1906.] on Ore Deposits and their Distribution in Depth. 307 



to which these ores extend depends on the depth at which these second- 

 ary processes can take place. 



The lodes are generally first found and worked on the surface. 

 The lodes may occur as walls of rock, left upstanding by the wearing 

 away of the soft slates beside them. Lode mining began in Australia 

 by a party of miners chipping out the pieces of quartz that contained 

 visible gold from such an outcrop, and then breaking up the quartz 

 with hammers. 



The exposed part of tiie lode having been used up, the miners 

 follow it downward as deeply as it pays them to go with the appliances 

 available to them. Mining has shown that there are two main types 

 of lodes ; some ores are in flat sheets or veins, others in huge masses, 

 which may consist of solid blocks of ore, or of rock impregnated by 

 veins, veinlets, or even scattered grains. Both types of ore-deposits 

 when followed down, are often found to become thinner and perhaps 

 to pinch out altogether. Even if the lode continue downward, there 

 is soon a marked change in the character of the ores. Thus, in a 

 gold-quartz lode, the work is easy near the surface, till the miner 

 reaches the level where the rocks are charged with water ; this change 

 occurs at a depth which varies indefinitely, and may be as much as 

 1600 feet, but is often met at from SO to 150 feet. Then comes a 

 great increase in the difficulty of mining, and a great decrease in its 

 profit. 



Above water level the rocks are soft and decomposed, and full of 

 cavities ; below they are hard, and compact ; above that level they 

 are dry, below they are wet, so water oozes into the excavations which 

 have to be kept empty by costly pumping. The ore above water 

 level is stained rust-red, and the gold is visible, so the good and 

 barren grounds are readily distinguished ; below that level the ore is 

 dark in colour, and usually charged with pyrites, with which the 

 gold is combined, so it is no longer to be seen by the eye. The 

 gold, moreover, may be associated with minerals, which may hamper 

 its extraction. Hence, instead of the miner having to deal with an 

 ore from which the gold can be easily obtained, it becomes refractory 

 and may require difficult metallurgical treatment. Again, the upper 

 part of the lode being full of cavities, from which heavy materials 

 have been removed, a cubic foot of it weighs less than a cubic foot 

 of the compact lode below, so the gold, if equally distributed, is 

 richer per ton weight of ore above than below water level. 



Hence a party of miners finds that a lode, which has paid them 

 handsomely at first, in time becomes quite unprofitable ; and the only 

 thing to be done, when the incoming of the sulphides announces the 

 end of the easily mined ore, is to sell the mine to a company, and 

 start fanning with the money. 



X 2 



