96 PROCEEDINGS OJF THE GEOLOGICAL SOCIETT. 



Cambrian series at St. David's. But on the north-west side, where 

 they have in large measure escaped the eiFects of the cleavage- 

 movements, their original structures are well preserved. 



One of the first features of these detrital deposits to arrest 

 attention is the amount and variety of the fragments of igneous 

 rocks in them. Some of the conglomerates, though enclosing 

 pebbles of quartz, quartzite, granite, and other rocks not found in 

 situ in the immediate district, are mainly composed of the debris of 

 the quartz-porphyry of the ridge. Indeed, this latter material 

 appears to have contributed a large proportion of the detritus of 

 which the general body of strata here is made up. But there are 

 to be noticed among the contents of the conglomerates and breccias 

 pieces of many volcanic rocks not to be found on the porphyry 

 ridge. Among these, besides felsites showing sometimes beautiful 

 flow-structure (rhyolites) and various quartz-porphyries, there occur 

 abundant fragments of less acid lavas (andesites) and pieces of older 

 tuff's. Some of the fragmental rocks are green in colour, probably 

 irom the abundance of fine basic volcanic dust in them. Certain 

 bands are full of large angular pieces of shale similar in character 

 to the Cambrian slates, and doubtless due to the disruption of 

 pre-existing Cambrian strata by volcanic explosions. It is clear 

 that from vents in this neighbourhood there continued to be an 

 abundant discharge of dust and various andesitic and other lapilli, 

 which, falling on the sea-floor, mingled there with the ordinary 

 mechanical sediment that was being deposited at the time. 



But we have evidence that during the period when these showers 

 of volcanic detritus were thrown out streams of lava, though on a 

 greatly diminished scale, continued to be poured forth. The hill of 

 Clegyr, near the lower end of Llyn Padarn, consists mainly of 

 cleaved tuff's, slates, and conglomerates overlying the quartz- 

 porphyry. Near the summit a band of f elsite is intercalated in 

 these rocks. But still more striking are the sections on the south 

 side of the lake. Starting from the porphyry of the ridge we cross 

 a zone of conglomerate and grit largely composed of the de^bris of 

 the porphyry, until we reach a baud of felsite or quartz-porphyry, 

 which at its eastern end is about ten feet thick, while it seems to 

 increase in dimensions westwards. In the centre the rock is dark 

 purplish-red, exceedingly compact or flinty, sprinkled with a 

 variable proportion of quartz-blebs and felspar crystals. Towards 

 its southern or upper edge (for the rocks, though nearly vertical, dip 

 southwards) it has been cleaved into a variety of purple slate, and 



