312 THE JOURNAL OF GEOLOGY. 



larger percentage of the gold production of the world. Hence, 

 it will be seen, that in gold deposits, surface alteration not only- 

 plays an important part in freeing the gold from the iron pyrites, 

 but also in forming placer deposits. Detrital deposits similar to 

 gold placers and carrying various other materials are not at all 

 uncommon, as in the cases of the platinum group of metals, cas- 

 siterite, diamonds and many other gems, chromite and magne- 

 tite sands, and, in fact, even with some of the more common ores, 

 as with the iron conglomerate at Iron Mountain, Missouri. 



Alteratio7i in tin deposits. — In tin deposits, the typical mode of 

 occurrence of the metal is in veins, dikes, or country rocks, in 

 the form of the oxide known as cassiterite. Cassiterite is not 

 easily affected chemically by surface influences, so that it is not 

 much changed by superficial alteration, but for this very reason, 

 its concentration is most markedly affected by surface alteration, 

 for in the erosion of tin-bearing deposits the masses of cassiter- 

 ite are broken up and carried off mechanically by surface waters, 

 to be deposited somewhere else in the form of gravel beds, 

 instead of being dissolved and possibly disseminated. In this 

 transition, the fragments of cassiterite are largely separated from 

 the accompanying materials by reason of their greater specific 

 gravity, and hence, gravel deposits rich in cassiterite frequently 

 occur. These represent the stream tin of the miner, and have 

 been formed in much the same manner as have the placer gold 

 deposits. Some chemical action, however, has gone on in the 

 tin ore itself, but this seems to have been simply a process of 

 solution and redeposition, as is seen in the pseudomorphs of 

 cassiterite after other minerals and in the impregnations of 

 animal remains in Cornwall, such as antlers, with oxide of tin. ^ 



Alteration i?i a?itimo?ty deposits. — In many antimony deposits, 

 alteration similar to that described in some of the deposits already 

 mentioned frequently occurs. The metal occurs most commonly 

 as the sulphide known as stibnite. By alteration, however, this 

 passes into the oxides valentinite, senarmontite, cervantite, 

 stibiconite, etc., or into the combined sulphide and oxide known 



'J. H. Collins, Mineralogical Magazine, Vol. IV., 1882, p. 115. 



