-M) THE UNITED STATES GEOLOGICAL SURVEY. [bull.227. 
a process of double concentration. The processes of concentration 
and reconcentration were fully treated and their economic importance 
was emphasized. 
The gold and copper deposits of the Ketchikan mining district were 
investigated in 1901 and found to be closely associated with igneous 
rocks. The origin of the gold in the Chistochina placers in the 
Copper River Basin has been referred to locally metamorphosed areas 
of Permian shales. Along the northwestern slope of the Alaskan 
Range, north of Mount McKinley, colors were found in the stream 
gravels, which may indicate a mineralized area in that region. 
The extension of the work in 1903 established the fact that work- 
able placers are present in the northeastern part of Seward Peninsula, 
and also in the Fairbanks district, in the Tanana Valley. A detailed 
examination of the Juneau gold district of southeastern Alaska showed 
that the deposits fall into two groups — those occurring in a well- 
marked zone skirting the mainland, and those occurring in less per- 
sistent belts, which are isolated from the first. 
In 1898 the copper deposits of Prince William Sound were investi- 
gated, and the copper was found to be widely distributed as sulphides 
in shear zones of greenstone rocks or associated with diabasic intrusives. 
In 1899 the copper deposits of Kletsan Creek, at the head of White 
River, were investigated. Copper had long been known to occur in 
the stream gravels of this region, and the locality had been visited by 
one of the earlier expeditions. The source of placer nuggets was 
found in veins along the contacts of crystalline limestone and intrusive 
greenstone, where it occurs in the native or metallic form with calcite. 
In 1900 one of the most important economic problems was the 
investigation of the Copper River country. Copper was found to 
occur native in cavities and stringers in the Nikolai greenstone (an 
old volcanic now), and as sulphides, bornite, and chalcopyrite. in 
fissure veins near the contact of the igneous rock with overlying lime- 
stones. The different localities were visited and copper was reported 
to exist in amounts sufficient to justify exploration. Further work 
was done in this area in 1902. The Nikolai greenstones and the over- 
lying Chitistone limestones were mapped and copper-bearing localities 
along the contact were indicated. The copper deposits of the upper 
Tanana were found to be associated Avith diabasic intrusives, and to 
be similar to the Kletsan Creek deposits, already noted. 
In the investigation of the mineral deposits of the Ketchikan mining 
district in 1901 copper was found to occur both in veins and in min- 
eralized shear zones, chiefly as chalcopyrite, malachite, bornite, and 
chalcocite. These were found in feldspathic schists, probably intrusive 
in greenstone, in the greenstones themselves, and along the contacts 
of crystalline limestone with intrusives. 
While work was in progress in the York region in 1900, attention 
