576 ME. H, WOODS ON THE IGNEOUS KOCKS [Nov. 1894, 



grains ; there is a large amount of chlorite and some leucoxene. 

 Minute crystals of brown mica and magnetite occur clustered 

 together in little groups. A section taken from the southern end 

 of the sill, near Park Wells, is similar to the one just described, but 

 the place of the augite is taken by chlorite. 



The largest sill is the one to the east of the two preceding, 

 forming the high ground on which Welfield is situated. It is not 

 exposed in any quarries, but may be seen in the bed of the Wye and 

 at Pen-maenau Rocks. A section from the Wye, near Pen-ddol 

 Rocks, shows the ordinary minerals. 



The sill to the north-west of Llanelwedd Church, on which Upper 

 Llanelwedd Wood is situated, is more irregular in form than the 

 others ; it is not well exposed, and in hand-specimens resembles the 

 rock at Pen-cerig Wood. In a section from near ' w ' in Upper 

 Llanelwedd the augite is rather scarce ; ilmenite, leucoxene, and 

 chlorite are plentiful. 



We come now to the diabases on the east of the main volcanic 

 series. The most important is that which forms the ridge known 

 as the Castle Bank. 1 This stretches from Llwyn-Madoc northwards 

 almost to the Cannant Brook. Specimens collected from various 

 parts of the Bank exhibit to the naked eye a fairly uniform ap- 

 pearance, the differences being due mainly to the amount of 

 weathering ; the rock is always rather fine-grained. In a specimen 

 from 70 yards north-north-east of the ' Castle ' the felspar is 

 abundant and very much decomposed. The augite is in rather 

 small quantity ; it is almost colourless and not ophitic. There is a 

 large amount of a green chloride mineral, which in some cases is 

 derived from the augite ; in others it shows a fibrous structure, and 

 possibly represents a rhombic pyroxene. Secondary quartz is 

 abundant ; magnetite, ilmenite, and leucoxene also occur. The 

 silica-percentage of the rock is 54*54, and its specific gravity 2*73. 

 Another section taken from the south-eastern part of the Bank, just 

 north of the ' Cam,' shows similar characters ; but the augite is 

 sometimes idiomorphic. 



Another mass of diabase occurs a little to the south of Llwyn- 

 Madoc; it extends from the Caer to Cil-y-berll an. A specimen from 

 near the road south of the Caer is similar in appearance to the 

 Castle Bank rock. But under the microscope it differs from that in 

 the augite being more abundant and generally ophitic ; there is a 

 little secondary quartz. Another section, taken from about 150 yards 

 north-north-east of Cil-y-berllan, was very similar to the last. On 

 the west side, south of Cwm-berwyn, the rock has rather the structure 

 of a dolerite than that of a diabase : it is much finer-grained, darker 

 in colour, and comparatively fresh ; the augites are mostly very 

 small, and never ophitic, and there are two generations of felspar, 

 although the earlier is not well marked; the specific gravity of 

 this rock is 2-81. 



1 The northern part of this ridge is not shown in the map which accompanies 

 this paper. 



