574 MR. H. WOODS OK THE IGNEOUS ROCKS [Nov. 1 894, 



Maen-cowyn shows under the microscope a cryptocrystalline ground- 

 mass, with here and there small microcrystalline patches. The 

 porphyritic felspars are twinned on the albite type, sometimes 

 combined with the Carlsbad or with the pericline type. The other 

 slides differ from this chiefly in the relative proportions of the 

 cryptocrystalline and microcrystalline material in the groundmass. 

 In some sections there are bands of quartz. Spherulitic structure 

 has not been met with in any case. 



The rhyolite has been found to be nodular only at one spot, 

 namely, § mile east of Newmead Farm, and a little more than 

 I mile north of Caer Einon. The nodules stand out on a weathered 

 surface of the rock, and can be easily detached : as a rule they are 

 very irregular in form, and of small size, sometimes being 2 inches 

 in diameter, but often less. They are solid throughout, and in 

 section are seen to be formed of microcrystalline quartz and felspar, 

 aud to show no trace of either a radial or concentric arrangement ; 

 in fact the structure does not differ from that of the groundmass 

 of some of the ordinary rhyolites. The silica-percentage of one 

 nodule was found to be 72 - l. 



lihyolitic ashes have been met with at the following spots : — 

 Immediately north of Maen-cowyn ; north of Caer Fawr ; at | mile, 

 and also at a little more than ^ mile west of Cwm-berwyn. 



VI. The Diabases. 



The diabases are of later date than the rocks previously described, 

 and may therefore conveniently be considered last. They are all 

 intrusive in the Llandeilo Shales, and are found on the east and 

 west of the main volcanic series. The minerals which occur in the 

 diabases are plagioclase-felspar, augite, magnetite, ilmenite,leucoxene, 

 apatite, chlorite, and secondary quartz and calcite. The plagioclase 

 is generally much decomposed, and the augite in most cases ophitic. 



On the west, between the town of Builth and Pen-cerig, there are 

 four sills of diabase. The northernmost stretches from Pen-cerig in 

 a south-westerly direction for about | mile. It is well exposed in 

 the quarries at the north-western edge of Pen-cerig Wood, and at 

 Pen-cerig Lake ; also in the road-cutting at the back of Pen-cerig 

 House (fig. 2, p. 575). At the two last-named localities the junction 

 with the Llandeilo Shales, containing the following fossils, is seen : 

 Ogygia buchi (Brongn.), Ampyx nuclus (Murch.), Trinucleus fim- 

 briatus, Murch., Barrandia cordai, M'Coy, Siphonoireta micula, 

 M'Coy, etc. 



The rock in the quarry at the north-west of Pen-cerig "Wood is a 

 medium-grained and rather decomposed diabase, having a specific 

 gravity of 2 - 75. The augite is colourless, and occurs in ophitic; 

 plates. Leucoxene forms large grains and is often associated with 

 ilmenite. 



The rock at Pen-cerig Lake is finer-grained than the preceding, 

 and in hand-specimens differs considerably in appearance. The 

 microscope shows that the augite is usually scarce, and in one slide 



