308 Professor •John W. Gregory [April 27, 



The Latekal Secretiox Theory. 



The conversion of successful working miners into farmers was once 

 stimulated by the widespread belief that these changes in the lodes 

 indicated that gold is a surface formation, and that gold quartz lodes 

 would not persist below the depth of about 300 feet. Thus according 

 to Sir Roderick Murchison, " as frequently as deep mines enriched 

 the speculators who sought for copper and silver, so surely gold 

 mining in the solid rock proved abortive, owing to the slender, 

 downward dissemination of gold in a hard and intractable matrix." 

 And he maintains that it is an "indisputable fact that the chief 

 quantities of gold, including all the considerable lumps and pepitas, 

 have been originally imbedded in the upper parts of the vein-stones." 

 Murchison insisted repeatedly on the essentially superficial distribu- 

 tion of gold ores. 



It is true that many of the earliest students of ore-deposits 

 regarded all ores as derived from a " Pluto's Hoard " in the inner 

 recesses of the earth. But this view was opposed to many of the 

 most obvious facts. Thus lodes are often found to be closed below, 

 in which case it is easier to explain them as filled from above, or from 

 the sides, than from below. Again, lodes are often poor below and 

 rich above, whereas the contrary would be expected if the heavy ores 

 came from the interior of the earth. The veins known as gash-veins, 

 which are entirely confined to one bed of rock, can, therefore, 

 only have received their contents from it ; yet the contents of these 

 gash-veins are identical with those of the great fissure lodes, and so 

 they also may have obtained their materials from the rocks beside 

 them. 



A further strong argument in favour of the local origin of the 

 lodes is the character of the vein-stuffs, the non-metallic lode 

 materials in which the metallic minerals occur. These vein-stuffs 

 vary according to the rock beside the lode. Where a lode traverses 

 sandstone or other sihcious rocks, the vein-stuffs consist mainly of 

 quartz ; when it traverses limestone or igneous rocks rich in lime, 

 calcite is a common vein-stuff. In sucli cases it is obvious that the 

 vein-stuffs have been brought from the sides and not from below ; 

 and the ores are so intimately mixed with the vein-stuffs that it was 

 natural to infer that both had been introduced by the same agency, 

 and had been derived from a common source. 



The lodes, moreover, in many mining fields show essentia] de- 

 pendence on the rocks beside them, and apparent complete independ- 

 ence of the rocks below. Thus in Cumberland, the lead lodes are 

 apt to be broad and rich where they occur in limestone, and narrow 

 and barren where they traverse shale and sandstone. Again, in the 

 famous copper-field of Thuringia, the ores are confined to those parts 

 of the veins that occur in a narrow band of slate, above and below 



