THE GENESIS OF ORES 541 



The time element enters in a variety of ways into the problem of 

 ore formation by descending circulations. Thus an ore deposit formed 

 in its primary, low grade constitution during earlier geological periods, 

 such as the Cambrian or Huronian, and during all of the subsequent 

 ages exposed to the action of superficial agencies unhampered by sub- 

 sequent covering of later rocks, has a thousandfold the opportunity 

 for concentration of its ores that is presented by similar rocks and 

 ores formed during later geological epochs, say the Tertiary. This is 

 exemplified by the iron ores of the Mesabi range as contrasted with 

 the glauconitic deposits of New Jersey or Texas. During almost all 

 the ages since the Cambrian the iron ore formation of the Mesabi has 

 been exposed to the weather, covered only for a geological moment dur- 

 ing a part of Cretaceous time. The result is the largest and purest 

 deposits of iron ore ever discovered, while rocks of similar composition 

 but much more recent formation exhibit only the initial stages of ore 

 formation. 



Another way in which time affects ore deposition is in connection 

 with the rate at which the waters move in a vein. Solutions of a 

 given composition may move so rapidly as to produce but little effect, 

 or may move so slowly that they clog up or retard other active waters 

 after their own power is exhausted. Upon a steep drainage slope or 

 mountain the waters may pass off so rapidly, even below the actual 

 top of the ground, as to exert but little influence, or they may move 

 with just sufficient rapidity to accomplish their maximum of chemical 

 effect. 



Our third factor, temperature, is of great importance. In the first 

 place, oxidation, which is but another name for combustion, is greatly 

 accelerated or retarded by slight changes in temperature. Sulphides 

 which remain immersed for centuries in water under a glacier in 

 Alaska would be completely oxidized in a few years exposed to the 

 heat of the sun on a southern slope in Colorado or California. In the 

 next place, the rate of solution depends directly upon temperature, in- 

 creasing as the temperature rises, and, itself a process of heat consump- 

 tion, is greatly facilitated by heat from external sources. Thus in 

 warm rocks, in mild climates, upon the sunny side of mountains, there 

 will be the most favorable conditions as regards temperature, for the 

 formation of secondarily enriched ore deposits. The experienced pros- 

 pector will tell you that it is in precisely these localities that they are 

 found, although he never before heard any explanation for it. 



Lastly, the physical structure and solubility of the rocks and ores 

 affect their susceptibility to later enrichment for perfectly obvious 

 reasons. A dense rock is not readily entered by mineralizing solutions. 

 Likewise an insoluble one is not easily replaced and does not afford 

 lodgment for ores. And if the ores themselves are not readily attacked 



