Yol. 50.] IGNEOUS ROCKS NEAK BUILTH. 569 



III. The Andesites. 



The andesites are widely distributed in this district ; there are 

 four areas, which will be described in tbe following order: (1) 

 surrounding the three southern masses of diabase-porphyrite ; 

 (2) around the northern part of the largest mass of diabase-porphy- 

 rite ; (3) north of Gelli-Cadwgan ; and (4) extending from Caer 

 Einon to Llwyn-Madoc, except where broken into by the diabase 

 near Cwm-berwyn. 



(1) The andesite between Llanelwedd and Carneddau Farm, on 

 the west of the Llanelwedd diabase-porphyrite, forms a well-marked 

 type : it is, when fresh, a dark-green compact rock, and on a 

 ■weathered surface shows a thin white crust. Good specimens may 

 be collected at between | and k mile north of the Rock House ; in 

 these, crystals of felspar and augite may be seen with the unaided 

 eye. Under the microscope, sections show a groundmass formed 

 mainly of minute felspars, exhibiting in places flow-structure ; in 

 this porphyritic plagioclase-felspars occur ; they are rather decom- 

 posed, and sometimes contain irregular inclusions of the groundmass. 

 Augite is also rather abundant ; it is colourless and quite fresh. 

 A pale green, rhombic pyroxene, showing distinct pleochroism and a 

 slightly fibrous structure, is fairly common ; it is probably an altered 

 enstatite. Ilmenite associated with leucoxene is present, and also 

 small crystals of magnetite. The silica-percentage of this rock 

 is 50 - 8 and its specific gravity 2- 74. 



The andesite on the eastern side of the Llanelwedd diabase- 

 porphyrite differs considerably from that on the west just described : 

 it is much more decomposed and of a dull greenish colour. Under 

 the microscope it is seen to be formed of porph) 7 ritic plagioclase- 

 felspars in a base of minute felspars ; there is very little augite, 

 and enstatite is absent ; chlorite is abundant, and there is a little 

 calcite. The rock has a specific gravity of 2-702. 



The andesite, extending north of the Llanelwedd diabase- 

 porphyrite up to the southern border of the largest mass of the 

 same rock, is green or greyish in colour, sometimes light, some- 

 times dark; the felspars in many cases are seen as glistening 

 lath-shaped crystals, in others they are milky- white and rather 

 irregular. Sections show that the groundmass of the rock is made 

 up of a brownish isotropic material containing numerous small 

 felspars generally having a flow-arrangement ; sometimes there are 

 also irregular granules of a semi-opaque substance. The phenocr} T sts 

 consist of plagioclase-felspar, and, in some slides, of an altered 

 rhombic pyroxene ; augite is rare and often absent. 



This same mass of andesite is continued southwards by Caer 

 Fawr down to the Big Wood. A section from the south-western part 

 of Caer Fawr is similar to those just described, but is moi'e decom- 

 posed and contains much chlorite ; another from the southern part 

 of the Big Wood shows a particularly good flow-structure in tbe 

 later generation of felspars, and also contains a large quantity of 

 calcite. 



