1906.] oil Ore Deposits and their Distribution in Deptli. 319 



the ore shoots may l)e quite shallow. The disappointing result in 

 the second case is due to the upper part of the lodes having been 

 removed by denudation, so that only the barren roots remain. 



To plunge into deep mining regardless of the geological structure 

 and history of the field is to invite failure. And deep mining to 

 be profitable must be so organised as to work low grade ores economic- 

 ally, and to reduce dead work to a minimum by a plan of mining 

 adapted to the geological structure of the field. 



Gold mines worked thus at Bendigo have already reached the depth 

 of 4250 feet, and the success there is the more remarkable as the mines- 

 are not working continuous lodes, but isolated masses of quartz. The 

 reefs are saddle-shaped folds of quartz, floating in folded l)eds of slate 

 and quartzite. It was not until the clue to the distribution of these 

 isolated quartz masses was discovered that there was any chance of 

 working the mines deeply. And though the field shows the usual 

 decrease in grade, there is no indication that the lowest reefs have 

 been approached. 



In some fields the mines will be doubtless more shallow. Thus 

 ores in lavas and horizontal stratified rocks, whether formed as contact 

 deposits near igneous intrusions, or along fault planes and fractures, 

 up to which hot solutions have ascended, are likely to be patchy, owing 

 to the sensitiveness with which such liquids respond to sHght variations 

 in the conditions. The depths of the ores will be directly deter- 

 mined l)y the thickness of the stratified rocks, and this is usually 

 a simple problem of field geology. There is less probability of the 

 ores continuing to as great a depth, as in infolded uptilted masses 

 of ancient rocks. 



The vast masses of pyritic ores represent a third type, which is 

 of a special historic interest. They were at one time regarded, as 

 they still are by some geologists, as bedded ores, precipitated in ancient 

 lakes, ])y the action of decomposing organic matter, on materials intro- 

 duced by streams. The ores were expected to continue to vast depths, 

 because they are often hundreds of feet in width, and the horizontal 

 extent of a sedimentary deposit is usually dozens, and may be hundreds, 

 of times greater than its vertical thickness. So the depths of the 

 ore-body was expected to be many times its thickness, and as one 

 lenticular ore-deposit thinned out, others were expected to come in 

 at sHghtly different levels. But these pyritic ore-masses have been 

 found to be comparatively shallow in depth ; none has yet been 

 worked to the depth of 1000 feet, and for their shallowness there are 

 excellent reasons ; they are secondary deposits, formed by ore-bearing 

 solutions replacing, particle by particle, shattered masses of rocks. 

 The replacement has been so slow that the aspect of the original 

 bedding is often still preserved, leading to the view that the pyrites itself 

 was a bedded deposit. The shattered rock masses, necessary for 

 the formation of these huge ore-bodies, can only occur near the 

 surface ; for at greater depths the weight of the overlying rocks 



