306 Professor John W. Gregory [April 27, 



haul up a ton of gravel from a river bed, sort it, wash it, and extract 

 its gold, at the cost of three half -pence. It has been predicted that 

 this invention, which we owe to New Zealand, will so greatly increase 

 the gold yield of the world, as to repeat the revolutionary effect on 

 prices occasioned by the gold discoveries of California and Australia 

 in the middle of the last century. 



But economical though these dredges be it is doubtful whether 

 they can maintain an adequate metal supply, and we are becoming 

 increasingly dependent on the mining of deeper ore-deposits. They 

 belong to two main groups : — 



1. Ores of alluvial and sedimentary origin ; 



2. Ores deposited in lodes and masses. 



Deep alluvial and sedimentary ores only pay to work for precious 

 metals, and practically only for gold. They are of two main types : 

 deep leads, and buried sheets of gold-bearing conglomerates. The 

 deep leads are the beds of old rivers, which have been buried under 

 thick sheets of river deposits or lava flows. In working these deposits 

 the course of the old rivers has to be discovered by boring through 

 the overlying rocks ; and when the old river system has been mapped 

 out, the miner can pump the gravels dry, then dig them out, and 

 extract from them their gold. 



The only deeply buried, wide sheet of gold-bearing sedimentary 

 rock, which is of first-rate mining importance, is the banket of the 

 Transvaal. The banket is a marine conglomerate, composed of 

 pebbles of quartz, and some pebbles of what is now pyrites. The 

 rock was doubtless formed on a sinking shore-line. According to 

 the explanation which seems to me most probable, the Rand gold 

 was derived from the wearing away of gold-bearing quartz lodes. 

 The sheets of pebble-reef have been traced east and west through the 

 length of the Rand ; and they may occur to the south of Johannes- 

 burg, in the bottom of a great basin, far deeper than they can at 

 present be profitably worked. Preparations are being made for 

 mining the banket at the depth of 5000, and even 6000 feet, and if 

 the gold be alluvial in origin, the limit of working will be determined 

 by expense, and not by any limit in its distribution. It is, however, 

 authoritatively held that the Rand gold is not alluvial in origin, but 

 has been formed by impregnation, as in ordinary gold-quartz lodes. 

 If so, the banket is only a very abnormal lode, and the downward 

 limit of its ores will depend, not on the depth to which the con- 

 glomerate goes, l)ut on the factors that control the deposition of ores 

 in lodes. 



Lode-Caps. 



The future metal supply of the world will doubtless come from 

 ores deposited l)y secondary processes in lodes or masses, and the depth 



