Chap. I.
GEOLOGY.
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diminishes. Granite is often found to yield tin ore in abundance, whilst in the same dis-trict the slate abounds with copper, lead, and tin.
In Derbyshire the lead is obtained from the carboniferous limestone and toadstone rocks,and the veins, which are valuable in the first, become poor as they pass into the other. InCornwall copper is most abundant where the granite and kellas unite; and in Alston Moorthe veins of lead are richest in the limestone, a bed of which measuring 70 feet in thicknessbeing much more abundant than those in other districts which extend to a depth of 1000feet, through eight varieties of limestone, eighteen of grit, and twenty-six of shale.
In a country abounding with ores, both the geologist and the miner have a guide to leadthem to the veins : they are characterised by their positions and contents. In Cornwall the earliest are those of tin; they underlie to the north, and arc traversed by others of asecond class, and of smaller value: from these two all the tin ore is obtained ; and thewidth of the lodes varies from that of a mere thread to 30 or 40 feet in thickness; the mostproductive lie east and west. The copper lodes in Cornwall also lie east and west, andtraverse the tin ; there are others termed contra , because they cross at right angles.
The Slides consist of imperfectly formed veins, and run in various directions, though themost productive are usually east and west, and the cross courses north and south.
Lead in England is found in veins running from east to west,“which are traversed bycross courses at right angles to them less productive. Out of 300 observations made inCornwall , the direction taken by most of the veins was between west and south-west or westand north-west.. Mineral veins, as has been observed, are quite independent of the rock inwhich they are found, and differ also from them in age; being filled up long after thefissure was caused, or the disturbing force of the several strata applied. The three chiefmining districts of England are found in mountainous regions: the granite rocks inCornwall rise to an elevation of 1300 feet above the level of the sea, in which are the tin,and a considerable proportion of the copper used in commerce.
The Devonshire mica slate and granite dykes offer to the traveller a surface of countrybarren, and apparently unprofitable, but under it is a variety of minerals ; and in Derby,shire the region around the Peak affords abundance of lead. The high moors of the northin Northumberland, Durham, and Cumberland, yield a rich return of the same ore. Inthe Island of Elba iron ore is obtained from veins in sedimentary rocks, which are con-tiguous to igneous rocks. Wherever the metals are found, their deposit seems owing toelectric or magnetic influences; but the hypotheses on this subject are not at present satis-factory. After the mineral vein is found, the engineering operations commence, which mustbe carried on to a considerable extent before any advantages can be derived. A pit or shaftis sunk, and an horizontal gallery or tunnel driven, through which the drainage of the mineis effected, and which is called the adit level; the horizontal drivings , the downward sinkings,and the upward risings , are set out with reference to the locality or direction of the strike oftlie vein. There are usually two sets of galleries, both horizontal, at right angles to eachother, one being in the direction of the strike of the vein, and the other at right angles to it,which serve the double purpose of extracting the ore and carrying off the water, which risesfrom the springs or drains from the several strata. The great adit or drain in Cornwall ,which receives the water from the G wennap and Redruth mines, is nearly 30 miles in length,and discharges its waters into the sea, about 40 feet above high water-mark.
When the works are extensive, many shafts are requisite, though there is usually one ofgreater diameter than the others, which communicates with all the horizontal galleries ;this principal shaft is generally double, or divided into two compartments, one of which isdevoted to the engine, which pumps the waters from the deep workings into the adit level;the other is called the whim shaft, through which the ores are drawn up. The workingsof the several lodes are commonly connected by horizontal shafts, or by cross-cuts, so thattheir contents may be brought to the shaft and drawn up. From the extraordinary depththat many of these shafts are sunk, the greatest care is requisite; that of the Dolcoath Minein Cornwall is upwards of 240 fathoms in depth, 210 of which are below the adit level,this being 30 below’ the surface. The total amount of sinking of these shafts in Cornwall hasbeen estimated at 12 miles of perpendicular height, and the horizontal galleries as extendingaltogether to 40 miles in length. It is not only necessary for the engineer to direct his at-tention to the best method of extricating the ores, or draining off the w’ater, but also tointroduce a supply of fresh air into the workings, to lower the high temperature whichis encountered at the greater depths beneath the earth’s surface This is effected by makinguse of the several shafts and galleries which have communication with them ; the currentsof air, produced by the variations of temperature, set in different directions ; these by me-chanical contrivances are made to keep up a constant ventilation ; where gunpowder is usedto blast the rocks, a considerable quantity of gas is evolved, which is injurious to the at-mosphere of the mine, and therefore requires to be dissipated by ventilation, the gaswhich is evolved at each explosion being equal to at least five thousand times the volumeof the powder made use of.