VIII.

GEOLOGY AND MINE ENGINEERING.

THERE is no profession that comes so intimately in contact with geological phenomena, or stands so much in need of a knowledge of geological truths, as that of the miner and mining engineer. It is true that mining was practised, and in many instances successfully, long before geology had shaped itself into a science; but even the most successful practice was local and limited, and wanted that grasp of general truths which could enable it to pronounce on other districts, and deal successfully with their altered phenomena. The discrimination of the same formation in distant localities, the varying nature of sedimentary deposits, the relations of the eruptive to the stratified rocks, and the laws regulating the direction, character, and effects of faults, dykes, and veins-these, and many kindred problems, can be solved only by a pretty extensive acquaintance with the facts and principles of geology. It is true that much in successful mining depends on mechanical appliances-sinking, lifting, hauling, pumping, and ventilation; but the most skilful appliances will not compensate for ignorance of the nature, position, variations, and interruptions of the substances the miner may be in quest of. It is in this way that a knowledge of geology becomes of use to the miner and mining engineer, whether working among stratified deposits like those of the coal-formation, or in metalliferous veins like those which traverse the older formations. The mining of a bed of coal and the mining of a vein of lead-ore are two very different and dissimilar operations, and the geological information applicable to the one may have slender bearing on the other. For this reason it will be necessary to treat Geology and Mine Engineering under several sub-heads, and the following are perhaps the most natural and intelligible—viz., Quarrying or Open-Working, Mining in Stratified Deposits, Mining in Veins, and Stream or Placer Working.

I. QUARRYING OR OPEN-WORKING.

Quarrying or open-working is carried on both in stratified. and unstratified rocks; but the methods followed in the one case differ considerably from those adopted in the other. The stratified rocks usually raised in Britain are sandstones, flagstones, slates, limestones, and marbles; the unstratified, basalts, greenstones, porphyries, and granites. We pass over sand-pits, clay-pits, and other superficial diggings, which, though often of considerable extent and local value, can scarcely be regarded as quarrying in the ordinary sense of the term. The stratified rocks are sought along their outcrops, or where they come to the surface; the unstratified are taken from hillsides, cliffs, and not unfrequently from intersecting dykes. But whether stratified or unstratified, there are some general considerations which should receive attention, both from proprietor and lessee, before the ground is broken. What, for example, is the nature of the rock? judging from its position and structure, can it be raised at moderate cost? is it durable and suitable for the purposes in view? is the demand likely to be permanent or merely temporary? is there sufficient supply to warrant preliminary outlay on opening, on suitable machinery, and on roads? are there facilities of transport by road, rail, canal, or harbour? These and similar questions should receive the attention of the quarryman if he means his undertaking to be more than a mere temporary, local, and limited burrowing.

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Stratified Quarries.

In quarrying stratified rocks, the usual plan is to follow the dip till the superincumbent matter-"callow," "cover," "overburden," or tirr" of the quarryman—becomes too thick and expensive to be removed, and then to follow the strike or outcrop; and thus long lines of old workings are to be witnessed on many of our hillsides. As removal of the cover or overburden is often an expensive part of the process, the quarryman should always endeavour, if possible, to utilise the material. When it consists of clay, it may be converted into bricks, as is sometimes done in the neighbourhood of Newcastle; when of shingly or rubbly debris, it may be used as road-metal; and in all cases it may be employed in filling up the old workings, and restoring again the agricultural surface. In some cases where the bed is valuable, and the "tirr" too thick to be profitably removed, mining is adopted, and then

the usual plan is followed of removing a portion, and leaving other portions as pillars to support the roof. Such galleryworkings are most frequent in limestones, being unsuitable for freestones and flagstones, which have to be raised in large blocks for the builder. Burdiehouse and Starleyburn near Edinburgh, and Box near Bristol, may be pointed to as successful instances of stone-mining, the consideration of which belongs to the following rather than to the present section.

In sandstone quarries where there are beds of different qualities, and all saleable, it is usual to follow what is termed "bench or stage working "—that is, to keep the quarry open on one or more stages, so as always to have in readiness a

b

Bench or Stage Working.

supply of the different sorts. In the preceding sketch, b b' b" represent three working stages,-the first, of flagstones; the second, of medium sandstones for ashlar; and the third, of strong thick-bedded sandstones, suited for basements, pedestals, columns, and other large purposes. Good free stages of this sort are always most cheaply worked, and attended with less waste of material, there being nothing so laborious and wasteful as tearing away in confined nooks and corners. For slates, flagstones, and large blocks of freestone, this mode of quarrying is indispensable; but where stones are sought merely for rubblework, for the kiln, or for road-metal, blasting, or any other expeditious plan, may be adopted.

As many stratified rocks have a jointed structure, and all are less or more traversed by fissures having definite directions -the "backs and cutters" of the quarryman-these natural partings should be carefully studied as at once facilitating the working, and allowing the largest amount of available stones to be raised from the same space. In slate-quarries this is

especially necessary; the unbroken jointed blocks always yielding, on splitting with wedge and mallet, the finest slabs and slates. And yet in many of these quarries, the waste of good material, through injudicious and reckless blasting, is simply disgraceful. Though not a very artistic operation, quarrying requires a certain amount of skill and care,skill in following the natural lie or structure of the rock, so as at once to lessen labour, and to secure the greatest amount of useful stones from the same bed-and care to reject all faulty or doubtful portions. Though it is the duty of the architect and builder to select the materials suitable for special structures, yet valuable assistance may be rendered in the quarry by throwing aside blocks of unequal texture and blocks containing silicious concretions, nodules of iron sulphide, and other objectionable ingredients.

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Another important point is always to keep a good free face in the quarry—that is, to allow no redd" or rubbish to be shot nearer the face of the rock than 20 or 30 feet. Every proprietor of a valuable quarry should insist on this observance, otherwise a careless outgoing tenant may so choke up the face that a new lessee could not enter upon the workings without an outlay of several hundred pounds in removing the rubbish so accumulated. Indeed, we have known instances where it was thought easier and cheaper to open a new quarry, than to remove the waste and rubbish which years of careless or of selfish working had permitted to accumulate.

In stratified quarries of any depth, water is apt to collect, unless where the rock is sufficiently fissured and jointed to afford a natural drainage. Where there is sufficient declivity, a water-level can be driven; where there is a point in the neighbourhood lower than the bottom of the quarry, a siphonpipe may be employed; but where the excavations are deeper than any of the surrounding land, recourse must be had to pumping.

The raising or lowering of large monoliths from their native beds is now a comparatively easy task-steam-cranes performing in a few minutes what formerly took hours to accomplish, and this often with severe strain and toil alike to men and horses. Indeed, vast improvements have recently been made in all the adjuncts of stone-quarrying-in lifting, drilling of blast-holes, the use of new explosives, and in the application of machinery to the sawing, planing, and polishing of the blocks and slabs when raised. A large modern quarry with its steamcranes, steam-drills, sawing, planing, and polishing machinery, and electrically discharged explosives, more and more assumes

the aspect of a factory; and it is only by such appliances that labour can be saved and the materials for high-class architecture reduced to a reasonable price. These, however, belong to the mechanical, and not to the geological, aspects of the subject.

Unstratified Quarries.

Quarrying for unstratified rocks, such as basalts, greenstones, granites, and porphyries, generally takes place in hillsides and the faces of cliffs, as well illustrated by our Vignette;* but occasionally in flat situations when the rocks occur as dykes intersecting the stratified formations. Though all hard and crystalline, their raising is greatly facilitated by their structure -the basalts being less or more columnar, the greenstones often sub-columnar, the granites tabular and cuboidal, and the porphyries always much fissured and jointed.

When a good roomy opening is once effected, the columns of basalt are not very difficult of removal, whether perpendicular in cliffs or horizontal in dykes. Being mainly raised for road purposes, the great secret is to get fairly behind the columns, when lever and mash-hammer will be all that is requisite to effect their removal and reduction. The columns are always separable, and though pentagonally or hexagonally interlocked in the mass, are easily removed when once a good free face has been given to the quarry.

The greenstones, also mainly used for road purposes, being only irregularly columnar and often massive or amorphous, require more labour, and for the most part are broken up by blasting. When raised for kerbstones, for which some varieties are admirably adapted, they are cut by wedge and hammer to prevent waste of material. In general, however, there is little waste in basalt and greenstone quarries, every chip and fragment being of use, if not for causeway coursers and kerbs, at least for macadamising.

The same remarks are applicable to the porphyries and felstones, which are usually much cracked and fissured, whether appearing in dykes or in eruptive masses. When raised for macadamising there is no waste, however blasted or broken up ; but when coursers and large blocks are needed, the porphyries and felstones require some skill in hammering, as they are not

*From the Dalbeattie Quarries-the granite of which is easily squared and tooled-a large amount of material has of recent years been extracted, not only for street and tramway pavings, for dock and sea walls, as the Thames embankment, and for lighthouse structures, as the Great and Little Basses in Ceylon (the blocks of which were tooled and fitted before exportation), but also for ornamental architecture and monumental purposes.