ANyiVEKSAKY ADDKKSS Or TUK PKESIDENT. 97 



would there at once be classed among the ordinarj' slates of the 

 neighbourhood. But the fissile character is merely a marginal 

 structure which the rock shares with the highly cleaved tuffs 

 that follow it. Traced westwards, this bed is found to enclose a 

 quartziferous core, which is also cleaved along the margin as well 

 as partially in the interior. It would not be possible to distinguish 

 parts of this intercalated band from portions of the porphyrj' of the 

 main ridge. The difference of colour does not count for much, for 

 even in this band the purple tint gives place to greenish grey, and 

 what in the centre at the east end is a solid dark purplish-red 

 felsite passes westward into a greenish slate, like that already 

 noticed on Mynydd y Cilgwyn. 



The microscopical examination of this rock shows it to be a true 

 felsite of the rhyolitic type, which in the central uncleaved part 

 exhibits a wavy flow-structure like that found in the quartz- 

 porphyry of the ridge. So intense has been the cleavage in its 

 upper part that the original structure of the rock is there eff'aced. 

 The immediately overlying tuffs, which are likewise so thoroughly 

 cleaved that it is not easy to draw a sharp and continuous line 

 between them and the intercalated lava, precisely resemble those 

 found below the conglomerate on the opposite side of the lake. 

 They include bands of coarse volcanic breccia as well as fine compact 

 material, showing the varying intensity of the volcanic discharges. 

 Their included stones consist of various felsites, andesites, and 

 slates. 



The thin sheet of interstratified quartz-porphyry here described 

 is not the only one to be found in the section. Others thinner and 

 more intensely cleaved lie among the tuff's higher up. They have 

 been sheared into mere pale unctuous slates, bat the remains of 

 their quartz-blebs may still be detected in them. 



The tuffs, with their interstratified bands of porphyry, become 

 more and more mingled with ordinary argillaceous and sandy sedi- 

 ment as they are followed in upward succession. Among them 

 occur bands of grit and fine conglomerate containing pebbles of por- 

 phyry and pieces of slate. Some of these grits are mainly composed 

 of white felspar, felsite, and clear grains of quartz, evidently derived 

 from the disintegration of a rock like the porphyry of the main 

 ridge. As the ordinary sediment of the Llanberis group sets in, 

 the tuffs are restricted to thinner and more widely separated bands. 

 Some thin layers of felspathic breccia, seen among the slates close to 

 the Glyn Peris Hotel, probably mark the last discharges of the 



