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INTRUSIVE AND ALTERED ROCKS, LLANBERIS. 



211 



184. — Greenstone amygdaloid. 



Tremynydd range, 



from 3 to 4 miles north-east of St. David's. Dull green fel- 

 spathic base, with large black hornblende crystals and 

 kernels of calc spar. 



A line of greenstone, about 2 miles in length, intruded 

 between the beds of Llandeilo slate in their strike. 



Series of rocks illustrative op the alteration of 

 the Cambrian rocks of Llanberis, in contact with 

 or near quartz porphyry, described at No. 124. 



185. — Quartz porphyry. — North-east side of Llyn 

 Padarn, near Llanberis, Caernarvonshire. Map 78, S.E. 

 Grey compact felspathic base, with interspersed quartz 

 crystals, and a few crystals of felspar. (See No. 124.) 



186. — Cambrian grit, much altered. It was probably 

 nearly in a state of igneous fusion by the close proximity 

 of No. 185, into which it almost imperceptibly passes. 

 Like No. 185, it contains granular quartz which may 

 either be grains of silica in the original grit, or attempts at 

 crystallization in the silica of the altered mass. Locality as 



above. 



187. — Cambrian grit, talcose and felspathic, and con- 

 taining numerous granules of quartz similar to those in 

 No. 185, but less crystalline in form. This specimen on 

 the ground is a little further removed than No. 186 from 

 No. 185. Locality as above. 



188. — Cambrian conglomerate, altered by proximity 

 to No. 185, and a little further removed from it than No. 

 187. The pebbles of the original conglomerate are indi- 

 stinctly visible in a greenish -grey felspathic looking matrix, 



full of granular quartz crystalline grains. 



Pebbles and 



matrix are alike highly altered, and the approach to absolute 

 fusion has been so great that the forms of the pebbles seem 



Upper 

 Gallery. 



Wall-case 4 







