416 REV. E. HILL ON THE ROCKS OF GUERNSEY. 
marks a quarry worth clearing of water is usually a sure token of a 
diorite, syenite, or gabbro. 
6. Dykes AND VEINS. 
Guernsey furnishes a numerous assortment of minor igneous 
rocks. These vary in size from the smallest strings and threads up 
to the great masses at Bon Repos Bay and Pleinmont, which last 
covers several acres. They are so numerous that an observer has 
estimated them to occupy three per cent. of the cliff faces round the 
island*. Their diversity of constitution and character may be 
gathered from the following review. 
Acad Rocks.—Granite dykes are numerous except along the 
southern cliffs. Many are grey or white, and though they contain 
hornblende, are probably offshoots from the Lancresse mass. As an 
instance may be taken one about 8 inches wide close to the bedded 
rock at Fort Doyle. There are many pink or red macro-crystalline 
dykes, extremely straight and uniform, which might be attributed to 
the Cobo mass, but that I fancy I have seen them cutting it. As 
they consist mainly of felspar and quartz they may be considered 
elvans. They are often rather decomposed, and in that state some- 
times bear a good deal of resemblance to sandstones. ‘To this group 
belongs the large dyke in quarry on the south-west of Delancy Hill, 
which, as mentioned in section 4, has been supposed to be a bed of 
quartzite. Felstonesarerare. In the quarry on the Grande-Maison 
Road the hornblende-gabbro is cut by thin sheets of a crypto-crys- 
talline felspathic rock, which meet and appear to be offshoots from a 
vertical dyke of grey granite. Round Castle Cornet are several 
intrusions of a beautiful pink felsite, so compact and glassy, and 
with so splintery a fracture, that perhaps it may be arhyolite. One 
of these, on the north side of the breakwater near the timber bridge, 
contains very perfect double pyramids of quartz almost a quarter of 
an inch long. ‘There are others of this class on the shore beneath 
Fort George. 
Basic Rocks.—These are by far the most numerous and most 
various. Several types may be distinguished. One very pretty 
rock consists of well-shaped fresh-looking crystals, -1 or -2 inch 
long, of white felspar and hornblende. This occurs at Castle Cornet, 
near the boundary of the gneiss, at Portinfer, in the extreme out- 
crop of the Cobo granite, and abundantly at the north promontory 
of Fermain Bay, where a sudden change of strike probably indicates 
a dislocation of the gneiss. In this last area the intrusions are 
usually lenticular or irregular in boundary, but are sharply separated 
from the other rock, and cut its bedding or foliation obliquely. I 
think it very probable that the rocks of this group are simply off- 
shoots from the diorites. Another type is grey, fine-grained, with 
grains, rarely exceeding °05 inch, of somewhat greenish felspar. 
This occurs in large extremely regular dykes with straight walls, 
and is frequent over the whole island. A dyke of this kind quarried 
* Quoted in Fisher’s ‘Physics of the Earth’s Crust,’ p. 186. 
