156 ALASKA AND ITS MINERAL RESOURCES 



rived from a much older series of rocks, for the gold-bearing 

 slates of the coastal region have not yet been recognized there. 

 While the exact age of these gold-bearing rocks has not }'et been 

 determined, the}^ are known to be older than the limestones sup- 

 posed to represent the Carboniferous and Devonian formations 

 of the cordilleran system ; hence they are probably pre-Paleozoic, 

 and in part are possibly as old as the Archean. The grounds 

 for assuming this derivation are that these rocks contain abun- 

 dant auriferous quartz veins, and that the richest placers thus far 

 discovered are so situated that they must have been derived from 

 them. These rocks are classified by Spurr as follows, commenc- 

 ing at the base : 



Basal granite-schist. — This, so far as known, is the fundamental 

 rock formation of the region. The granite has characteristically 

 a somewhat schistose or gneissic structure, thus showing evidence 

 of having been subjected to dynamic action or intense compres- 

 sion, and it may pass into a gneiss, or even a mica-schist, where 

 this action has been most energetic. On the other hand, it is 

 sometimes massive, showing no parallel structure planes, and 

 then is with difficulty distinguishable from the massive younger 

 granites, which are also of frequent occurrence in the region in 

 the form of dikes and intrusive masses cutting across older rocks. 

 As distinguished from the granites of the coastal region, which 

 are intrusive, these older granites are generally of reddish color 

 and crumbly nature, while the later ones are dark gray from the 

 abundance of hornblende as a constituent mineral. 



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 quartzitic rocks, 

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

 schists ; in some places they contain carbonaceous matter and 

 develop 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 



