^^^' 55'] SEKPENTINE AND ASSOCIATED EOCKS IN AN'GLESEX. 293 



(2) Gabbro. 



Immediately north of Dinas-bacla a dyke cuts the serpentine, 

 extending along the shore for about 20 yards or more, roughly from 

 east to west. The mass in the field is seen to vary from a pale 

 green or grey rock with small green crystals, to a coarsely holo- 

 crystalline rock, apparently consisting of three distinct minerals, 

 respectively dark, pale-green, and whitish. Microscopic examination 

 shows in all the varieties : — (1) unaltered augite (the dark crystals 

 in the hand-specimen), which in one is intercrystallized in small 

 patches like a micropegmatite ; (2) felspar much kaolinized (the 

 above-named whitish mineral) ; (3) serpentine, somewhat abnormal, 

 not resembling that derived from olivine : it sometimes shows a 

 parallel structure probably due to the alteration of enstatite, and 

 the relations with angite would agree with this interpretation ; and 

 {4) a constituent formed of small aggregated granules, dull drab in 

 colour, with high refraction, and tending to an isometric outline. 

 They sometimes form strings bordering the pyroxene or crossing 

 the serpentine, often in a double cluster, as if deposited at the sides 

 of a crack, or they constitute rather large patches at limited spots. 

 Perhaps these represent more than one mineral. Some are clustered 

 around spots of iron-oxide, and thus suggest perofskite ; but a 

 larger number occur along cracks in, or even replace parts of, a 

 mineral like felspar, and thus are more probably garnets. Tbe rock 

 may be termed an enstatite- gabbro. 



The ordinary gabbro, as all observers have noticed, generally forms 

 abrupt masses. It passes from the usual coarse structure, with 

 diallage-crystals f inch long, to a fine-grained or even a compact 

 variety. The different forms may occur in a patchy manner within 

 the same large mass, as at Graig-fawr ; but it is often difficult to 

 identify a compact form, when it occurs in a small outcrop. 



A search along the margin of the gabbro-masses^ was not rewarded 

 by specimens giving a direct proof of the relations of the gabbro to 

 the serpentine ; this has to be inferred from the distribution of the 

 two rocks, as seen on the map. One mass, risiug like an islet from 

 the marsh north-west of lAjn Dinam, is roughly oval in outline, 

 about 30 feet long, nearly perpendicular on its western face, but 

 sloping eastward. The mass is mainly gabbro, though a laminated 

 rock is adherent on the sloping surface, apparently a dull drab- 

 coloured flinty argillite. Microscopic examination shows this rock 

 to consist of rather sparse lenticular streaks of a mosaic of minute 

 quartz or secondary felspar, with intervening darker bands, some- 

 what dusty-looking but crowded with epidote ; a minute micaceous 

 mineral (? sericite) is often intercrystallized in both parts. The 

 streaky structure and the crumpling of veins show that the rock 

 lias been greatly crushed. Whether the original mass was one of 

 the schists or a gabbro it is not possible to say. 



^ Specimens from two localities ars described in § Y, p. 295. 



