24 ALASKAN MINERAL RESOURCES IN 1906. 
highly altered rocks which locally have proved to be gold bearing. It 
is presumable that some of these will be found to cover larger areas 
than here indicated. 
The experienced prospector need not be told that it does not follow 
that because a certain formation is gold bearing gold will be found 
wherever it occurs. A tyro, however, may interpret the accompany-' 
ing map as an absolute indication of the distribution of gold rather 
than as a guide to localities where the precious metal is likely to be 
found. Although the laws governing the distribution of gold in this 
field are but imperfectly understood, it seems certain that the occur- 
rence of mineralization is due to causes that have in many places 
acted very locally. There appear to be no facts which bear out the 
assumption often made that there are one or more well-defined gold 
belts which can be traced across Alaska, though the formations with 
which gold is associated may be found to be continuous over extensive 
areas. The work so far accomplished appears to justify the statement 
that within the areas of metamorphic rocks there are zones of miner- 
alization. These are, however, usually of very slight extent, ranging 
from only a few hundred yards to rarely a few miles in length. 
There is but little information on which to formulate a law for the 
occurrence of these mineral zones, and it is quite possible that in the 
different districts different causes have been operative. 
It appears to have been definitely established by Mr. Wright (see 
pp. 49-50) that in southeastern Alaska there is a causal relation 
between the intrusion of the Mesozoic granites and the ore bodies. 
As he sets forth, the zones of mineralization thus far discovered all 
occur along or near the margins of the intrusive granite masses. 
There is some evidence that a similar association of the zones of min- 
eralization and the granite exists in the -Yukon district. Prindle has 
shown that granitic rocks are common in all the gold-placer districts 
of the Yukon-Tanana region and that in at least one locality the gold 
is closely associated with intrusive phenomena. He has also suggested 
that intrusion and the formation of quartz veins took place at different 
periods. a During the last summer the writer found evidence of min- 
eralization accompanied by deposition of gold in the so-called Aucella 
beds (lower Cretaceous) on Washington Creek, a tributary of the 
Yukon. This appears to be the first instance in this province where 
definite proof was obtained of a post-Paleozoic mineralization, and is 
significant because it appears to belong to the same period as the 
intrusion of auriferous veins in southeastern Alaska. 
Mendenhall 6 has shown that in the Chistochina placer district of 
upper Copper River the mineralization is post-Permian and pre-Eocene, 
a Prindle, L. M., The gold placers of the Fortymile, Birch Creek, and Fairbanks regions, Alaska: Bull. 
U. S. Geol. Survey No. 251, 1905, p. 37. 
b Mendenhall, W. C, Geology of the central Copper River region: Prof. Paper U. S. Geol. Survey No. 
41, 1905. p. 115. 
