GEOLOG Y. 21 
resuit of compression, and veins of similar granite intrude in the 
gneiss. 
_ The grey rocks, which are largely quarried all over the northern 
part of Guernsey, are locally termed granite: but, strictly speaking, 
they are diorites or syenites. They make excellent blocks for street 
paving, and very durable road metal. A rock known as hornblende 
gabbro, of a dark colour, is also extensively quarried, and is locally 
called ‘bird’s eye.’ ‘I'rue granite is found in the vicinity of Cobo and , 
Grandes Rocques, as well as at Lancresse and a few other places. 
To the south of a line drawn from the town across the island 
the various gneissic rocks are traversed by intrusive veins of trap, 
porphyry, and greenstone. Quartz rock protrudes in picturesque 
masses on the cliffs at Jerbourg on the western side. The rock 
which prevails at the south-western extremity of the island is a 
granitoid gneiss, and along the cliffs eastward it acquires a coarse 
porphyritic character and an agreeable tint, owing to the presence in 
abundance of large red crystals of orthoclase felspar. Dykes of 
quartz felsite are abundant in Guernsey, and still more so in 
Alderney, but are rare or absent in Sark. Mica-traps, which are 
interesting on account of their extreme rarity in England, are 
frequent. They are composed chiefly of mica and felspar, and from 
their colour and roughness, and the glitter of the particles of mica, 
have probably given rise to the statement found in old books that 
‘emery was one of the products of these islands. 
Notwithstanding the complete absence of limestone or calcareous 
rock in Guernsey, the average quantity of lime in drinking water is, 
by analysis, not less than seven grains per gallon, while from ten to 
fifteen grains is by no means an unusual quantity. Clays of various 
kinds exist in large quantities on the high ground, as well as in many 
of the lower levels, and in some places there are distinct traces of 
stratification. Some kinds are only found in veins, like the Kaolin 
clay, which does not occur as a superficial deposit in the island. 
The yellow clay, the unctuous blue clay, and others used in the 
manufacture of bricks, occur in deposits of variable thickness, and 
occasionally contain waterworn pebbles and flints. 
Evidences of the ancient changes of sea-level are found in the 
“raised beaches,’ which exist nearly all round the coast, as well as in 
several places in the interior. Their height ranges from twenty-three 
feet to seventy-five feet above the present mean sea-level. 
The existence of a kind of peat under the sea-beach has lon 
been known. The beds extend along the low shores of the nort 
and west coasts, but the richest deposit occurs beneath the sands at 
Vazon Bay, where it has been dug into to a depth of twenty feet. 
This peat, which is locally known by the fazozs name of gorban, 1s 
dried by the cottagers and used as fuel. ‘Trunks of full-grown trees, 
still erect, occur in this peat bed, as well as acorns and hazel-nuts, 
and also the teeth of hogs and horses; but the presence of pottery 
