Tol. 50.] OF THE NEIGHBOURHOOD OF BUILTH. 571 



well jointed in a roughly cuboidal manner. A specimen from this 

 quarry was found to contaiu 57'73 per cent, of silica, and to have 

 a specific, gravity of 2'fc>39. Under the microscope it is seen to be 

 formed of a base of small, rather irregular felspars, with a few 

 porphyritic crystals of plagioclase ; there is also some secondary 

 quartz. Other sections taken from specimens collected at different 

 spots between the quarry and Gauallt Wood are similar to the 

 last, but the porphyritic felspars are larger, and the smaller felspars 

 of the groundmass have more regular outlines and show a well- 

 marked flow-arrangement. 



(4) The last area of andesite extends from Caer Einon in a 

 north-easterly direction to Llwyn-Madoc, a distance of a little more 

 than 2 miles ; but this is broken into by a mass of diabase between 

 Cwm-berwyn and Cil-y-berllan. 



Sections from various parts of Caer Einon show numerous rather 

 large porphyritic plagioclases, and a base composed mainly of small 

 felspars, generally with a good flow-structure, and containing 

 vesicles of irregular shape filled with a pale green, fibrous, chloritic 

 mineral. The same features were seen in a section from near 

 Bwlch-y-trawspen. 



Another slice was cut from a specimen collected at a spot £ mile 

 north of Rhiw-las and 70 yards east of the stream which runs to 

 Rhiw-las ; this shows distinct evidence of crushing, the felspars have 

 a parallel arrangement, and numerous bands of quartz are present. 

 Limonite also occurs, sometimes showing eight-sided sections, 

 apparently as a pseudomorph after augite. 



Between Cwm-berwyn and Llwyn-Madoc I was only able to find 

 exposures near Gaer; the rock is here rather dark in colour, and 

 contains felspars, chlorite, and a small quantity of augite. 



IV. The Andesitic Ashes. 



A belt of andesitic ash extends from Llanelwedd to Orl Wood, 

 forming the western boundary of the volcanic series, and being 

 overlain by Llandeilo Shales; just north of Wern-to its outcrop is 

 shifted by a dip-fault. When fresh the rock is of a light bluish- 

 grey colour, and has a gritty feel ; it is well exposed in numerous 

 quarries between Llanelwedd and Wye Cottage, and is largely used 

 in the neighbourhood for building. Specimens from these quarries 

 were found to have a specific gravity of 2-66 and a silica-percentage 

 of 51 - 10. Under the microscope, sections taken from Tan-y-graig 

 and other quarries to the south show numerous irregular fragments 

 of plagioclase-felspar, embedded in a fine-grained base containing 

 chlorite and calcite ; some fragments of andesite are also present. 



The lower part of the ash in some of the quarries, particularly 

 at Tan-y-graig, contains numerous well-rounded pebbles of 

 andesite ; they are usually 5 or 6 inches in diameter, but one 

 example I saw had a length of 2 feet and a breadth of 1|. Under 

 the microscope the pebbles are seen to resemble more closely the 



