34 PROr. T. G. BONNET AND THE EEV. E. HILL: [Feb. I912, 



weld, from the paler more normal rock. On the more southern 

 side this dark kind appears occasionally in patches, with irregular 

 outlines, among the more normal diorite, which itself is rather 

 variable in coarseness. All are cut bj veins of the exceptionally 

 felspathic kind. This rock from Fort xilbert was described in 

 1889 ^ as closely related to the picrites, but a slice, made from an 

 apparently average specimen collected last year, shows less olivine, 

 and distinctly more felspar : so the extreme basicity, as at Little 

 Knott,^ is obviously local. 



In the field the rock is irregularly jointed, weathers with rather 

 rounded edges, and varies in both the amount of felspar and the 

 size of its hornblende-crystals, which sometimes reach half an inch 

 in diameter. Here and there it much resembles the Guernsey 

 ' birdseye,' and passes on either side into a normal ' speckly ' 

 diorite. Commonly the junction between the two varieties is 

 sharp, a little wavy, and with a perfect weld ; but occasionally it 

 is irregular. We consider the normal diorite to be the intruder, 

 for now and then, near a junction, it contains streaks, sometimes 

 almost ' wisps,' occasionally forking, of ' picrite ' or a solitary 

 hornblende, quite half an inch in diameter ; while one of the 

 streaks, less than an inch thick and a few inches long, may 

 contain felspar, in a way that suggests an impregnation from the 

 other rock. 



All these varieties are veined by the rather cream-coloured 

 felspathic one, mentioned above, in which the hornblende is about 

 the size of a mustard-seed. Thus we infer that the ' picrite ' 

 had only just crystallized when it was followed or carried up 

 by the diorite, and that both were still at a high temperature 

 when the more felspathic material was injected. Diabase dykes, 

 like those common in Guernsey, cut the mass, and at Bibette Head, 

 granite,^ as was described by Mr. Hill, is clearly intrusive in the 

 ordinary diorite. The former rock is of normal coarseness at the 

 junctions, and sends a vein into the other which is about 3 inches 

 thick. 



In the Guernsey instance, at Bon Repos Bay, a rather broad 

 mass of diorite, obviously intrusive in the gneiss, passes from a 

 coarse, very hornblendic form (in the lowest part of the clifi") to 

 a fairly normal one, the intermediate zone being now and then 

 very like the 'birdseye.' In the field the first rock much resembles 

 the dark one at Fort Albert, but under the microscope it proves 

 to contain more felspar (plagioclase) and some quartz, occasionally 

 forming an intercrystalline mosaic with hornblende. The latter 

 mineral is abundant, and is often associated with small flakes 



^ Q. J. Gr. S. vol. xlv, p. 384, where the results of the microscopic examination 

 are given. 



2 3id. vol. xli (1885) p. 511. 



^ A slice shows it to consist of quartz, felspar (not very well-preserved, but 

 in part at least orthoclase), biotite, dominating over hornblende (though often 

 closely associated with it, as if formed at its expense), a little sphene, and 

 iron-oxide. 



