Vol. 6S.^ PETROLOGICAL NOTES ON GTJEKNSEY, ETC. 45 



imperfectly idiomorphic, are generally brown in colour, but some- 

 times green : in one case the former includes rounded grains of the 

 latter, but in another (the larger crystals) the enclosures are com- 

 posite — the small constituents giving rather exceptionally high 

 polarization-tints.^ The ground-mass is microcrystalline, consisting 

 of much-decomposed felspar, small grains of augite or hornblende, 

 and many granules of iron-oxide. This rock not improbably 

 represents an offshoot from the ' birdseye ' variety of the diorites. 



The next specimen is from a dyke on the north side of Fermain 

 Bay, 3 to 4 feet thick, which runs up the cliff from the shore, 

 cutting gneiss. This is chiefly interesting because it is cut by a 

 thin aplitic dyke. Yery near it, eastwards, is a mass of diorite, 

 and westwards are two diabase and other aplitic dykes, a thin vein 

 from one of the former distinctly cutting a similar vein of the latter. 



Under the microscope the structure is microcrystalline. The 

 pale-green hornblendes, with occasional rusty stains, prismatic in 

 form (more abundant than the felspar), suggest a slight fluxion - 

 foliation. The felspar is granular and interstitial, not twinned, 

 rather decomposed, and the clear parts may sometimes be reconsti- 

 tuted. Iron-oxide is small and not very abundant. Perhaps this 

 dyke also is an offshoot of the diorite. 



The following specimens represent the more ordinary types. 

 We take first a compact dyke, which cuts the gneiss at the south 

 end of Yazon Bay. The hand-specimen is of a greenish-black 

 colour, very minutely granular. A slice shows the rock to be 

 microcrystalline. It contains numerous prismatic, sometimes 

 almost acicular, greenish crystals exhibiting a general resemblance 

 to hornblende but often granular in structure, especially towards 

 the centre, so that it is difficult to observe the extinction-angles. 

 There are small grains of iron-oxide. An interstitial mineral, pro- 

 bably felspathic, possibly a devitrified glass, contains numerous 

 belonites (? hornblende). 



The next specimen is from a dyke on the east side of Petit Bot 

 Bay. The hand-specimen, a dark blackish-green sub-porphyritic 

 diabase, is minutely granular, and contains specks of pyrite. The 

 microscope shows a fair amount of a plagioclastic felspar, measuring 

 up to about three-fifths of an inch in length, tending to occur in 

 groups, but not well preserved. The ground-mass consists of a 

 felspar, also too altered for determinatioii, with augite, almost 

 replaced by the filmy hornblende described above, and a little 

 iron-oxide. A few grains of calcite occur, perhaps slightly dolomitic 

 and probably secondary. 



The next specimen, from a dyke about 9 feet wide, on the 

 side of the Lihou causeway, is selected to represent the more 

 porphyritic examples. The ground-mass (somewhat minutely 

 ophitic) consists of decomposed plagioclase and rather small augites 

 partly replaced by ' viridite,' together with little grains of iron- 

 oxide, resembliug magnetite. In this are scattered crystals of 



^ This may signify that the enclosed mineral was once au augite, poor in 

 iron, like diopside. 



