64 ORIGIN AND DEVELOPMENT OF ORB-DEPOSITS. 



stance, more or less of the ore-material sought by the miner. 

 This is expressed by him in the phrase " mineralized ground." 



6. If then we consider that the ore-deposits were, in the 

 main, formed as we now have them, at depths of many thousands 

 of feet below the present surface and that we are now only able 

 to command at most 4,000 feet of depth, it follows that our 

 working zone forms but a very insignificant portion of the original 

 circulation-zone. Consequently we have not much reason to 

 expect any marked general increase of richness, as the workable 

 zone is followed for a few hundred feet in any given mine, 

 except so far as it is due to (a) comparatively recent and local 

 enrichments above the water-level from downward percolation or 

 upward efflorescence, and {b) to the coming together of converg- 

 ing veins. The general enrichment which miners usually 

 believe in is easily accounted for, so far as it has any reality at 

 all, by the fact that veins which are not found to be especially 

 rich as they are followed down, are soon abandoned perforce in 

 most instances owing to the greater cost of working. 



Variations of richness, as well as of mineral character, will of 

 course be constantly noticed, both in depth and laterally ; the 

 reasons for such variations have already been fully set forth; but 

 in the absence of definite knowledge as to the varying characters 

 of the rock-masses themselves, in age, composition, structure, 

 permeability, &c., it would seem that, setting aside the excep- 

 tions noted above, the chances of improvement are practically 

 equal to those of deterioration in any given case as greater 

 depth is attained. 



Chap. IV. — Detritad Ore Deposits. 

 Sec. 1. — Defitiition. 



All clastic rocks, such as conglomerates and sandstones, and 

 even some clays, may be regarded as detrital deposits, whatever 

 may be their geological age. But the term is by custom generally 

 restricted to the but little consolidated sands and gravels of the 

 later geological periods. Such beds are often found to contain 

 distributed particles of ore-matter, apparently derived from pre- 

 existing rocks ; and when these occur in workable quantities or 

 nearly so, the beds themselves may be classed as detrital ore- 

 deposits. 



