Yol. 50.] OF THE NEIGHBOURHOOD OF BTTILTH. 567 



Builth and a Hue drawn from Cwm-amliw, eastward to Garth, 

 and then round the Castle Bank ; and shown in the accompanying 

 map (p. 568). The rocks met with are diabase-porphyrite, 

 andesite, andesitic ash, rhyolite, rhyolitic ash, and diabase. These 

 give rise to characteristic hummocky ground, with here and there 

 a few prominent peaks, forming a marked contrast to the regular 

 features of the neighbouring Ludlow escarpment. A good example 

 of the scenery of the district is given in the ' Silurian System ' 

 (plate facing page 330), from a drawing made by Lady Murchison. 



II. The Diabase-Porphyrite. 



This is the most conspicuous rock in the district ; it consists of a 

 dark-coloured, fine-grained base, much decomposed, containing 

 numerous light felspars of large size, and often having a greenish 

 tinge ; in some cases these porphyritic felspars become much 

 smaller. The diabase-porphyrite occurs in four masses, extending 

 in a north-and-south direction, and is intrusive in the andesites and 

 andesitic ashes, possibly in a laccolitic form. The first of these, 

 commencing in the south, forms the central part of the tract known 

 as the Llanelwedd Rocks, extending from the Rock House nearly 

 to Carneddau Farm, and is surrounded, except on the south, by 

 andesite. The second, which is more irregular in form, occurs to 

 the north-east of Carneddau Farm, and is intruded into the 

 andesite. The third is the smallest mass, and forms the eastern 

 part of Caer Fawr, which reaches a height of 1267 feet. The 

 fourth is the largest, and extends along the western part of the 

 Carneddau Hills for a" distance of a little over a mile ; it is intruded 

 into the andesite and andesitic ash. 



Under the microscope the earlier plagioclase-felspars in this rock 

 are seen to be considerably decomposed ; the base consists of small 

 plagioclase-felspars and augite, with some, and frequently much 

 pale green chlorite. In a few cases, as for instance in a specimen 

 from near the Rock House, the augite has entirely disappeared, the 

 base being composed of felspar and chlorite, llmenite and leu- 

 coxene are often present. Frequently there are large irregular 

 vesicles filled with chlorite or calcite. In some sections the later 

 felspars are markedly lath-shaped, but they do not exhibit any flow- 

 arrangement. 



The silica percentage of the diabase-porphyrite is 48*36, and its 

 specific gravity 2" 78. 



On the Geological Survey map (56 S.W.) a continuous mass of 

 ' greenstone ' is shown, extending from Llanelwedd Church north- 

 wards, to near Cwm-amliw ; this was probably intended to 

 represent the diabase-porphyrite, since on the horizontal section 

 (Sheet 5, No. 1) it is spoken of as ' greenstone porphyry,' but it 

 includes, in addition to the diabase-porphyrite, a considerable area 

 of andesite and andesitic ash. 



