Vol. 68.] PETROLOGICAL NOTES ON GTJEENSEY, ETC. 43 



and of a rather pale hornblende, in which the extinction-angles 

 are smaller than is usual, and transverse sections are few (both 

 suggestive facts). The rock is finer in grain than the normal 

 diorite, from which it has probably been broken, but has perhaps 

 been almost re-melted. The red part is mainly felspar, variable in 

 size, the larger grains being hypidiomorphic. It is decomposed, 

 but microcline and plagioclase are recognizable. Also present are 

 a fair amount of a pale yellowish-green mineral, rather irregular in 

 outline, with high refractive index, probably epidote, quartz (not 

 much), and the usual little flakes of greenish mica, which in the 

 hand-specimen are hardly visible. (See PI. I, fig. 4.) 



We come, lastly, to two dykes which present some rather marked 

 differences in structure from the others. One is an uustreaked 

 dyke about 12 feet thick, just north of the angular bastion on the 

 western side of the castle. A slice from the middle part contains 

 only a few flakes of altered biotite. Quartz and felspar occur por- 

 phyritically : the former often retaining its prism-angles, the latter 

 fairly idiomorphic, varying in length from "1 inch to "05 inch, and, 

 as usual, decomposed, though generally with clear borders. The 

 ground-mass often exhibits a rather spherulitic structure, irregular 

 in its outline, somewhat brush-like in growth, and apparently 

 connected with one or two of the larger minerals (see PL I, fig. 5). 

 The remainder of it is a mosaic of the same minerals, the granules 

 being about '005 inch in diameter, mixed with some flakelets of a 

 white mica. Some peculiar, thin, granular, elongated bodies are 

 perhaps only cracks subsequently filled up. Another slice, cut 

 from the chilled edge about 2 inches from the exterior, shows quartz, 

 felspar, and altered biotite much as before, in a quartzo-felspathic 

 ground-mass, the constituents of which measure not more, and 

 generally less, than "001 inch in diameter ; under a high power it 

 shows a tendency to a spherulitic or graphic structure, and some 

 minute belonites, rather irregularly shaped and of a pale yellowish- 

 green colour. 



Another very compact dyke, of a purplish colour, almost flinty 

 in structure, with small phenocrystals of quartz and felspar, 

 occurs at the north-western corner of the Castle-Cornet rock near 

 the pier-bridge.^ We were at first inclined to suppose it an offshoot 

 from a thick dyke of normal character, which for some reason had 

 cooled more rapidly, but further study excited suspicions \Ahich 

 were strengthened by microscopic examination ; and Mr. Hill, on 

 his last visit, satisfied himself that the dykes were independent, 

 although for part of their course the}' occupy the same fissure. Here 

 then, as in one or two other places,^ there has been a second and 

 later intrusion of acid rock. 



1 It is described in the paper of 1884 at pp. 416 & 423. 



- For instance, near Fort Clonque in Aklernev and Fort Regent in 

 Jersey. 



