Geology and Mineralogy. 307 



Later Tertiary beds ) Tert; 



Kenai series j J 



Mission Creek series — Cretaceous. 



Tahkandit series — Carboniferous and Devonian. 



Rampart series ) 



Fortymile series > ? pre-Paleozoic. 



Birch Creek series ) 



Basal granite-schist. 



Birch Creek series, — Resting upon the fundamental granite is 

 a series of rocks, roughly estimated as possibly 25,000 feet in 

 thickness, named the Birch Creek series, from the place of their 

 typical occurrence. They consist mainly of quart zitic rocks, 

 generally thin-bedded or schistose, so that they pass into mica- 

 schist ; in some places they contain carbonaceous matter and de- 

 velop graphitic schists. There are also bands which probably 

 originated as intrusive rocks, but which by compression have 

 become schistose like the other members. These rocks have 

 abundant quartz veins ; they are generally parallel to the schis- 

 tosity or bedding, small, and not persistent, but some cross the 

 bedding and are then wider. They carry gold with abundant 

 pyrites, and sometimes galena. They are often broken and 

 faulted. 



Fortymile series. — Younger than the Birch Creek series, but in 

 general closely associated therewith, is another thick series of 

 rocks, called the Fortymile series because of their development on 

 Fortymile Creek. They are characterized by alternations of beds 

 of marble, from a few inches up to 50 feet in thickness, with 

 quartzites, which pass into micaceous, hornblendic, or garneti- 

 ferous, and sometimes graphitic schists. They are traversed by 

 abundant dikes of eruptive rock, mostly granites and diorites. 

 Two sets of quartz veins are developed in these rocks: (1) an 

 older set, which are generally parallel to the schistosity or lamina- 

 tion, like those in the Birch Creek series, and like them are 

 broken by later movements and carry pyrite and occasionally 

 galena ; (2) a set of larger veins, which form an apparent transi- 

 tion from dikes of aplite. They cut across the bedding and are 

 not disturbed by later rock movements, hence are younger in age. 



Rampart series. — This still later series is primarily distin- 

 guished from the preceding by the darker color of its rocks, 

 which are dark-green when fresh and become a dark-red by 

 weathering. They consist largely of basic eruptive materials, 

 beds of diabase and tuflaceous sediments, with hard green shales 

 and some limestones containing glauconite. They also contain 

 novaculites and jasperoids, or secondary quartzose rocks which 

 are at times glauconitic. Serpentine and chlorite are frequent 

 alteration products. These rocks also contain a few quartz and 

 calcite veins, which are generally developed along shear zones. 

 The basic character of these rocks and their large content of 

 pyrite seem favorable to the concentration of ore deposits ; but 

 the observed veins are younger than the joints and shear planes, 



