ECONOMIC GEOLOGY. 37 
becoming- graded; in part to more abundant precipitation, as is sug- 
gested by the much greater extension of the glaciers of the Alaska 
Range, and in part, perhaps, to a higher average temperature, though 
it would seem that with the other factors present no essential differ- 
ence in the temperature would be required. Whatever the conditions 
of formation — and these are only imperfectly known — the dominant 
facts of economic importance are that in general but one pay streak 
has been laid down; that this is next to bed rock beneath a considera- 
ble thickness of other deposits, and that its formation is for tic most 
part, a closed incident. 
SOURCE OF Till: GOLD. 
The origin of the gold in the placers, although not definitely deter- 
mined, is suggested by the character of the gold itself and by its asso- 
ciation. Most of that found near the heads of the creeks is rough and 
practically unworn; much of it is flat, as if derived from small 
seams: most of the coarse pieces are intimately intergrown with 
quartz and many of them are flat, like the small fragments of thin 
quartz seams which are common in the schists. That mineralization 
has not been confined to gold is shown by the occurrence of native 
bismuth intergrown with gold, of veins of stibnite, and of the cas- 
siterite often found in the gravels. The most acidic igneous rocks 
observed in the Fairbanks region are intrusive porphyritic biotite 
granite. The acidic dikes so common in the Fortymile region are ab- 
sent and the gold of the placers has probably been derived from small 
quartz seams in the schists. 
It is often a subject of surprise to the miners that when gold is 
abundant in the placers it should be found so rarely in the bed rock. 
It might be said that if gold were commonly encountered in the bed 
rock the proportion of it in the placers, considering the amount of 
bed rock that has been removed, ought to be much greater. There 
is the possibility also that the veins in the country rock which con- 
tributed the material for the first deposits of the valleys were richer 
in gold than those now exposed. Be that as it may. it is certain that 
through long-continued weathering and sorting of the rock material 
a concentration of the heavier indestructible contents, including the 
gold, takes place, yielding auriferous detrita] deposits which are made 
richer in gold than the parent rock. Furthermore, much of the gold 
may have been contributed by the bed rock that has been removed in 
the formation of the present valley-. 
PLACERS. 
The productive areas of the Fairbanks region at presenl (1907) 
include the valleys of streams draining the ridge that extend- north- 
east and southwest between ( natanika and Little Chena rivers. The 
distance between these two streams i> about 25 mil' 1 - and th«- gold- 
