part 1] OX THE GEOLOGY OF THE OLD RADNOR DISTRICT. 7 



Dr. H. H. Thomas has kindly furnished us with the following 

 notes on the structure of these rocks as exhibited in thin slices : — 



All these grits and greywackes are linked together by their general 

 felspathic character and angularity of grain. 



Microscopically they consist of a closely-packed aggregate of quartz, grains 

 of quartzite. and igneous rocks rich in soda-felspars. The quartz is of two 

 kinds, one practically devoid of strain and originating most probably in quartz- 

 porphyries or quartz-keratophyres, and the other highly strained and probably 

 derived from pre-existing sedimentary rocks that have been subjected to 

 shearing stresses. In addition, broken crystals of albite are of common 

 occurrence, and these are certainly derived from soda-granites and soda- 

 aplites, or from porphyritic keratophyric rocks, fragments of which are 

 occasionally met with. 



All these rocks, whether collected from the north of Dolyhir Station, from 

 Harp-Inn Quarry, or from Gore Quarry, present characters that are quite 

 distinctive but yet unite them closely with the matrix of the conglomerates 

 described below, and the small pebbles that are of frequent occurrence in the 

 grits are identical with, or of rock-types similar to. those found in the 

 coarser deposits. 



Locally the rocks become highly micaceous. The mica is arranged with 

 well-defined parallelism, and is obviously detrital in character. In some of 

 the rocks it is mainly muscovite ; but usually, as in the green grits north of 

 Dolyhir Station, it is an intimate parallel intergrowth of muscovite and 

 chlorite, the chlorite being pseudomorphous after biotite. 1 In some bands a 

 pale leek-green pleochroic mica is met with, similar to that which is so 

 abundant in certain beds of the Lower Old Red Sandstone of the Welsh 

 Borders. 



From the general abundance of minute detrital garnets in all these rocks, 

 and especially so in those containing much mica, it is probable that the micas 

 had their origin in a series of garnetiferous schists or granulites. 



The conglomerates. — Although the shattered condition of 

 the Pre-Cambrian rocks makes it difficult to correlate the different 

 outcrops one with the other, or to assign them to any definite 

 horizons in the series, a study of the constituent pebbles shows 

 clearly that at least two distinct types of conglomerate occur. Of 

 one of these we may speak as the 'pink ' conglomerate, and of the 

 other as the ' greA* ' conglomerate. 



The pink conglomerate is characterized by its prevailing red tint 

 throughout ; the majority of the pebbles are composed of bright- 

 pink quartzite and dull-red felsite, some of the latter being visibly 

 banded in hand-specimens ; among them occur also a few pebbles 

 of dark-grey granitoid rocks, which are stained red to a depth of 

 about yV mcn fo' om the surface. The matrix is deeply impregnated 

 with haematite, which gives it a uniform tint, except where it has 

 been bleached by surface- weathering. The pink quartzite-pebbles 

 are fairly uniform in size, of a blunt ovoid form, with well-smoothed 

 surfaces; they have an average longer diameter of 14 to 2 inches, 

 and exhibit a bright lustre on freshly fractured surfaces. The 

 felsite-fragments are usually more elongate, with approximately 

 circular cross-sections; and the same is often the case with the 

 granitoid rocks. A few small well-rounded pebbles of grey and 



1 C. Callaway. Q. J. G.S. vol. lvi (1900) p. 513, regards the mica in these 

 rocks as authigenous. but its nature and arrangement militate against this view. 



