ALASKA AND ITS MINERAL RESOURCES 163 



gold-bearing formations that had furnished the richest placers 

 in the districts visited by him. A brief statement of the prom- 

 inent characteristics of these districts as given by him will there- 

 fore probably be of value. 



The hills surrounding the gulches of the Little Mynook and 

 Hunter creeks, on the Lower Yukon, are formed of rocks of the 

 Rampart series. The bed-rocks are of diabase, tuffs, impure 

 shales, and quartzites, and in the bottoms of the gulches there 

 is from 10 to 20 feet of gravel. The gravel consists in part of 

 angular fragments of rocks that form the walls of the gulch, in 

 part of waterworn pebbles of Birch creek schist, schistose granite, 

 and other rocks. The gold is generally in rounded, bean-shaped 

 grains and nuggets, and less frequently in unworn particles. 

 This points to a two-fold origin of the gold, as derived in part 

 from the rocks immediately about and in part from distant and 

 older rocks, which may have been worn down, possibly along an 

 old seashore, into terrace gravels, and then by subsequent erosion 

 brought into the present stream beds. Further exploration in the 

 hills to the south may disclose the true source of these pebbles 

 and of the gold that accompanies them. On American creek, 

 in the Mission creek district, the gold-bearing placers are also 

 derived from rocks of the Rampart series — quartzitic schists, ser- 

 pentines, and chloritic rocks — and the gold is said by Spurr to 

 have been derived mainly from the schistose zones in the bed- 

 rock. 



The richest gravels have been found in the Birch creek and 

 Fortymile districts. In the entire Birch creek district, which 

 lies south of Circle City, and on Miller, Glacier, Poker, and Davis 

 creeks of the Fortymile district, near the international boundary, 

 the bed-rocks are always the quartzite-schists of the Birch creek 

 series, containing veins of quartz. The gravels rest, as a rule, 

 directly on the schist, though in some cases, as on Harrison and 

 Eagle creeks, in the Birch creek district, there is clay beneath 

 the gravels, and the gold, as a rule, does not extend into the bed- 

 rock, but occurs chiefly at the top of the clay. Generally, how- 

 ever, the schist is rotted and reddened from oxidation for a few 

 inches to several feet below the surface, and in this part the gold 

 has settled into the cracks and joints. The pa}' gravels lie 

 mostly next the bed-rock, ,in an average thickness of perhaps 

 two feet, though sometimes up to ten feet, while the overlying 

 gravels average eight or ten feet, with a maximum of 25 feet. In 

 the gravels the schist is in quite large, flat fragments, and the 



