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i«95] 



NATURE 



21 



ihilc j;ol<l does not occur in important quantity except in its 

 upper or Silurian strata in Victoria. Near Zilmantown (lat. 

 17' 20' S., loni;. 144' 30' E. ) there are interesting <levelopments 

 •I these rocks, which now form steep ranges with flat-bottomed 

 \.illeys, in which coralline limestone of the Devonian period rests 

 unconfonnably, and in places rises abruptly several hundred feel, 

 presenting the form of ancient coral reefs, such as now exist on 

 the great Barrier Reefs. In fact, they indicate that at some re- 

 mote time a passage existed from the ea.st coa-st to the southern 

 part of the (lulf of Carpentaria, under similar conditions to those 

 of the present Torres Straits, and that the subsequent elevation 

 ■of the land has now placed it more than 500 feet above sea- 

 level. This description of the present slate of these rocks is, 

 however, a digression in regard to geological sequences of the 

 «arly [»riod. 



.More Favoi rable CoNnrnoNS. 



The Cambrian and Silurian period was succeeded by the 

 Devonian, during which there is little evidence of any great 

 ■variation in the limits of the sea and land, but organic remains 

 show that the conditions were becoming more favourable for the 

 development of marine life. The rocks consist principally of 

 fine-grained slates, which must have been deposited in a deep 

 sea, and in some places the now visible sections indicate a thick- 

 ness of 10,000 feet. 



The upper strata connected with the Devonian series have 

 been cla.s-sed by geologists as belonging to the Permo-carboni- 

 ferous, on account of the marine fo.ssils which have been found 

 in the Gympie series of rocks. Some difficulties, however, arise 

 in regard to the identification of Australian rocks with those of 

 I''nroj5e on the .sole Ijasis of the occurrence of nearly the same 

 •\ 'Lcies of mollusca, and it may be remarked that in Central North 

 America the appearance of fossil mollusca and plants, which 

 miuld in Europe indicate a definite horizon, often occurs in 

 r ' >cks which lithologically and stratigraphically are of an earlier 

 •Lite ; and the same conditions of the earlier appearance of species 

 uul genera seem to obtain in .-Xustralia, and if ultimately estab- 

 li>he<l would clear away many of the existing ilifficulties in the 



nqKirison of .-Vustralian and .-Vmericaii fossils with tho.se of 

 I iiro[>c. .-Accepting the classification of the Gympie rocks as 

 I'rrmo-carboniferous, there was no important alteration in the 

 ■^rographical limits during the Devonian period, or in the earlier 

 Permo-carlxmiferous (jympie beds, but shortly after this there 

 were very decided variations in both the area and altitude of the 

 Jand. The whole of the present continental area was raised 

 sufficiently to lift large portions of the previous sea-bottom above 

 its .surface. The principal elevation was on the eastern co,ast, 

 where the rise must have been several thousand feet ; whi'e on 

 the west it was less pronounced, though the area added to the 

 land appears to have included nearly the whole of what is now 

 Western Australia, .■^nd in regard to the intervening space be- 

 tween it and the eastern ranges there is only the negative 

 evidence, of no later marine deposits to indicate that it also was 

 above the ocean. Although the general elevation of the conti- 

 nent appears to have been quiescent in the western and central 

 parts, there were violent disruptions on the eastern coast, and 

 the strata were apparently crushed by a force from the east which 

 lifted them into a series of waves showing the faces of dislocation 

 to the east and strata sloping to the west, the most easterly wave 

 being near the present coast-line, and the succeeding waves more 

 gradual as they recede to the west, both in angle and height, 

 ■until they merge into the level of Central .Australia. It is also 

 probable that the South Australian range was also the result of 

 this compression, causing the strata to rise in abrupt mas.ses on 

 an axis nearly north and south. It was at this stiige of disruption 

 and elevation of strata that the more important auriferous 

 deposits of both the eastern and western p.irts of the continent 

 were formed, and these may be divided into two classes -true 

 fissure veins, or lodes, in which the deposits of ore are found filling 

 fi.ssurcs in the slate strata, and generally nearly vertical ; and floors 

 of ore which occur in sheets dipping at a less angle from the 

 horizontal than the vertical, the including rock being of cry.slal- 

 line character, being, in fiict, intrusive granites. The dip of these 

 sheets of ore is in the direction of the huge dykes of intrusive 

 rock in which they occur. 



.-\uRiKERous Deposits i.n Lodes. 



There was not only great disruption of the strata, but igneous 

 rocks forced themselves into the fissures in the sedimentary beds, 

 ■ ind the resulting metamorphism of the adjacent rocks increased the 



NO. 133 I, \OI.. 52] 



confusion, as beds of slate may be traced through the transforma- 

 tion of their sedimentary character, by the recrystallisation of 

 their component elements into tiiorites having that peculiar 

 structure of radiating crystals which usually characterise rocks of 

 volcanic origin. .-Xs regards the auriferous ileposits in these 

 lodes, it appears that first simple fissures were filled with water 

 from the ocean or deep-seated sources ; but in either case the 

 powerful electric currents which continually traverse the earth's 

 surface east and west met resistance at the lines of disruption, 

 and electric action being developeil, the mineral and metallic 

 salts in the water in the fissure ami the a<ljacent rocks would be 

 decomposed, and the constituents deiwsited as elements, such as 

 gold and silver, or as compounds, such as quartz, calcsp;ir, and 

 sulphide of iron, all which were in course of deposit at the same time 

 as the angles of the crystals cut into each other. There have been 

 many speculations as to the .source from which the gold was de- 

 rived, but that which best accords with the actual conditions is that 

 the metal exists in very minute quantities in the m.ass of the adjacen I 

 rocks, from which it h.as been transferred thrcjugh the agency of 

 electric currents and the solvent action of alkaline chlorides, 

 which dissolve small quantities of the precious metals, and would 

 be subject to decomposition at the places where fissures caused 

 greater resistance to the electric current. One remarkable cir- 

 cumstance is that the character of the rocks forming the sides of 

 the fissures has an evident influence on the richness of the ores 

 in metals where lime, magnesia, or other alkaline compounds, or 

 graphite, enter into their composition ; the gold especially is 

 more abundant than where the rocks contain silica and alumina 

 only. 



Queensland's Testi.monv. 



In Queensland, Gympie affords some instructive examples of 

 fissure lodes. In some, large masses t)f rock have fallen into the 

 fissure before the ore was deposited, and have formed what 

 miners term " horses,'* where the lode splits into two thin sheets 

 to again unite below the fallen mass. The Mount Morgan mine 

 may also be cited as a case where several fissure lodes rise to the 

 surface in close proximity. The ore was.origuially an auriferous 

 pyrites, but the sulphide of iron was largely decomjio.sed, leaving 

 the gold disseminated through the oxide of iron. In other cases 

 the sulphur and iron have both been dissolved out, and left 

 cellular quartz, with gold in the cavities or as fragments of gold, 

 mixed with minute crystals of quartz, presenting the aspect of 

 kaolin, for which it has been mistaken. The auriferous deposits, 

 which occur in the intrusive granites, appear tmder conditions 

 differing from the true lodes in sedimentary riKks, as the intrusive 

 granitoid rock forms dykes which fill fissures in the older true 

 granites, and also cut through the sedimentary slates. It bears 

 evidence of intrtision in a state of fusion, or, at least, in pla.stic 

 condition and subsequently crystallised, after which there has 

 been shrinkage, causing cavities as the sides of the dyke were 

 held in position by the enclosing rock. The vertical shrinkage 

 being greater than the horizontal, the cavities were nearer the 

 horizontal than the vertical, antl being afterwards filled with ore, 

 formed what are called " floors," one characteristic of which is 

 the tendency to lenticular form, or .a central maximum thickness 

 with thinner edges. The Charters Towers goldfield exhibits a 

 good illustration of this class of auriferous intrusive granite. 

 Here the intrusive granite appears as a dyke of great thickness, 

 exceeding a mile, with a length of twenty miles ; the rock is 

 well-crystallised quartz and felspar, with very little mica or 

 hornblende. <Jne shaft has been sunk 2000 feet to a floor showing 

 gold, and similar to the floors that outcrop on the surface. The 

 clip of these floors is north, about 30 <legrees from the horizontal, 

 and the strike across the ilirection of the tlyke. There are, 

 however, no good natural cross-sections, as the watercourses are 

 small, so that the length and breadth have to be estimated to 

 some extent by the character of the soil derived firom the 

 decompo.se<l rock, it Iicing more fertile than that of the other 

 rocks in the Iix:alily. The exploratory shafts which have been 

 sunk are in jxisitions selecle<l for the purpose of reaching known 

 sheets of ore at greater depth, or under the impression that the 

 ore de|x>sits were true fissure lodes, and would have extension in 

 the ilirection of the discovered outcrops, and therefore not 

 calculated to exteml our knowledge of the auriferous deposits. 

 The most instructive instance of the occurrence of auriferous 

 intrusive granite jxLsts in the valley of the Brisbane River, near 

 Eskdale, where a granitoid dyke, fifty yards wide, cuts through 

 a slate hill for a dLslance of three miles, and in places shows thiu 

 : sheets of (|uarl/ containing gold ; the strike is at right angles to 

 the length of the dyke, ami the dip is 30 degrees. .Some of the 



