purington.] GRAVEL AND PLACER MINING. 33 
the terms gravel deposit, gravel washing, and gravel mining arc 
employed they must be understood, for the sake of brevity, to include 
the consideration of all classes of deposits in which gold of detrital 
origin is found. 
The term u elluvial" has been applied to placer deposits formed by 
the rotting of rock in place to greater or less depth." Such deposits 
do not occur in the portions of Alaska visited, and may be excluded 
from consideration. 
In regard to the valuable contents of the deposits, it should be 
stated that, in all the cases here considered, gold is the mineral sought. 
Platinum or any minerals of the platinum group have not been found 
in paying quantity in any part of Alaska. Alluvial tin has been 
found and mined in the western portion of the Seward Peninsula. 
The deposits were not, however, visited by the present expedition.^ 
CLASSIFICATION OF ALLUVIAL GOLD DEPOSITS IN ALASKA. 
The alluvial gold deposits of Alaska may be classified as follows: 
Classification of alluvial deposit* in Alaska. 
Creek placers . . 
Hillside placers 
Bench placers 
River-bar placers. 
Placers in, adjacent to, and at the level of small streams. 
Placers on slopes, intermediate between creek and bench 
claims. 
Placers in ancient stream deposits from 50 to 300 feet 
above present streams. 
Placers on gravel flats in or adjacent to the beds of large 
streams. 
Gravel-plain (tundra) I Placers in the coastal plain of Seward Peninsula, 
placers. 
Sea-beach placers Placers adjacent to the seashore to which the waves 
have access. 
Lake-bed placers Placers accumulated in the beds of present or ancient 
lakes; generally formed by landslides or glacial 
damming. 
aEng. and Min. Jour., vol. 77, May 5, 1904, p. 722. 
bSee the report of A.J. Collier (Bull. U. S. Geol. Survey No. 229, 1904) for an exhaustive account of 
the York tin deposits. 
Bull. 259—05 3 
