80 ECONOMIC GEOLOGY 



as large as those of the Klondike field which lies east of the Inter- 

 national boundary. Mining in Klondike is said to have passed 

 its zenith while the maximum yearly output of Seward Peninsula 

 is still to be reached. The Klondike placers were discovered in 

 1896 and those on the Seward Peninsula in 1897. 



According to A. H. Brooks, three conditions are usually opera- 

 tive in the formation of placers: (1) The occurrence of gold in 

 bed rock to which erosion has access; (2) the separation of gold 

 from bed rock by weathering or abrasion; and (3) the transporta- 

 tion, sorting and deposition of the weathered and eroded auriferous 

 material. 



Origin: In some parts of Europe, in the tropics and in the 

 southern Appalachians some workable placers have been formed 

 solely by the weathering of the bed rock in place. T. A. Rickard 

 recognized placers in Australia that have been concentrated 

 through the agency of the wind, the lighter material having been 

 removed. In the formation of the true placers transportation, 

 sorting, and deposition of material furnished by the weathering 

 of the rocks are important agents. Uplift may revive the forces 

 of erosion and render these agencies repeatedly effective, which 

 results in the reconcentration of the alluvial gold. 



The classification of placers should be based both upon origin 

 and form.- According to their origin there are three types of 

 placers: (1) Residual; (2) sorted placers; and (3) re-sorted placers. 

 The residual placers are those in which there has been no water 

 transportation, the concentration of gold being due solely to rock 

 weathering. The gold of the sorted placers is the result of trans- 

 portation, sorting and deposition of auriferous material by water. 

 Re-sorted placers are those in which the gold has passed through 

 two or more cycles of erosion before its final deposition. 



Residual placers are practically all of one type. Sorted placers 

 may be subdivided into hillside, creek and gulch, river bar, gravel 

 plain, bench and high bench deposits. Re-sorted placers may be 

 divided into creek and gulch, beach and elevated beach deposits. 

 Intermediate types may be found which belong to either one of 

 the last two groups. Hillside placers occur on hill slopes and 

 do not occupy any well-defined channels. They grade on the one 

 hand into placers of residual origin and on the other into placers 

 of the stream or gulch type. Creek and gulch placers occur 

 both in material that has been assorted once and in that which 

 has passed through several cycles of erosion. 



