502 H. G. STYiith — Basic Intrusions 



The Diabase. 



This type makes an imposing show on the ground. One sill 

 (aligned with the Castle Bank intrusion described by Mr. Woods ^) 

 commences just north of Camnant Brook, runs north-east between 

 Blaenkerry and Garn-fach, turns due north, and is continued on the 

 eastern side of Gelli Hill to disappear under the ashes near the Bog, 

 a distance of about two miles. The Llanvirn Shales are in contact 

 throughout most of this distance. Another intrusion is exposed 

 north of Frank's Bridge. It runs N.jN'.E. for about a mile and stops 

 short before reaching the River Edw. The same line is continued on 

 the other side of the river at Graig-fawr, exposed as a sill dipping 

 west, running just on the western side of the farm, and dying away 

 in the vicinity of Llanwefr Pool. The northern part of the sill is 

 shifted to the east by a dip fault. A shorter intrusion with the same 

 trend is found west of Pye Corner, and, finally, a parallel sill runs 

 from Cwm-maerdy to the Edw south of Pye Corner, but a portion of 

 the sill is shifted to the east by trough faulting. The behaviour of 

 some of these intrusions on approaching the rivers suggests that the 

 serrated upper edges of the sills have not yet been obliterated, and 

 the rivers have selected those places where the sinuous edge makes 

 a downward bend. 



The rock is medium-grained with a general greenish tinge, showing 

 a pale-grey network on a black background. 



In thin section the rock is seen to be made up principally of pale- 

 brown augite and lath-shaped felspars related ophitically. ISTo fresh 

 specimens have been obtained, and in all the sections examined the 

 felspar is more or less decomposed ; but as far as can be determined 

 the refractive index is never high enough for labradorite, and this 

 impression is supported by the symmetrical extinction angles of the 

 albite lamellae ; the maximum value obtained is 16°, and it is 

 suggested that the felspar is andesine. Pericline twinning is rare, 

 and zoning has not been observed. 



The augite varies from colourless to a pale brown. It has the 

 usual refractive index and cleavages, and alters, as a rule, to chlorite, 

 which polarizes in ultra-blue or first order grey, but in one section 

 a brown amphibole represents an intermediate stage in the alteration. 

 There is an occasional suggestion of pleochroism. The polarization 

 colours are of the first and second orders, and twinning, though seen, 

 is rare ; the double refraction is positive. The individuals polarizing 

 in very low colours are invariably found to be approximately 

 perpendicular to an optic axis, and for this reason it is considered 

 that the fresh pyroxene is exclusively monoclinic. There are, 

 however, some pseudomorphs which show good cleavage, polarize in 

 bright colours, and extinguish as a single unit which may be altered 

 hypersthene. Some of the pseudomorphs suggest derivation from 

 olivine, but proof is wanting. 



Another constituent locally abundant is ilmenite. It commonly 

 shows the characteristic white alteration product and is moulded on 

 the felspar and augite. A few idiomorphic crystals of sphene 



1 Q.J.G.S., vol. 1, p. 576, 1894. 



