76 John Parkinson—Rocks of Northern Guernsey. 
A thin section cut from one of these shows that the characteristic 
mineral is obscured by secondary products, but such extinctions as can 
be seen indicate labradorite. The crystals are occasionally zoned. 
A few grains of apatite and ilmenite are the only accessory minerals. 
The felspathic dykes contain also darker shreds and inclusions, 
sometimes clearly, sometimes indefinitely outlined, suggesting that 
a local absorption of derived fragments attended the intrusion. 
Occasionally a more definite arrangement of parts produces a rude 
banding. Thus in one instance the centre of such a dyke, cutting 
the ordinary ‘ bird’s-eye,’ was dark-coloured and close-grained, about 
14 inches across, and bordered by a narrow felspathic edge, not 
sharply marked off from the surrounding rock, while one or two 
identical felspathic patches were contained in the central part and 
were disposed roughly parallel with the direction of the dyke. 
The dioritic rock, referred to under the heading Hornblende 
Gabbro (a), is found also as ill-defined fragments lying in a more 
felspathic matrix, which nevertheless contains a considerable quantity 
of hornblende. Other inclusions occur, petrologically inseparable 
from the hornblende dykes, while the rocks rich in felspar in which 
they le are indistinguishable from the later dykes when these contain 
hornblende. 
A note by the Rev. Edwin Hill and Professor T. G. Bonney on 
a banded specimen of ‘long-grain’ and felspathic rock from here, is 
of interest in this connection. The inferences drawn as to the 
possible derivation of the hornblende from augite and the occurrence 
of biotite as a derivative from the former mineral are entirely in 
accord with my own work, as is also the conclusion that the rock 
‘‘exhibits an imperfect mixture of two magmas” (p. 137). A slight 
difference in the physical state of either or both of the magmas 
may give rise on the one hand to a brecciation, on the other to 
interbandine. 
Examination of Bordeaux Harbour and the shore between Hougue 
ala Perre and St. Peter’s Port did not appear to me to warrant the 
conclusion that the hornblende gabbro was genetically distinet from 
the dioritic rocks to the north and south. 
(2) Secrions on rae Norraern AND WestTeRN Coasts. 
(a) Hast Shore of Grande Havre.—The quarry at the south-east 
angle of Grande Havre is excavated in a fine-grained mica-diorite 
containing a little quartz. A thin section shows that the hornblende 
is intimately associated with biotite, which apparently replaces it, 
and forms a kind of matrix to the other minerals. A coarser rock 
consisting of plagioclase, some orthoclase, and a considerable quantity 
of quartz (sometimes a full third of the rock) is intrusive into it. 
The junction, which is not very clearly defined, is made apparent by 
the greater size of the felspars of the more acid rock, accompanied by 
the development of biotite to the exclusion of hornblende. The 
quartz has corroded the felspar. The more basic side of the junction 
consists of plagioclases varying from oligoclase to labradorite, usually 
1 Quart. Journ. Geol. Soc., vol. xlviii (1892), p. 1385. 
