SUPERFICIAL ALTERATION OF ORE DEPOSITS. 301 



increased depth they become more numerous and continuous, until 

 they predominate over the altered products, and finally, when the 

 limit of alteration is reached, they entirely replace them. The 

 planes of contact between an ore deposit and the country rock, 

 that is the walls, afford, when well defined, easy passages for the 

 downward percolation of surface waters, and therefore alteration 

 frequently continues down along these lines for considerable dis- 

 tances after the limit of alteration in the main part of a deposit 

 has been reached. Any other possible channels, such as the 

 planes of contact of different minerals in banded deposits or the 

 series of drusy cavities often found in the central parts of ore 

 deposits, may act in the same way as passages for water. Hence 

 the not infrequent abundance of alteration products, such as 

 hydrous sesquioxide of iron, and native copper and silver, along 

 the walls and elsewhere in certain deposits. 



Classificatio7i of the products of alteration. — The products of 

 superficial alteration may be divided into two general classes : 

 ( I ) Those which occupy the same position as the materials from 

 which they were derived, or are only slightly removed, and 

 possess the same general environment. Thus the altered out- 

 crops of auriferous quartz and iron pyrites, of argentiferous 

 galena, of sulphides of copper and many other similar deposits, 

 represent alteration-products occupying the same general posi- 

 tion as the original sulphide ores ; while the iron ore bodies of 

 the Lake Superior region represent alteration-products changed 

 somewhat in position from that occupied originally, but yet in 

 the same series of rocks and sometimes with somewhat similar 

 environment. (2) In the second class are included those deposits 

 which have been entirely removed from their original position 

 and redeposited under totally different environments. Thus, 

 placer gold deposits, stream tin, most of the deposits carrying 

 platinum and the allied metals, magnetic and chromite sand, 

 the gravels and sands carrying precious stones, and many other 

 similar deposits represent this class. They have been derived 

 by the decay and erosion of veins, dikes or country rocks carry- 

 ing the materials now concentrated in these fragmental deposits. 



