THE CHINA CLAY INDUSTRY OF CORNWALL AND DEVON. 135 
Many of the so-called “ deposits ” of clay extend for a dis- 
tance of a quarter of a mile, half-a-mile, or even more, in the 
direction of the veins, while their breadth may be only a few 
inches, and seldom exceeds a few fathoms. It is true that 
very wide masses of china clay are wrought in many places, 
but these are invariably associated with a group of parallel 
veins. 
The granite rock is usually covered by a layer, from 4 to 30 
feet thick, of brown or yellow sandy earth, often full of angular 
pieces of hard granite, schorl rock, tourmaline schist, with 
sometimes a little tin ore, &c. This layer is called by the 
workmen “overburden,” and it must be removed before the 
clay can be got at.* The process of working is usually as fol- 
lows : — Let us suppose that a patch or band of suitable decom- 
posed granite, called by the workmen a “ bed of clay,” has been 
discovered in a hill-side. The first thing to be done is to drive 
an “ adit-level ” horizontally right into the hill beneath the bed 
of clay, the position and extent of which has been more or 
less accurately determined by systematic “ pitting ” through the 
overburden. This adit-level is a sort of tunnel — from 6 to 9 
feet high, and from 3 to 6 feet wide. While this level is being 
driven, a large piece of the overburden is removed so as to expose 
a considerable area of the bed of clay.f A vertical opening or 
shaft is then made from the inner end of the adit, to the surface 
of the uncovered clay bed — partly by digging downwards from 
above (“sinking”), partly by digging upwards from below, 
(“putting up a rise”). A square wooden pipe, having holes at 
regular distances of a few feet in one of its sides, is then placed 
in the vertical opening, so as to keep open a communication 
with the level below ; J the remainder of the shaft is then either 
filled in or kept open for the removal of the coarse sand or 
stones produced in working; and the regular washing of clay 
may be proceeded with. Of course the arrangements for obtain- 
ing the clay vary very much in different works. These different 
cracks of the granite — now occupied by the solid matter of the veins re- 
ferred to above — and not as commonly stated in geological works by carbonic 
acid acting from above. 
* This covering closely resembles some glacial deposits ; but neither organic 
remains, nor scratched stones, nor stones of foreign origin have been found 
in it, to my knowledge, although many acres have been removed in the various 
clay works. 
t The term “ clay ” is applied indiscriminately in Cornwall to the decom- 
posed granite rock, and to the true clay washed out of it. 
% The holes are — except the top one — temporarily covered with pieces of 
board nailed over them. The whole contrivance is called a 11 button-hole 
launder.” 
