SUPERFICIAL ALTERATION OF ORE DEPOSITS. 317 



bility of the compounds formed by the action of the materials in 

 the surface waters on the constituents of the ores. 



(3) The physical effect of superficial alteration is generally 

 to make the deposit more open an porous, to cause it to shrink, 

 and, in some cases, to convert it to a loose material of the con- 

 sistency of sand and clay. In some cases, however, especially 

 where hydration is active, and expansion may be caused. 



(4) Superficial alteration extends downwards as far as sur- 

 face influences are able to act, though generally alteration is not 

 complete down to the possible limit. The depth of alteration 

 depends on the topography of the region, the nature of the 

 rocks, and on the climate. In glaciated regions, the glacial 

 action has swept away the products of alteration, and sufficient 

 time has not yet elapsed since then for alteration to have gone 

 on to any great extent, but in many other regions the products 

 of alteration have accumulated to considerable depths. The 

 depth of alteration, under different conditions, varies from a frac- 

 tion of a foot to 1,500 feet, or possibly more. 



(5) Superficial alteration is well illustrated in iron, man- 

 ganese, copper, lead, zinc, silver, gold, tin, and many other 

 deposits. For special descriptions see text. 



(6) The accumulation of soluble saline materials, like salt, 

 on the surface has a very important effect in converting certain 

 materials in underlying ore deposits to chlorides, etc. 



R. A. F. Penrose, Jr. 



