Yol. 62.] IGNEOUS AND SEDIMENTARY ROCKS OF LLANGYNOG. 235 



900 feet of andesitic rocks, which, generally speaking, occupies a 

 less elevated tract of country. The best exposures of these rocks 

 are situated on the western side of the Coomb Dingle, east and north- 

 east of Tre-hyrn Farm, where the beds, which consist chiefly of 

 olive-green banded tuffs, are well seen along and near the footpath 

 leading from Tre-hyrn to Llwyn-celyri. These tuffs undulate con- 

 siderably in strike, but have a general dip of 35° to 45° in a north- 

 westerly direction. 



Surmounting the slope occupied by these tuffs, and forming con- 

 spicuous crags west of Llwyn-oelyn, is a pale massive rock, probably 

 of extrusive character, which microscopical examination has shown 

 to have the composition of a hornblende-andesite. Its lower limit 

 is exposed along the side of the dingle for some distance, but its 

 summit is nowhere seen ; however, there is sufficient evidence to- 

 prove that it is followed by another series of tuffs similar to that 

 which preceded it. There are very few exposures of the andesitic- 

 rocks on the west towards Gelli, and such as there are evidently 

 consist of the usual types of banded and pumiceous tuffs. 



(iii) The Diabase of Tre-hyrn. — This rock is intruded into the- 

 andesitic tuffs below the hornblende-andesite, and in character 

 varies from a dark-green, finely-crystalline rock to a more compact 

 variety, which is hard to distinguish in the hand-specimen from 

 some of the andesitic tuffs. The rock is well-exposed south- 

 east of Tre-hyrn farmhouse, and occupies the footpath for about 

 50 yards in the direction of Llwyn-celyn ; it may then be traced 

 along the bank for another hundred yards to the north-east. At 

 this point the intrusion breaks across the andesitic tuffs, and con- 

 tinues its course as a much thinner sheet at a higher horizon, tailing 

 off to a band a few feet thick, and occupying a position on the side 

 of the dingle midway between the hornblende-andesite and Llwyn- 

 celyn. Its maximum thickness is exposed at Tre-hyrn. 



(iv) The Diabase of Pentre-newydd. — This rock is exposed 

 only in the road east of Pentre-newydd, appearing on the imme- 

 diate north of the Old Red Sandstone and merely a few feet away 

 from its base. 



Passing northward up the hill to a gate at a bend in the road, in a 

 distance of 30 yards, the section is seen to be as follows : — The most 

 southerly exposure consists of a pale-blue rock with conspicuous 

 pyrites-crj^stals ; the rock is very much decomposed, but has a 

 parallel structure which strikes about south 25° east. A little higher 

 in the road the mass becomes much fresher in character, and lath- 

 shaped felspars, small patches of chlorite, and crystals of pyrites 

 may be detected in the hand-specimen. This is succeeded by a 

 dark-greenish-grey rock with lath-shaped felspars, and that by a more 

 compact marly-looking rock, which northward gives place to the blue 

 variety first mentioned. Just at the bend of the road, the section 

 is occupied by sedimentary rocks, consisting of grits with fairly- 

 large grains of opalescent quartz, and a few thin beds of badly- 

 weathered greenish shales. The section continues from the bend 



