SCIENCE AND MINING.

Otago Witness, Issue 1912, 13 July 1888, Page 11

SCIENCE AND MINING.

FI3SILITY OF ROCKS.

By J. A. Miller.

So much depends upon the knowledge of the nature .of the rocks in the successful prosecution of mining works that a further dissertation upon the subject may possibly be found of use.

Professor Geikie, in his " Text-book of Geology," recognises four kinds of fiesility, arid says of them :—" Lines due to original stratification must be carefully distinguished from other divisional planes which, though somewhat like them, are of entirely different origin. Four kinds of fissility may be recognised among rocks — first, ' laminations' of-original deposit; second, '• cleavage,' as in slate ; third,' shearing,' as near faults ; fourth, « foliation,' as in schists." The first term—" laminations"— means the arrangement in layers lying one over another, allowing of cleavage in one direction only. The second term—" cleavage"— designates that structure of many stratified rocks, such as slates, which renders them capable of being split indefinitely into thin plates. The third term— " shearing "—refers to those five divisions which run at greater or lesser angles across the cleavage or foliations of rock, cutting it up into angular pieces. Shearing occurs most frequently near lodes and faults, and wherever there has been great lateral pressure. The fourth term— " foliation "—is properly used only in connection with crystalline or metamorphosed rocks, and indicates the planes along which these kinds of rock split most readily.

It will thus bo seen that although tha terms of laminations, cleavage, and foliation are generally u&ed as synonymous, there attaches to them in reality very different meanings. So also aro the schist rocks of the Obago goldfields erroneously called " slate rocks " by many miners, and as it is often desirable to distinguish these rocks correctly one from anotheri it mayb e mentioned that whereas slates split tin even surfaces, the plates being of an equal thickness, schist rooks show rough and uneven surfaces when split;, the plates being irregular in thickness, and mostly mn out in wedge form. This definition, if not strictly scientific, is the best that can be given' for practical purposes. The above -will assist the practical miner' in distinguishing the different layers upon both sides of a fanlfc, and armed with a good microscope he should have no difficulty in recognising the corresponding portions on each side.

JOINTING AND DISLOCATION.

The following extract from the above quoted authority will make further clear what has been left to inference in the foregoing : — " Almost all rocks are traversed by vertical" or highly inclined divisional planes termed ' joints.' These have been regarded as due in some way to contraction (luring consolidation (fissures of retreat), and this is ro doubt their origin in .innumerable cases. But, on the other hand, their frequent regularity and persistence across materials of very varying texture suggest, rather the effects of internal pressure or movement within the crust. ... < " But in many cases the rupture of continuity has been attended with relative displacemeni of the sides, producing what is termed a fault. . . . As the solid crust settles down the subsidence, where unequal in rate, may cause a rupture between the less stable and more stable areas. When a tract of ground has been elevated the rocks underlying it get more room by being pushed up, and are placed in a position of more or less stability. As tbe-.y cannot occupy tbe additional space by any elastic expansion of their mass, they accommodate themselves to the new position by a series of dislocations. Those segments having a broad base rise more than those with narrow bottoms, or the latter sink relatively to the former. Each broad bottom segment is thus bounded by two sides sloping towards the upper part of the block. The 1 plane of dislocation is nearly always inclined from the vertical, and the side to which the inclination rises, and from which it * hades,' is the upthrow side. Faults of this kind are termed 'normal,' and are by far the most common in nature. In moirifcainous regions, however, instances FrHju**ntlv orcur in which one side 'has been pushed over th« other, so that lower are placed above higher beds. Such a fault js said to be ' reversed.' It indicates ah upward thrust witbin tbe crust, and is offcpn found associated with lines of plication. Where a sharp fold, of which onn limb is pushed forward over the other, gives way u'or-£ 3. line of rupture, the result Js a reversed fault."

In many nistnnnps the very lines or indications of demarcation between the layers of rocks are valuable quid* 1 -? to the clc termination of the corre?poncliug portions on the opposite sides of a fault. Be ifc that the cTivifcion occurs as a free partition of the two adjoining kinds of rock, both coming pparfc without any cohesion, or be it that thpy are or-imented together by felspathic or quartzose substances, they are always valuable aids in reading the facts sought after. Aud be it observed that the foliations of metamorphosed rock— that is, such rock as has changed into, crystalline state— have been developed parallel with the divisional planes which guided the crystallisation or arrangement of the minerals, and in some instances these planes coincide with those of the original deposit, while in others they may represent cleavage, but in most cases foliation would be induced along the principal divisional planes

The width of faults differs from a mere chink up to many yards in thickness. A wide cro.°<?course intersecting and displacing a lode will preoerally show the connection between the two displaced portions of lodes by a string of staias ov by a small stream of water running diagonally across the crosscouise, and when fchese indications are present and hrivn V>ppn discovered it is not difficult io follow them, whinb is always advisably as they will iuMiibly lead to tbe recovery of tl"> lost lore. As a rule dip faults have' had a distntbinnr effect upon the strata through whir}) they pass, and there has generally been a displacement attended with a crumpling and deranging of the rock next the fault, so that the "fault rock" is found lying at others "than its true angle with the horizon, and before its true angle can be ascertained a little driving along the rock will be necessary. ' . < .„■ SEMMABY.

(1) That, there is an important distinction, between landnation, cleavage, and foliation, 4 which must fc^stricfjly observed in reading the rooks upon botfrlwfea.of a 'fault. ' (2) That the prevailing" rock of the Ofcago

goldfields is erroneously called slate, whereas it is Bchisfc (mostly mica schist). (3) That the distinction between the, two kinds of rock is most readily made by the planes of the cleavage. (4) That the origin of faults is due to subsidence in innumerable, cases, but still more frequently to internal pressure or movement within the crust of the earth.

(5) That normal faults are the most common in nature.

(6) That ;in mountainous* districts reversed faults are frequent, (7) That the lines of demarcation between the layers oi rocks are valuable guides to the determination of the corresponding portions on the opposite sides of a fault.

(8) That in most cases foliation would be induced along the principal divisional planes. '