190G.] on Ore Deposits and their Distribution in Depth. 309 



which they are practically always barren. It was claimed that the ores 

 had been originally precipitated from the water of the sea during the 

 deposition of the copper-slates, and subsequently concentrated in the 

 veins. 



The first adequate scientific statement of the theory that the ores 

 in lodes are leached out of the rocks in which the lodes occur, was by 

 Bischof in 1847. His pupils endeavoured to justify this lateral 

 secretion theory by demonstrating the existence of metals in ordinary 

 rocks ; but for thirty years their efforts were practically in vain. 

 Then Fridolin Sandberger varied the method of search. 



Most sedimentary rocks contain small quantities of accessory 

 minerals, consisting of fragments of the more durable constituents of 

 igneous rocks. The bright idea occurred to Sandberger that if any 

 metals occur in sedimentary rocks, they would be found in these 

 accessory minerals. An element present as a mere accessory con- 

 stituent, in a scarce accessory mineral, would form an infinitesimal 

 proportion of the rock, and it would thus escape detection on a bulk 

 analysis. So Sandberger first concentrated the accessory constituents, 

 and then analysed these concentrates. The result was the discovery 

 that minute quantities of various metals, including lead, zinc, copper, 

 cobalt, nickel, etc., are widely distributed in rocks and especially in 

 the old slates, in which most ores occur. 



This discovery removed the one objection to the lateral secretion 

 theory that had appeared insuperable, for it showed that the common 

 rocks, in which lodes occur, contain a supply of metal which lateral 

 secretion can collect into lodes. 



It is always a pleasure to solve a difficult problem by substituting 

 known forces, working on accessible materials, for unknown agents, 

 acting under conditions and on materials that must ever remain 

 beyond the range of experiment and direct observation. So the 

 lateral secretion theory at once sprang into popularity ; and it was 

 applied in one after another of the great mining fields of the world. 

 Thus Emmons attributed the origin of the silver-lead ores of Lead- 

 ville to lateral secretion from the overlying porphyrites. Becker 

 derived the silver of the Comstock bonanzas from the adjacent 

 diorites and andesites ; Chamberlin and Kendal taught us to look 

 for the source of the lead in the gash-veins of Wisconsin and of the 

 north of England, to the limestone in which the veins occur ; Howitt 

 and Rickard advocated the view that the gold in the quartz lodes of 

 Australia has been leached out of slates, into which it had been pre- 

 cipitated from the water of the Silurian seas. 



But this fascinating and simple lateral secretion hypothesis is now 

 generally abandoned. To mention only one strong objection, it does 

 not explain the occurrence of lodes with different ores in the same 

 country rock. Moreover, it supplies no explanation of the original 

 source of the metals and the ores. For the metals found in the 

 accessory minerals of sedimentary rocks are not necessarily present in 



