196 



IGNEOUS ROCKS. 



Upper 

 Gallery. 



This is also the equivalent of the Aran and Penmaen fel- 

 spathic trap in its north range, on the west side of the River 

 Wnion fault (see section above, No. 7); and in this neigh- 

 bourhood the thick beds of ashes disappear that, further 

 south, underlie these felspathic lavas. From Nos. 91 to 99 

 the rocks belong to the loicer set of lavas that lie near the 

 base of the Llandcilo flags, and, associated with many beds 

 of ashes, circle in a crescent form from the neighbourhood 

 of Dolgelli to Tremadoc, including the mountain of Cader 

 Idris, the Arans and Arenigs, the Manods, and Moelwyn 

 Mawr and Moelwyn bach. See maps 59, N.E., 74, S.W. 

 and N.W., 75, S.E. and N.E., and sections No. 4, 7? 8 , and 

 9, above. 



Felspathic Lava Beds of the Higher or Snowdon 



Series, lying in the Caradoc or Bala rocks. 



100. 



Felspathic trap. 



Snowdon, Caernarvonshire. 

 greenish-grey felspathic rock 



Clogwyn du'r Arddu, 



Map 75, N.E. 



Compact 



felspathic substance. The latter resist weathering, and 

 form projecting lines on the weathered surface. The lines 

 probably originated from the same kind of cause that pro- 

 duced the laminated structure in the lava from Ascension 

 Nos. 26 and 50, Case 2. 



Clogwyn du'r Arddu forms a lofty cliff about a mile 

 north-west of the peak of Snowdon, in which the felspathic 

 traps (old lava beds) are faulted against the calcareous 

 ashy beds that form the mass of the summit of Snowdon. 

 See section No. % above. 



101.— Felspathic trap.— Clogwyn du'r Arddu, Snow- 

 don, Caernarvonshire. Map 75, N.E. Compact bluish- 

 grey felspathic rock, with banded lines, showing viscous 

 flowing. These lines, invisible in the freshly fractured 

 interior, become visible on weathered surfaces, and are pro- 

 bably analogous to those shown in specimens No. 45 and 



Case 2 



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