of heating of the host-rock during metamorphism.