8 PROF. E. J. GARWOOD AND MISS E. GOODYEAK [vol. lxxiv, 



white quartz also occur. The matrix, which forms only a small 

 proportion of the rock, is fairly coarse, and consists of smaller 

 and more angular fragments of pink felsite and quartz set in a 

 sandy ferruginous paste. 



The best exposures of this conglomerate are found at two points 

 near the northern margin of the inlier. The northernmost of 

 these outcrops occurs in a field about a third of a mile due 

 west of Stockwell Farm, and about 150 yards south of the mound 

 known as ' Castle Nimble,' at a spot marked ' boulders ' on the 

 6-inch map. The second outcrop lies about 300 yards south of 

 this, and some 280 yards west of the church ; it forms a con- 

 spicuous feature on the north side of the cart-track leading from 

 Old Radnor to Stone's Farm. These 'two outcrops both strike a 

 little to the east of north, and have a similar dip of 70° west- 

 north- westwards ; they both consist of two layers of conglomerate 

 separated by a bed of pebbly grit, and appear to form portions of a 

 continuous band lying at a definite horizon in the series. The 

 exposure of this conglomerate in Stone's-Farm Lane is cut off: 

 abruptly at its southern end by an east-and-west fault; but it 

 reappears at the surface about 200 yards away to the west, 

 immediately north of Stone's Farm. 



Dr. H. H. Thomas, who has kindly examined thin sections of 

 these conglomerates, reports as follows : — 



As has already been stated, the matrix of the conglomerates is practically- 

 identical with the material that forms the grits and greywackes ; but, owing 

 to the greater size of the rock-fragments, these conglomerates afford a better 

 opportunity than the finer-grained deposits for determining the types of rock 

 undergoing denudation at the time. The bulk of the pebbles are quartzitic 

 and felsitic in character, but coarser granitoid rocks, microlithic acid eruptive 

 rocks, and even basalts are of frequent occurrence. Quartzite is present in 

 two distinct varieties, one pink and the other grey to white in colour. The 

 pink quartzite which predominates in this conglomerate, has a fine even 

 texture, and a characteristic red staining throughout its whole mass — a 

 staining which was undoubtedly completed before the denudation of the rock 

 and its formation into pebbles. Another characteristic feature is the occur- 

 rence in some instances of minute but highly conspicuous rhombs of chalybite, 

 scattered through the rock, though generally altered, completely or in part, 

 to ferric oxide. The grey quartzite is of similar texture, but appears to have 

 suffered in general some slight deformation. It is often stained superficially ; 

 yet the staining never extends far into the rock, and is evidently subsequent 

 to the formation of the pebbles. The felsitic rocks are present in great 

 variety, ai*d present striking microscopic structures. The result of an exami- 

 nation of an extensive series of pebbles collected from the pink conglomerate 

 of Castle Nimble indicates that soda-rhyolites, in the form of devitrified rocks 

 with small porphyritic crystals of quartz and albite. constitute a prevalent 

 type. Microscopically they often show strong fluxion-banding, and in some 

 cases a beautiful perlitic structure has been retained. 



Among the coarser igneous pebbles, rocks ranging from albite-granite to 

 what may be termed soda-aplites and quartz-albite-porphyrites, are well re- 

 presented. The soda-granites and aplites are closely related, and consist 

 essentially of albite and albite-perthite with quartz, often in pcecilitic rela- 

 tionship, together with occasional subordinate muscovite. In some instances 

 the structure approaches hypidiomorphic, and if quartz be present the rocks 

 approximate to what may with propriety be termed quartz-albite-porphyrites. 



One rock from this same locality calls for special mention, indicating as it 



