570 ME. H. WOODS ON THE IGNEOUS EOCKS [Nov. 1894, 



(2) The second area of andesite surrounds the northern part of 

 the largest mass of diabase-porphyrite. Specimens of this, from a 

 spot just south of Cwm-amliw and east of Oil Wood, are dark grey 

 or almost black in colour, and one can distinguish with the unaided 

 eye felspars, calcite, and a dark green mineral. Under the micro- 

 scope the earlier plagioclase-felspars are seen to be very much 

 decomposed ; usually they have a length of about 3 mm. The 

 later felspars are very much smaller ; the groundmass contains, in 

 addition, a semi-opaque material, and a large quantity of magnetite, 

 the latter generally having the form of skeleton-crystals. Augite 

 occurs very sparingly, but not as a constituent of the groundmass. 

 Numerous large and often irregularly-sbaped vesicles are present, 

 having a diameter varying from '7 to 2 mm. These in some cases 

 contain calcite in the centre, around which is a narrow band of a 

 pale green mineral, probably delessite, and external to this is a zone 

 of quartz. In other cases the vesicles do not contain calcite, but are 

 occupied by delessite with a border of quartz. At other times, 

 almost the whole of the vesicle is filled with calcite, with here and 

 there a little quartz or delessite at the margin. Quartz and calcite 

 also occur in the groundmass as secondary minerals. The specific 

 gravity of the rock is 2-77. 



The rocks on the east of the diabase-porphyrite, which form the 

 main part of this area of andesite, are occasionally dark in colour, 

 but mostly light grey with sometimes a bluish or greenish tinge, 

 and generally showing crystals of felspar. A section from the 

 ridge immediately north-west of Penrubulla shows a groundmass 

 composed mainly of small felspars, containing porphyritic crystals of 

 plagioclase, which are considerably decomposed ; there is also some 

 secondary quartz. Similar features are seen in sections taken from 

 the ridge north-west of the last ' a ' in Carneddau (and north of 

 Ji.M. 1322), 1 and from the ridge south-west of Carneddau House 

 and immediately north of the first ' L ' in Llansantffraid ; the first 

 of these, however, differs in containing a large quantity of secondary 

 quartz. Specimens from the ridge |- mile west of Carneddau 

 House were also sliced. In these the porphyritic felspars are very 

 large, and so much decomposed that the twinning is only indistinctly 

 seen and is sometimes quite absent ; not unfrequently they have 

 been entirely replaced by secondary quartz. The groundmass of 

 the rock consists of a semi- opaque substance, and small felspars 

 more or less felted together ; it also contains some secondary quartz 

 and a little calcite. A few small vesicles containing quartz or 

 chlorite are present. 



(3) The field-relations of the andesite north of Gelli-Cadwgan 

 are not very well seen, but apparently it is intrusive in the 

 Llandeilo Beds. The rock is quarried near its south-western 

 extremity by the roadside, and is used as road-metal; it is compact, 

 of a light grey colour, with here and there dark streaks, and is 



1 These are references to the 6-inch Ordnance Maps. 



