"Vol. 62.] IGNEOUS AXD SEDIZVIEXTARr ROCKS OF LLAXGYNOG. 245 



pyroxene. Therefore, we are led to infer that at some period in its 

 history this rock was more or less completely uralitized. 



In its intrusion it has included a thin lenticular mass of grits and 

 shales ; the grit consists of closely-packed subangular grains of 

 quartz, felspar, and fine sediments, with a small quantity of 

 interstitial chlorite and iron-ores. 



2. The Rocks at Capel Bethesda. 



(a) The rhyolites of Capel Bethesda are almost identical in 

 character with those of Coomb, especially those on the eastern side 

 of Coomb Dingle (p. 241), and have a mean specific gravity of 2'64. 

 The spherulitic type is the more common, although the section in 

 the road opposite the Chapel has yielded a rock poor in phenocrysts, 

 the grouudmass of which presents that patchy appearance that 

 was noticed in some of the rocks from the Castell-Cogan area 

 (p. 242). 



The small patch of conglomerate which rests upon these rocks 

 is largely made up of partly-rounded masses of the underlying- 

 material. Many of these pebbles were collected, and several sections 

 cut from them. They showed, in most cases, marked spherulitic 

 structures : the spherulites inordinary light standing out in contrast 

 with the groundmass, on account of their being slightly stained by 

 oxides of iron. Under crossed nicols they present exactly similar 

 characters to the spherulites from Coomb. 



(/3) The Diabase north of Pen-gelli-uchaf. — This rock 

 is so rotten, even at its best exposure, that little can be said with 

 regard to its original composition, except that it was at one time a 

 diabase, and probably preserved an ophitic structure. 



jNow it consists entirely of almost completely-decomposed felspar- 

 laths, together with chloritic and serpentinous minerals resulting 

 from the decomposition of the original ferromagnesian mineral, 

 while much calcite is developed in patches and veins. A. sufficient 

 number of felspars have portions left such as to make determination 

 possible, and these are seen to be identical in composition with those 

 in the diabase of Tre-hyrn. 



The chlorites include the two varieties mentioned in connection 

 with the andesites of Coomb (p. 239). 



3. The Lambstone Porphyry. 



The rock at Lambstone, Llangynog, is of a light greyish-blue 

 colour, very tough, and has a specific gravity of 2'68. There is no 

 sign of bedding, but two sets of rough joints may be noticed 

 traversing the mass approximately at right-angles to each other. 



Under the microscope, the rock is seen to be considerably de- 

 composed, but it is still fresh enough to reveal its true character. 

 The constituent minerals are fairly-large idiomorphic plagioclase- 

 felspar and smaller orthoclase, with minute but well- shaped cry- 

 stals of hornblende, set in a fine felsitic groundmass without any 



