298 PHOF. A. H. COX AKJ) ME. A. K. WELLS OX THE [vol. lxxvi, 



also occurs in wisps, fibres, and sheaves, which penetrate alike 

 chlorite, hornblende, and felspar. The mineral was thus formed at 

 a late stage of the consolidation, and it may possibly be pneumato- 

 lytic in origin. Such criss-cross biotite never occurs in the normal 

 diabases of the district, but is characteristic of the minor grano- 

 phyric intrusions. 



Quartz is absent from some of the material collected at the 

 quarry, but towards the eastern extremity of the intrusion fairly 

 large granules become abundant. Its associates, however, are 

 calcite and chlorite, which latter invariably wraps round tl e quartz- 

 grains and sometimes crowds them with small inclusions ; some of 

 the primary quartz may, therefore, have grown larger by the 

 deposition of secondary material [C 235 & 245]. 



The small intrusion capping the hill a short distance to the east 

 of the Waen-Fechan sill was doubtless originally connected to the 

 latter, and in general the two rocks are identical. The hornblende 

 in this case, however, is distinctly bluish, owing to the presence 

 of a higher percentage of soda in the molecule [C 233]. An 

 interesting point is the occurrence of fine-grained xenoliths to 

 the extent of at least a third of the total mass of the rock. The 

 xenoliths consist of chilled basic material, and are similar to those 

 in the neighbouring Tyn-y-llwyn intrusion (see description below, 

 p. 299). 



The rock in the lower half of the Craig-y-Castell intrusion 

 approximates to a quartz-diabase in composition, or is intermediate 

 between a quartz-diabase and the markfieldites to be described 

 later. The rocks are somewhat weathered, and do not lend 

 themselves to an exact description. Dark minerals were abundant 

 in large crystals, which are now pseudomorphed by pale hornblende. 

 Twinning and other characters suggest that much, if not all, of this 

 was originally augite [C 276]. Small flakes of pale-green mica are 

 also present, distributed sporadically. Large crystals of plagioclase 

 and orthoclase are present in approximately equal amounts. The 

 plagioclases are, however, largely replaced by secondary products, 

 so that their characteristic lath-shaped outlines and a few traces of 

 albite-twinning alone remain. Primary quartz is fairly abundant 

 in large grains showing lines of fluid inclusions, also in the form 

 of a fine-grained granophyric intergrowth. Granophyric material 

 is almost wanting in the lowest part of the sill, but its amount 

 increases rapidly upwards. Of accessory minerals, apatite is the 

 most abundant ; the crystals sometimes contain a narrow zone of 

 pale-green chlorite similarly to apatites described by Dr. J. S. 

 Flett 1 from the albite-diabases of the Newton Abbot district. 

 Granular magnetite is the most abundant iron-ore. 



A similar type appears again in the lower half of the smaller 

 Tjm-y-llwyn intrusion, where the rock along the lower contact is 



1 ' The Geology of the Country round Newton Abbot ' Mem. Geol. Snrv. 

 1913, p. 60. 



