673
from the upper parts, producing similar displacements vertically as those which the crossveins occasion horizontally.
Taking the granite and slate with the lodes which traverse them, it appears thatthe largest part of the tin ore obtained in the west of England is from lodes in the gra-nite, and that of copper ore from veins in the slate, though the richest masses of tin ore yetdiscovered have been in slate, whilst the bunches of copper ore found in the granite have ina few instances been as large as any which have occurred in slate.
It is a prevailing and apparently well-founded opinion among practical miners, that thelodes are most productive near the junction of the granite and slate rocks; accordinglythe mines, instead of being irregularly distributed over the face of the country, are clusteredtogether near the lines of these junctions, and the heaps of rubbish separated from theores may be traced in such situations for considerable distances on the lines of the chieflodes, rising in some cases amidst rich fields, and destroying the vegetation like streams oflava from a volcano.
The St. Just mines form one group near the Land’s End, those near St. Ive’s another, atthe opposite ends of the same granite mass ; those of Breage a third, subordinate to thegranite of Godolphin and Tregoning hills. The Crowan and Gwinear mines stand at thewestern extremity of the Cairn Brea and AVendron granite, whilst those of Camborne andRedruth skirt it on the north, and those of Wendron on the south, and the Gwennap dis-trict occupies its eastern flank. In like manner many of the St. Agnes mines are locatednear a small patch of granite at digger Point; those of St. Austell are grouped on theskirts of the Hengsborough granite; whilst the mines near Callington and Tavistock arecontiguous to the Kithill and Dartmoor ranges.
Tin Ore is also found in deposits generally considered diluvial, mixed with the debris ofdifferent rocks, covered with an alluvial bed. Repeated washing by means of runningwater being the chief process to which such tin is subjected, the designation of stream-workis commonly applied to this method of obtaining the ore. In a solitary instance at Carnon,tins stratum of tin stuff is removed by subterraneous excavation, the alluvial bank or over-burthen being too thick to be taken off, and it is subject likewise to be covered by the sea athigh water.
Mines of iron and manganese, giving employment to a considerable number of persons,fall also within the district above described ; among them those near Lostwithiel are themost important. The ore lies in a vein, nearly vertical, and of an average thickness of10 feet. The greater part of this mine is worked open to the surface, and the access to theunderground part is by levels: the greatest depth does not exceed 50 fathoms. Themanganese mines, which are chiefly situated on the borders of the two counties, are like-wise very superficial, the workings being seldom carried more than from 20 to 30 fathomsfrom the surface. Antimony has also been raised to some extent, but the foreign ores ofthis metal have of late years almost monopolised the market, and very few personsare now employed in obtaining it. The mines of tin, copper, and lead, with the latter ofwhich metals silver is generally united, are those which present the characteristic featuresof the mining of the west of England.
"When it is known or thought probable that a lode which will repay the cost of workingexists in a particular locality, the usual course of proceeding is to sink a shaft verticallyto a certain depth, which shall intersect the lode. If this cannot be done, a gallery orlevel is driven or excavated at right angles to the shaft, in the assumed direction of thelode, and continued till it is reached: in cither case, when reached, a level is driven hori-zontally along its course, the miner working upwards and removing the rock from above.It must depend on the thickness of the vein and its inclination, whether it is necessary toexcavate any of the adjoining rock, and to what extent. Meantime the shaft being sunkstill deeper, another gallery or level is carried along the vein or lode, usually about 10fathoms below the former, and the metalliferous stone intervening between the two levels issubsequently removed. This process is repeated again and again, and as the workings becomemore extensive in length, additional shafts become necessary. Horse and water power aremade use of for effecting the earlier operations, but the steam-engine is employed in mostof these mines ; and as they increase in depth and extent, very powerful machinery is neededto raise the excavated rock and the water. Shorter shafts, called winzes, are formed atintervals between the levels, for the purpose of ventilation. In proportion to the dip orinclination of the vein, there must be an advance in a horizontal direction, as thedepth of the workings increases, which renders a communication necessary from thelower levels to the surface, in a more direct manner than can be furnished by the shafts.At a very early stage of this process, a separation is established between the shafts by whichthe men pass to and from their work, and those in which machinery is employed. Thisseparation is effected by a boarded division in a single shaft, or by devoting two distinctshafts to these purposes ; excepting the occasional raising of men and boys in buckets throughshort distances, ladders are the universal means of ascent and descent in these mines.Many of the shorter shafts, or winzes , are provided with ladders, so that the course taken
X x