Another species of stone very prevalent in Cornwall, is distinguished by the name of _Killas_. It is a schistus, and forms the most considerable substratum in the county. It varies in texture and colour, some being hard, others more pliable and laniated, and of a blueish yellow, and ferrugineous brown; but either forms an excellent material for building. The worst sort of stone found in Cornwall, is an opaque whitish debased crystal, generally called spar, and lies loose on the surface of the ground, in almost every parish. It is, however, useful for making fences and for repairing the roads. On the north and south coasts of the county, there are several _Slate Quarries_, the slate from which is generally adopted for the roofing of houses; but the best species is found in the celebrated quarry of _Delabole_ near Camelford, which is said to produce the finest and largest slates in England.—“The quarry is about 300 yards long, 100 broad, and upwards of 40 fathoms deep. The slate is first met with about three feet below the surface of the ground, in a loose shattery state, with short and frequent fissures, the _laminæ_ of unequal thickness, but not horizontal.—Thus it continues to the depth of 10 or 12 fathoms, when a more firm and useful stone is procured, the largest pieces of which are used for flat pavements. This is called the _top-stone_, and continues for 10 fathoms, after which the quality improves with increasing depth, till at the 24th from the surface, the workmen arrive at the most superior kind, called the _bottom-stone_.—The colour is grey-blue, and the texture is so close, that it will sound like a piece of metal. The masses are separated from the rock by wedges driven by sledges of iron, and contain from five to 14 superficial square feet of stone. As soon as this mass is freed by one man, another stone cutter, with a strong wide chisel and mallet, is ready to cleave it to its proper thinness, which is usually about one eighth of an inch; the pieces are generally from a foot square, to two feet long, by one wide, but the flakes are sometimes large enough for tables and tomb stones.”[3]