512 REPORT — 1887. 



It is important to bear in mind that the conditions under ■which gold 

 has been obtained have varied much in different periods. In the early 

 ages of the world gold was chiefly obtained by forced labour. African 

 slavery was first employed by the Carthaginians in working the gold and 

 silver mines of Spain ; and centuries later the Spanlai'ds revived this in 

 working the gold and silver mines of the New World. Moreover, the 

 influx of the precious metals which followed the discovery of Mexico and 

 South America was due to gold already raised, and which was stolen from 

 the natives, and not at first to actual mining by the invaders. The hands 

 of Englishmen have not always been clean In dealing with native races, 

 especially where gold has been concerned ; but our record is honour itself 

 when compared with that of those who preceded us in the New World. 



Again, the great influxes of gold have come from the discovery and 

 rapid development of alluvial deposits, which, in time, became exhausted ; 

 and a steady supply for the fature must, for the most part, be sought for 

 in ordinary mining, and in the metallurgical treatment of ores containing 

 small quantities of gold and silver. 



Another important point in regard to futui-e supply is the improve- 

 ment in mining, milling, and metallurgical processes. 



In the ordinary methods of alluvial working there are considerable 

 losses, and one source of future supply will be the re- washing of the waste 

 workings of former years. The Callfornlan method of ' Hydraullcking ' is 

 the most complete plan for extracting a high percentage of gold from 

 gravels, &c., but this can only be employed where large quantities of 

 water are available at considerable pressure, and where the debris can be 

 disposed of without Injury to rivers and cultivated lands. 



Mode of Occurrence of Gold. — Gold may be roughly classed under two 

 heads, descriptive of its mode of occurrence :— 1. In quartz-veins, cutting 

 through the rocks, though occasionally almost coinciding with the 

 bedding. 2. In detrltal beds, derived from the denudation of rocks con- 

 taining veins of auriferous quartz. 



Veins of auriferous quartz rarely occur except in association with 

 eruptive rocks; in the older rocks often with granites, and generally in 

 association with dykes of diabase or diorite. So close is this association 

 that we are led to believe that the eruptive rocks are the means by which 

 the gold has been brought up towards the earth's surface, and thence con- 

 centrated by slow aqueous action in the quartz- veins. 



That such has been the origin of the gold and silver In the Comstock 

 may now be taken as proved : — ' The diabase shows a noteworthy contents 

 in the precious metals, most of which is found in the auglte. The de- 

 composed diabase contains about half as much of these metals as the fresh 

 rock. The relative quantities of gold and silver in the fresh and decom- 

 posed diabase correspond fairly well with the known composition of the 

 Comstock bullion. The total exposure of diabase Is suflicient to account 

 for far more bullion than has been extracted from the mines. . . . Where 

 ore is found in diorite, or in contact with it, it is usually of low grade, 

 and its value is chiefly in gold. The notably productive ore bodies have 

 been found in contact with diabase, and they have yielded by weight 

 about twenty times as much, silver as gold.' ' 



' G. F. Becker, 'The Comstock Lode,' 2)id Ann. Ilc^f. U.S. Geological Survey, 

 1882, p. 309. 



