94 PROCEEDINGS OE THE GEOLOtilCAL SOCIETY. 



in intrusive sheets of similar material. Mr. Blake, indeed, has 

 brought forward the evidence of a section on the north or under 

 side of the Llyn-Padarn ridge, to show that the rock has there 

 been intruded into the Cambrian strata *. M3' own impression is 

 that these igneous masses were probably erupted to the surface as 

 long banks which rose above the waves ; that they were thus ex- 

 posed to prolonged subaerial and marine denudation ; that by this 

 means the upper more cellular portions of the lava were broken up 

 and pounded down into detritus, and thus that what is now visible 

 is a part of the eruptive rock which originalh' lay at some depth 

 within its body. This view is confirmed by a study of other lavas 

 which are found on different platforms in the detrital deposits that 

 overlie the Llyn-Padarn quartz-porphyry. 



That the material of each of the two main ridges is the result of 

 more than one eruption has been inferred from the intercalation of 

 bands of slate and of breccia in the rockf. Considerable lithological 

 differences may be detected in each mass, but they are not greater 

 than may be observed in single sheets. In some parts of the Llyn- 

 Padarn porphyry a distinct nodular structure appears which shades 

 off into bands and lenticular streaks, reminding one of the charac- 

 ters of some of the Bala rhyolites. Other portions are markedly 

 brecciated, the separated fragments being surrounded in a matrix 

 of the rock, which shows flow-structure sweeping past them. On 

 Moel Gronw angular fragments of a dark pinkish tint are scattered 

 through the general mass. Again, some parts are crowded with 

 quartz-grains, while others are comparatively free of these, and 

 occasionallj' a spherulitic structure has been observed J. 



The microscopic structure of this ancient eruptive rock has been 

 studied by Professor Bonney, who found that the general type was 

 a compact dull grey felsite with porphyritic crystals of felspar and 

 grains of quartz, closely resembling some modern rhyolites. Though 

 unable to detect any actual glass in the base, he had no doubt that 

 the rock was originally vitreous, and he found abundant and fresh 

 examples of the most perfect flow-structure §. 



Beference may be made here to the remarkable influence of the 

 intense cleavage of the district upon this rock j|. Along its southern 



^ Quart. Journ. Geol. Soc. vol. xhv. (1888) p. 283. 



t See for example, Bonney, Quart. Joui'n. Geol. Soc. vol. xxxt. (1879) 

 p. 312; Blake, ojj. cit. vol. xliv. (1888) pp. 277, 287. 



I Blake, ihld. p. 277. § Oix cit. vol. xxxv. p. 312. 



II The secondai'y planes due to cleavage must not be confounded with the 

 original flow-structure. 



