ASSOCIATED KOCKS OF THE BREIDDEN HILLS. 



541 



Fig. 3. — Fehpar-cnistal in andesite at N. end of Moel-y-Golfa, 

 ^00 ft. from summit^ showing fractarea of crystal (a) and inclitsion 

 of serjpentine (b). 



Fig. 4. — -Edge of felspar crystal showing inclusions of base. 



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2. Pyroxene. There are two forms of this mineral present : 

 (a) Rhombic pyroxene, in octagonal or elongated sections, indicating 

 a good development of pinacoidal faces truncated by those of the 

 prism. There is a good cleavage parallel to the best-developed 

 pinacoid, but no trace of any other cleavage. The crystals are ter- 

 minated by oblique faces, probably domes, which are generally only 

 developed at one end of the crystal. The structure is slightly 

 fibrous, and there are curious lamellae of darker green mineral gene- 

 rally parallel to the principal pinacoid. It is slightly dichroic, 

 passing from greenish yellow, when the cleavage is parallel to the 

 short axis of the nicol, to light straw-yellow when placed at right 

 angles. The maximum extinction takes place when the cleavage is 

 parallel to one of the vibration -planes of the crossed nicols, and the 

 polarization colours are not of a very high order. It occurs in 

 crystals of all sizes, retaining its shape do'w^n to the smallest prisms. It 

 is best observed in its unaltered state in the great crags running round 

 the E. and S.E. side of the hill, at a vertical distance of from 300 to 

 400 feet from the summit, but in an altered state it occurs in almost 

 all the lavas, being replaced by a green or brown serpentinous 

 pseudomorph, in the manner so admirably figured by Mr. W. Cross*. 

 The serpentine needles frequently have a radial arrangement, and 

 there is more alteration in the large than in the smaller crystals. 

 * Bulletin, United States Geol. Suit. I. 1885, pi. ii. fig. 8. 



