A Handbook of Gold Milling

Preface

The process of differentiation, so essentially characteristic of all modern development, has of recent years extended very markedly into the province of the gold miner. The time is not so long past since any ordinary miner was considered to be quite capable of taking a shift in the mill, and it was looked upon as part of his duty to be equally useful above and below ground. This state of things did well enough as long as rich reefs or rich portions of reefs, such as the superficial portions not uncommonly are, were alone worked ; enough gold was won to more than pay expenses, and no one knew or even seemed to care how great the loss might be in the mill. With the rapid exhaustion of high grade reefs and the introduction of more scientific methods of gold extraction, which (together with improved methods and machinery in mining proper) have rendered the exploitation of low grade ores profitable and therefore possible, a gradual change

PREFACE.

has made itself felt within the last few years. At the same time a class of men has gradually come into existence, and has been educated — by the hard lessons of practical experience for the most part — to carry out the duties of amalgamator, reduction officer, or mill man, these being the various names by which this post is designated in various parts of the world — the very divergence in nomenclature serving to show how very recently the need for and existence of such a trade have come to be recognised. Under these circumstances it is hardly to be wondered at that the technical education of the mill-man still leaves much to be desired, and the main object of the present volume is to impart so much of such technical instruction in his duties as can be included within the covers of a book.

Incidentally it is hoped that it may also prove of service to managers and managing directors of mines, not only by showing them the most general causes of loss both of money and material in gold-milling, and by indicating the readiest means of preventing such loss, but also by pointing out to them what are the essential portions of a mill-man's duty and to guide them in selecting a duly qualified man for this onerous post, upon the proper filling of which the prosperity of a mine is so largely dependent. For whilst it is quite true that no one can make a poor mine into a good one, the converse of the proposition is unfortunately by no means impossible ; and it is an undoubted fact that there is in existence to day a large number of mines which would give far

PREFACE.

IX better results than they actually do', were the charge of the mill handed over to a properly qualified mill-man. It is not too much to say that not one manager in ten knows how much gold he is losing in his mill — what he is getting is really a secondary question — and this, the fault in the first instance of an imperfectly trained mill man, renders it impossible to detect the shortcomings of the latter, whose very incapacity thus serves as its own shield. It is only quite recently that the science of gold-milling has come to be at all fully recognised. In most cases this work has been carried on by the light of untrained instinct, the ignis fatuus that has so frequently led the way to destruction. It is the object of the writer to apply the results of modern science so as to replace as far as possible the uncertain methods of crude empiricism, and especially to substitute scientific for unscientific modes of reasoning.

Facts can usually be trusted to take care of themselves, but the same cannot be said of the methods by which these facts are arrived at. Scientific reasoning is a process far less costly and quite as reliable as unscientific groping in the dark, and the first step towards the foundation of such science is the classification of our knowledge on a systematic basis. This is the main object that has been attempted in the following work, which commences by an account of the physical and chemical properties of gold and also of mercury, the knowledge of the latter being of scarcely less importance than that of the former

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X for the scientific amalgamator ; stamp mill construction is considered in detail, the mechanical principles under lying the design of each part being throughout eluci dated. The theory and practice of concentration, as far as it refers to gold-milling, is next considered, together with the most approved modern method of treating the concentrates and the other products of milling. A chapter on the economic considerations involved, and one on the assaying of gold ores and mill products are also appended. The art of gold-milling rests upon a twofold scientific basis, the comminution of the ore being a purely me chanical and the extraction of the gold a purely chemical process. The mill-man should accordingly have a know ledge of the theories of both Mechanics (including Physics) and Chemistry.

Practically he should have served his time as an engineer, or at least have gone through the shops sufficiently to be a good fitter and a fair carpenter ; he should also have had some experience in looking after machinery in motion. In addition to the above, he ought to have worked for a year or so in a chemist's or assayer's laboratory, where he will have acquired habits of accuracy in chemical manipulation, and where he ought to have learnt some assaying. He is then duly prepared to undertake the duties of a mill-man, and the writer hopes that a study of this volume will teach him exactly what are the problems which he will be called upon to solve, and what are at present the most approved methods of solving them.

PREFACE.

XI Much of the information here given may be found scattered through various British, American, and foreign works on Metallurgy, and more especially in the Trans actions of Societies devoted to the cultivation of this and cognate sciences. So very many of the facts con tained in this volume have become the common property, so to speak, of metallurgists, that, having re gard, moreover, to the essentially practical purposes of the present work, it has been considered advisable not to encumber its pages with references to all the sources whence information has been derived ; the author accordingly hopes that this general acknow ledgment of obligation to previous writers on this subject will be deemed sufficient. Henry Louis. London, November, 1893. CONTENTS (HAP. TAOn I. MODE OF OCCUr.r.F.NCE — DEPOSITS — PARAGENESIS — VARIOUS FORMS OF GOLD 1 II.

PHYSICAL AND CHEMICAL PROPERTIES OF GOLD .... 21 III. PHYSICAL AND CHEMICAL PROPERTIES OF MERCURY . . 44 IV. ALLOYS AND AMALGAMS OF GOLD 62 V. PRIMITIVE METHODS — OUTLINE OF MODERN METHOD — WEIGHING ORE — BREAKING ORE — ROCK BREAKERS — GRIZZLIES 8fi VI. GENERAL ARRANGEMENTS — THE MORTAR BOX AND ITS ACCESSORIES — SCREENS — DIES 10S VII. THE STAMP — TAPPET — STEM — HEAD — SHOE — CAM SHAFT — CAMS— CAM CURVE — POWER REQUIRED 14S VIII. FRAMES — GUIDES — HOISTING GEAR — WATER SUPPLY"" — BINS — GENERAL ARRANGEMENT — ORE-FEEDERS .... 203 IX. OTHER CRUSHING MACHINERY— ARRASTRA — HUNTINGTON MILL 242 X. AMALGAMATION — INSIDE PLATES— COPPER TABLES — MER CURY WELLS — AMALGAM TRAPS — LOSS OF MERCURY . 266 XI. PRINCIPLES OF CONCENTRATION — RIFFLES — BLANKETS — PUDDLES — VANNEPS — SIZING — SPPTZKASTEN .....

CONTENTS XIV CHAP. PAGE XII. TREATMENT OF CONCENTRATES — AMALGAMATION — CHLOR- INATION — SMELTING — CYANIDATION 333 XIII. CLEANING-UP — TREATMENT OF AMALGAM — CLEANING, RETORTING, AND MELTING 368 XIV. MODES OF TREATMENT — COST OF MILLING — GENERAL CON- SIDERATIONS — LABOUR — MILL SITE AND MILL BUILDING — POWER — ILLUMINATION — RETURNS 400 XV. SAMPLING AND ASSAYING OF ORE, TAILINGS, CONCEN- TRATES, AND BULLION 436 APPENDIX, ON THE CAM CURVE 479 TABLE I. THEORETICAL HORSE-POWER DEVELOPED BY FALL OF WATER 492 TABLE II. YIELD OF BULLION PER TON CORRESPONDING TO WEIGHTS OF PRILLS 493 TABLE III. VALUE OF GOLD ACCORDING TO ITS FINENESS . . 494 INDEX A HANDBOOK OF GOLD MILLING GOLD MILLING

Chapter I Mode Of Occurrence Deposits Paragenesis

— VARIOUS FORMS OF GOLD Gold is a metal that has been known to, and prized by, mankind from pre-historic times. Occurring as it does in the native state, its great specific gravity and well-marked colour and lustre must have attracted the attention of savages, even at a very early stage of the development of their powers of observation. At a period when the extraction of other metals from their ores would have proved a task impossible to their rudimentary resources, the great softness, malleability, and ready fusibility of gold (the latter probably discovered after brief experience) would have combined to render it of high value to them for many purposes. And yet, in spite of its having been known and worked for so long a period, the scientific metallurgy of gold is in reality only half a century old.

Up till then there seems to have been little or no improvement in the methods employed for its extraction since me diæval times, gold working and especially gold milling having IS ( Tables only. ) ■■ GOLD MILLING C.1AP. remained rude and neglected industries until the great dis coveries of gold in California in 1849, and almost immediately afterwards those in Australia, turned the energies of thousands of able, active, intelligent men into this new channel. In the mad race for wealth which then ensued, the primitive methods, all sufficient up to that time, were soon found to be too crude and too slow to satisfy the ambitions of modern gold diggers, whose inventive faculties, stimulated to the highest pitch, were prolific in new methods and improvements upon the old ones. When the rich shallow placers. and the rich superficial pockets of gold quartz had been exhausted, and it became necessary to work the poorer alluvials and low-grade reefs, the utmost resources of modern science became even more indispens able, and the present complex system of gold extraction is the ultimate result.

Of course this industry is even yet in a comparatively rudimentary and backward state ; the stamp-mill is at best a somewhat cumbersome machine and the science of the subject is even now but imperfectly understood. Nevertheless the art is by no means stationary, and steady progress is being made in various directions, with the result that sources of gold are now being profitably worked which are actually poorer than the tailings, which were a few years ago allowed to run to waste as worthless. Occurrence of Gold. — The manner in which gold occurs in nature may be divided for the purpose of the present work into two classes, this division being neither geological nor mineralogical, but simply technical, dependent upon the methods employed for extracting the precious metal from the matrix that carries it : Class A. — Gold occurring in a matrix capable of complete disintegration by the action of a stream of water. aMODE OF OCCURRENCE 3 r Class B. — Gold occurring in a firm matrix which re quires the employment of mechanical appliances for its pulverisation.

Class A. — In this is included the occurrence of gold in alluvial deposits of various ages, in river sands, on sea shores, and in gravels of various characters ; the gold, which occurs in extremely small proportions as a general rule, is readily extracted by the comparatively primitive method of washing. When the mass, consisting of gold in admixture with the lighter substances with which it is associated, is agitated in a stream of water, the relatively heavy particles of gold sink to the bottom, whilst the relatively light sand, pebbles, and clay are carried off by the current. It is by this method, or some variety thereof, that gold has always been obtained by primitive races of mankind. The washing was originally, no doubt, performed in a dish of some kind, and subsequently in a trough of greater or less length.

In time the former developed into the cradle, which consists of a short rectangular box to which a sieve is fitted, and which is supported upon rockers, thus receiving a lateral undulating motion, whilst the auriferous gravel, together with water, is thrown into it. The latter method developed into the ground sluice, which is simply a channel cut into the ground itself, and is often of very great length ; and finally American ingenuity produced the various methods of " hydraulicing," which consists in projecting a powerful stream of water, brought in pipes from a considerable elevation, against the face of a gravel deposit, and then washing clown the latter at a very rapid rate, and at a minimum of cost. Mercury is frequently used in most of the modern methods to assist in the separation of gold from the other materials with which it is associated.

All the above methods are usually b GOLD MILLING chap. 4 included under the general head of " Hydraulic Gold Mining." Their consideration is entirely foreign to the purposes of the present work, which is concerned exclusively with the methods employed for the treatment of such auriferous deposits as are included in Class B. Class __.. — This class comprises the following modes of occurrence, which differ, as will be seen, more in the manner of their formation than in their intrinsic composi tion ; in other words, this class is subdivided on geological rather than on mineralogical principles. (1.) Reefs (known also as lodes, leads, or veins). — These may be of any of the well-known types of mineral veins, fissure veins being perhaps the most abundant (Australia, Nova Scotia), though contact veins (Montana) and inter calated veins (Nova Scotia) also frequently occur ; a variety of this form of occurrence is that in which a stratified deposit, highly metamorphosed, is traversed by a reticulated mass of veinlets which carry gold.

Asa notable example of this form, the well-known " Great Mother Lode " of California may be quoted. The country rock is mostly slate or schist ; sometimes the auriferous veinlets are carefully picked out and treated separately, but more often the entire mass of rock and reef is sent to the mill, the method adopted varying with the richness, size, and physical con ditions of the deposit. The principal portion of the gangue of which auriferous veins are composed, is always quartz. (2.) Metamorphosed Ancient Stratified Deposits. — These are at times only an extreme variety of the occurrence last mentioned, where schists or shales, either in the neighbour hood of quartz reefs, or of which such veins form a larger or smaller portion, are impregnated with gold. Again, deposits which have by extreme metamorphism passed into quartzites, sometimes carry gold ; a notable example of this MODE OF OCCURRENCE I 5 is the so-called " Sheba reef," near Barberton, South Africa.

(3.) Gravel Deposits of Various Ages, such as are included under Class A, are sometimes found consolidated by metamorphic action, or cemented by various binding mate rials into an auriferous conglomerate, occasionally very hard. In these the gold may be either contemporaneous with the pebbles forming the deposit, and is then like them rounded and water worn, as is the case in the " Cement Mines " of California, or else it has been introduced subsequently, when it presents a crystalline structure, as in the well-known " Banket " of Witwatersrand, South Africa. Whatever their origin, all these divisions of Class B consist essentially of a siliceous matrix carrying gold, mostly in admixture with small proportions of other minerals. Quartz is the universal vein-stuff of gold, and as all the other modes of occurrence of gold appear to be derived from the disintegration of such veins in the first instance, clay being mostly present in some form, the chemical composition of all the above divisions is practically identical, being mostly silica, together with more or less silicate of alumina, oxide of iron, &c. Among gold miners the practice mostly obtains of speaking of an auriferous matrix, whatever be its real nature, as " quartz."

The term is of course inexact, but at the same time it is quite true that the principal portion of such matrix does really consist of silica ; it is also worth noting that in general the gold miner indiscriminately calls all these various occurrences " reefs," whether they be true veins or not. Minerals Associated with Gold, — There are a large number of minerals that accompany gold in the deposits of Class B, some of which are frequently, and some but rarely, associated with gold. It is important that these minerals should be thoroughly known, and their properties carefully ciiAr. i 6 GOLD MILLING studied, as their presence or absence may exert a very marked influence upon the milling of the ore. These associated minerals may conveniently be divided into four groups : A. Non-metallic minerals.

B. Metallic minerals, rarely or never auriferous. C. Metallic minerals, frequently auriferous. I). Minerals containing gold as an essential element of their composition. The minerals of Classes B and C occur with gold, and frequently show visible metallic gold enclosed in their substance ; those of Class B rarely contain gold in a state in which it is held invisibly, whereas Class C. often holds gold in this condition. The following is a list, as complete as possible, of all minerals that have been known to occur in gold-bearing veins. The rare minerals, which are only known in a few special localities, and need not be looked for as a general rule, are in small type, the common ones being distinguished by full-faced type. Many of these minerals are, it will be noted, decomposition-products of others that are more essentially the true original con stituents of auriferous reefs, and their occurrence is accordingly limited to such portions of the deposits as lie above water level and have been subjected to atmospheric influences.

Thus in Class A, sulphur is a decomposition product of iron pyrites, and selenite may result from the oxidation of the same mineral, the sulphuric acid so formed in its turn decomposing calcite. A very large number of the minerals comprised in Class B are the results of decomposition or oxidation of some of those in Class C. Whenever these secondary minerals are found in the upper portion of a deposit, the minerals from which they are derived may be expected to occur in depth. A.— NON-METALLIC MINERALS. Name. Chemical Composition. Crystallisation. Specific Hardness. Gravity. Localities. Quartz S102 Rhombohedral Clays Hydrated Silicate of Alumina ' Chlorite 4H20, 5(MgFe)0, Al.,0-,, (Si0.,)j Monoclinic Mica (Muscovite) KoO, 2H20, 3Al«03(Si0.>)6 .Monoclinic Talc (Steatite) H20, 3MgO, (3i02l4 iRhombic or Monoclinic.

Serpentine 3MgO, 2H20, (Si02)2 Massive 7. 2 to 2 5. 2 to 2*5. 1 to 1*5. 2 6 to 2 65. 2 6 to 2 63. 2 6 to 2*85. ■2*76 to 3. •2*7 to 2*8. (Theuniversal consti \ tuent of gold reefs (Occasionally formi the casing or wall! of auriferous quart: veins. 2*5 to 4. 2-8 to 2-65. Felspar (Orthoclase) K.O, Al.,0 i(SiO,)6 .*. Monoclinic Garnet 3FeO, Al203(Si02):! Isometric Grossularia 3CaO, AloO^SiO.'}- Isometric Calcite CaC03 'Rhombohedral Dolomite (CaMg)C03 Rhombohedral Aragonite CaC03 'Rhombic Magnesite !MgC03 Rhombohedral Apatite 3(Ca3P.,08)+CaF„ Hexagonal saline { »S££!$5?£r*+ "^"J j R-ombohedral Hornblende Silicate of magnesia, lime & iron. Monoclinic Fluorite 'CaF2 'isometric Barytes 'BaS04 Rhombic Prehnite 'h-jO, 2CaO, A1203, (Si02)3 Rhombic Pyrophyllite H20, Ai203, (Si02)4 Monoclinic (?) Sulphur S Rhombic Selenite CaSOj, (H20)2 Monoclinic Lignite Carbon, hydrocarbons, &c, &c....

Massive 6. 0*5 to 7*5. 6*5 to 7*5. 3. 3-5 to 4. 3 5 to 4. 3-5 to 4. 5. 7 to 7'5. 26. 3*9 to 4*2. 3*5 to 3.05. 2*71 to 2 72. 2*8 to 2*9. 2*93 to 2*99. 30 to 3*2. 3*17 to 3*23. 2*98 to 3*2. Colorado. Siam. /California, 1 1 Transylvania. iNew Zealand. Victoria. Ottawa. 5 to 6. 4. 2-5 to 3'5. 2*9 to 3*4. 3*01 to 3*25. 4-8 to 4*6. (Craddock, I Cape Colony. Beresovsk. 8 to 6-5. j 2*8 to 2*95. 1 to 2. 1-5 to 2-5. 1-6 to 2. 0-5 to 1-5. i2*8 to 2*9. 205 to 2.09. 2*31 to 2*33. 1*0 to 1*5. Transylvania. B.— NON-AURIFEROUS METALLIC MINERALS. Chemical Composition. Crystallisation. Hardness. Specific Gravity. Localities. ■ Name. Ilmenite Magnetite Haematite Limonite ' Siderite j Vivianite Manganese blende Rliodocroisite Bustamite Pyrolusite Chrysocolla Malachite Azurite Wolfram Scheelite Pyromorphite Mimetite Crocoisite Cervantite Valentinite Kermesite Bismuthite Native Lead Native Bismuth Native Arsenic Native Silver Aikinite Melanochroite i Vauquelinite Vanadinite Certissite Anglesite Scorodite j Jarosite FeTiO-j Rhombohedral 5 to 6. 4 5 to 5.

Fe-j04 Isometric 55 to 6 5. 5 17 to 518. |rr, , J c * [Thesearedecompos- Fc*oO;{ Rhombohedral 5'5 to 05. 4'2 to 53. lition products of, 2Fe.iOf, 3H.0 Massive, earthy, &c. ...5 to 5*5. 3 6 to 4. .... (and at times pseudo- !FeC03 , Rhombohedral 35 to 4. 3 83 to 3"SS. ( morphous after, Py- FeaP.jOg, 8H20 Monoclinic 1'5 to 2. 258 to 268. Victoria.; rites. MnS Isometric 3'5 to 4. 395 to 404. Transylvania. MnC<>3 Rhombohedral 3*5 to 4*5. 3 '45 to 3t>5. Transylvania. (MnCajSiOg Triclinic 5'5 to 6-5. 34 to 36. Mexico. MnOo Rhombic 2 to 25. 473to4"86. CuSi03, 2HoO Cryptocrvstalline 2 to 4. 2 to 224. Namaqualandj /These aredecompos- CuC03, CuH20-, Monoclinic 35 to 4. 39 to 403. Peru, &c. ] \,\ P1™1"0™ Oi 2CuC03, CuHoO* Monoclinic 3 "5 to 4. 377 to 8-83. Peru, &c. sulphuretted copper 0 * * ' / \ores.

(MnFe)W04 Monoclinic 5 to 5*5. 7"2 to 7*5. Victoria. CaW04 Tetragonal 4"5 to 5. 59 to 61. { I*^'' 3(PbsP.,08) PbCb> * Hexagonal 3*5 to 4. Go to 71. Wales.' 3(Pb3As.,08)PbCL> Hexagonal 35. 7 to 7*25. : Nevada. PbOrO*. Monoclinic 25 to 3. 5 '9 to 6*1. I Peru Sl>204 Rhombic 4 to 0. 40S. i Victoria.! (These are decompos- SbaOg Rhombic 2*5 to 3. 5*56. 'Victoria.! 'ition products of 2Kl"i.,s:l, Sbo03 Monoclinic 1 to 1*5. 45 to 46. .Victoria. J Istibnite. Bio03CO.»,HoO Amorphous, earthy 4 to 4'5. 686 to 0'9. 'Transvaal. Pb Isometric 1"5. 1137. Victoria. jBi Rhomboliedral 2 to 25. 97 to 9'83. As Rhombohedral 35. 5"63 to 5 73. Ag [sometric 2 5 to 3. 105. Mexico. PbCuBiS3 Rhombic 2 to 2'5. 6*1 to 6"8. Beresovsk. 3PbO, 2Cr03 Rhombic (?) 3 to 8'5. 575. „ 2(PbCu)C..O4(PbCu)3P.,08 Monoclinic 25 to 8. 5"S to G'l. 3Pb3V.,Oa^ PbClo Hexagonal 275 to 3. 660 to 723.

PbC03 Rhombic 3to3'5. '6"46 to 6*57. PbS04 Rhombic 275 to 3. 612 to 0"39. Fe.i03, As*,05, 4H..0 Rhombic 3 "5 to 4. 31 to 3*3. ,, :K20. 3Fe..b3(S03)4, 6HoO Rhombohedral 2"5 to 35. '3-15to3'20. „ C— AURIFEROUS METALLIC MINERALS. D.— MINERALS CONTAINING GOLD AS AN ESSENTIAL CONSTITUENT. Name. Chemical Composition. Crystallisation. Specific Hardness. Gravity. I Localities. Iron Pyrites Mispickel Lollingite Copper Pyrites Erubesoite Covt'lline Reilruthite Stibnite Galena Zinc Blende Cinnabar Bournonite Tetrahedrite Pyrargyrite Molybdenite Tetradymite Argentite Clausthalite Hessite Melonite Stroineyerite Native Tellurium FeS., FeS.., FeAs* I FeAso j CuFi-S., Cn3FeS*3 CuS Cu2S Sb2S3 PbS ZnS HgS PbCuSbS3 Cu3Sb2S7 Aft*SbS3 MoS2 Bi2(TeS)3 Ag.,S PbSe Ag2Te NUT**., Ag2SCu2S Te, with a little gold and iron ....

Isometric 6 to 6*6. 4*95 to 51. Rhombic 5*5 to 6. ;5*9 to 6*3. 'Rhombic 5 to 5*5. 0*8 to 7 4. Tetragonal 3*5 to 4. 4*1 to 4 3. Isometric 3. i-t-9 to 5*4. Rhombohedral 1*5 to 2. ,4 69 to 4*64. Rhombic" 2*.ri to 3. 15*5 to 5 8 Rhombic |2. 4 52 to 4*62. Isometric 2*5 to 3. 7 4 to 7*6 Isometric 3*5 to 4. 3*9 to 4 1. Rhombohedral 2 to 2*5. 8*0 to 8*2. Rhombic 2*5 to 3. 6*7 to 5*9. Isometric 3 to 4*5. 4*4 to 5 1. Rhombohedral 2*5. 5*77 to 5*S0. Hexagonal 'Ito 1*5. 4*7 to 4*8. Rhombohedral 11-5 to 2. 7*2 to 7*6. Isometric. l2to 2*6. 7*2 to 7*30. Isometric 2*5 to 3. 7*6 to 8 8. Isometric 2*5 to 3. S 31 to 8*89. Hexagonal Ilhoinbic 2-.1 to 3. 0*15 to 0*3. Rhombohedral 2 to 2*5. 6*1 to 0*3. Idaho. Russia. California, &c. Hungary. Siberia. Harz. Transylvania. California. Siberia. Transylvania. Specific Name.

Chemical Composition. Crystallisation. Hardness. Gravity. Localities. Petzite (AgAu)2Te Massive 2 '5 to 3. [8 7 to 9"02. Transylvania, Colorado, California. Kreiinerite Telluride of gold and silver Rhombic 25. jS'35. Transylvania. Calaverite ,, ,, Massive 2*5. ,9*04. California. Sylvanite (AuAg)Te.i Monoclinic 1*5 to 2. 7*0 to S"3 Transylvania, California, Colorado. Nagyagite Au2Pb14Sb3Te7Sj7 iRhombic il to 1*5. 6*85 to 7 2 Transylvania. Name. Localities. Of the physical characters of the above minerals, those of chief importance for the mill-man to bear in mind are their hardness and specific gravity, as these influence, the former the crushing, and the latter the concentrating of the ore that carries them. The hardness of minerals is their power of resisting abrasion ; thus a harder will always scratch a softer mineral, and if two pieces of mineral are rubbed together, the softer will lose more weight than the harder one.

Hard ness is indicated in the above list by Mohs's scale of hardness, well known to all mineralogists. It need only be said in this place that the numbers are quite empirical and have no quantitative value. They merely indicate the relative order, but not the proportion of hardness. A hardness of 3 on the scale is about that of a plate of copper, 6 is about that of ordinary glass, 7 is that of the hardest steel ; iron varies between 5 and 7. The hardness of quartz, the principal constituent of all gold-bearing reefs, being thus greater than that of the material of which the crushing machinery for the same is constructed, it follows that the wear of the latter must always be very considerable, and that this is unavoidable from the very constitution of the substances themselves. Disregarding the minerals which but rarely accompany gold, and turning our attention solely to those that are usually associated with it, it will be noticed that the average specific gravity of Class A is under 2-7 ; of Class B it is about 4-5 to 5, and of Class C it is over 5 ; an admixture of the telluride minerals that contain gold, comprised in Class D, would raise this figure still higher.

The im portance of this matter will be better appreciated further on ; for the present, it will be sufficient to note that as a geneial rule minerals with which gold is intimately I MODE OF OCCURRENCE 11 associated (Classes C and D), have a specific gravity of about 5. These minerals are usually the object of a special series of operations for their collection with a view to the extraction of the gold they contain. When thus collected they form the concentrates or sulphurets of the gold miner ; either term may be and is used indifferently, but the former is certainly the more correct. These concentrates may obviously contain any of the minerals of Classes C and D, and they will also at times contain some of the heavier ones of Class B, notably magnetite; these latter minerals must however be looked upon as diluents or impurities of the concentrates proper.

It is interesting to note that marcasite does not seem to occur in auriferous veins, although pyrites is such a very common constituent. Mode of Occurrence of Reef Gold. — Native gold some times occurs crystallised ; it crystallises in the isometric system, the commonest forms being the octahedron, cube, and dodecahedron. The crystals are rarely distinct, mostly aggregated, and frequently distorted and elongated in the direction of one of the axes. It more often occurs in irregular grains, plates, scales, filiform, dendritic, reticu lated, or spongy. Sometimes it is pseudomorphous after some other mineral, mostly iron pyrites. It has no cleavage and a hackly fracture. In nature gold never occurs in an absolutely pure state ; it is either alloyed with various metals or else, more rarely, is combined with a few metalloids in the scarce minerals of Class D. The principal metals with which it is alloyed are silver, copper, and iron, the former being universally present.

Native gold thus consists of an alloy of gold with silver or copper, or both ; the name of gold being applied to such alloys as long as the gold is present in such quantity that its value considerably exceeds that of the other constituents. No hard and fast line of division between e.g. argentiferous gold and auriferous silver has yet been drawn, the distinction being essentially a technical one, depending upon the methods by which the ore has to be treated (whether by goldor by silver-extraction processes) and upon the character and after treatment of the bullions produced. I would suggest that a useful practical line of demarcation could be drawn at the alloy containing forty-eight parts of gold to fifty-two of silver, which is of just about the same specific gravity as mercury. Argentiferous gold with more gold than the above would therefore sink in mercury, whilst auriferous silver, with less gold than the above, would float on it.

The alloy containing four parts of gold to one of silver is called electrum. Silver with 10 to 30 per cent, of gold is called kiistelite. The specific gravity, hardness, colour, and other physical characters of the alloy depend, of course, upon its composition. Some ambiguity may occasionally result from this, strictly speaking, incorrect use of the word gold, to designate both the pure and the native impure metal ; but it has been sanctioned by custom, and the context will mostly serve to show whether pure gold or native gold is meant ; wherever there might be any doubt I shall use the expression pure gold to distinguish it from the native alloy. The table on the next page gives the compositions and specific gravities of a number of different specimens of native gold from various sources ; these data have been collected from various publications and are the work of trustworthy observers.