216 GEOLOGY AS AN ECONOMIC SCIENCE 



detect delicate shades of lithological variation, and 

 to give to the world, for the first time, some idea of 

 the definite relationship that mineral-deposits bear to 

 the country-rock in which they occur, and to throw 

 new light on the genesis of the deposits as a whole. 

 The work that has been carried out, in recent 

 years, on the petrology of ore-deposits, and on the 

 genesis of metalliferous veins, marks a great 

 advance; for the association of metalliferous 

 minerals with certain rock-types, the increase or 

 decrease in richness of mineral-deposits in depth, 

 the secondary enrichment of the upper portions of 

 lodes, and the tracing of alluvial deposits to their 

 source, all present problems which the geologist has 

 been called upon to solve. It is thus that the 

 investigations into the mode of occurrence and 

 origin of the iron-ores of Northern Europe, the 

 lead-deposits of the United States, the zinc-ores of 

 Greece, and the gold-deposits of Canada, South 

 Africa and Victoria, are a few outstanding examples 

 in which the results of geological studies of an 

 academic character have proved of the greatest 

 value to the practical miner. 



Water-supply. 



With the centralization of industry and the 

 consequent segregation of population, the question 

 of the supply of pure water in sufficient quantity 

 is of ever-growing importance. Whether such a 

 supply be obtained from superficial or underground 



