GRANITES. 
17 
granite which contains only dark mica, a granitite. Hornblende 
is not infrequent either as an accessory constituent, or replacing 
the biotite, and the rock is then distinguished as a hornblende- 
granite. 
Certain varieties of granite rather rapidly suffer disintegration 
on exposure, but most granites are remarkable for their extreme 
durability. Combined with a considerable degree of hardness, 
this durability renders granite highly valuable as a building 
stone for bridges and other massive structures, while the 
toughness of the liornblendic varieties makes them especially 
suitable for road materials. At the same time the beauty of 
many granites, and the high polish which their hardness renders 
them capable of receiving, recommend their use for purposes of 
ornament. The industrial applications of granite are, however, 
restricted by the expense of working so hard a stone. 
Granites of Cornwall and Devonshire. — Pedestals and 
Columns Nos . 9, 61, 63, 72, 75, 78, 81, 85, 176, 180, 207, 
and 209. Cubes in Table Case III. and. Wall Cases VII. 
and VIII. 
The principal exposures of granitic rocks in England are in the 
counties of Cornwall and Devon, where five large areas occur, 
with a few smaller outlying masses. These districts form high 
ground, rising like islands of granite from the surrounding 
killas or clay -slate. The five principal areas are known, pro- 
ceeding from east to west, as Dartmoor, Drown Willy or 
St. Breward granite, Hensbarrow or St. Austell granite, Carn 
Menelez or Penryn granite, and the Land’s End or Penzance 
granite ; whilst still more westerly are the exposures of granite 
in the Scilly Islands. Lundy Island, in the Bristol Channel, is 
also composed of granite. 
Most of the West of England granites are coarse grained, and 
even porphyritic in texture. Their prevailing colour is grey, 
hut red granite is not unknown, as seen in the handsome rock 
from Trowles worthy on Dartmoor (No. 9). In some of the 
granites of Cornwall and Devon, the felspar passes into kaolin or 
china-clay ; this is especially the case on the southern margin of 
Dartmoor ; in the neighbourhood of St. Austell ; and at Breague, 
near Helston, in Cornwall. Schorl, or black tourmaline, is not 
unfrequently present as an accessory constituent, especially on 
the margin of an intrusive boss of granite. 
The following great works, amongst many others, have been 
constructed entirely or in part of Cornish granites : The Penryn 
and Lamorna granites have been used in Portland breakwater ; 
Keyham Docks, Devon ; Commercial Docks, London ; the Hull, 
Great Western, and Birkenhead Docks, and. the National Works 
at Chatham and Portsmouth, together with the Scutari monu- 
ment. The plinth for the railings of the British Museum is 
from the Carnsew Quarries, near Penryn ; and the towers. 
