PRE-OEDOYICIAN 269 



Gneissic rocks, -ffhicli form a belt some 20 miles wide, separating these 

 two areas of metam.orpliic sediments, are composed chiefly of granitic 

 rocks. Biotite and hornblende are the prevalent dark silicates of these 

 gneissoid rocks, and most of them show evidence of shearing. Porphy- 

 ritic phases dominate, and in many places these have been altered to 

 augen gneisses. Mica schists are not imcommon within the gneissic belt, 

 and these may be included masses of sedimentary rocks. As in the sedi- 

 mentary rocks, greenstone schists are not uncommon, intercalated with 

 the gneisses, and also occur in a belt several miles in width along the 

 southern margin of the crystalline area. The gneisses are for the most 

 part undoubtedly igneous intrusives which have been deformed subsequent 

 to their crystallization. In addition to these, massive intrusives, proba- 

 bly representing a later injection, are not uncommon. These are litho- 

 logically similar to the gneissoid granites. Some aplite dikes, apparently 

 the most recent injections, deserve mention. 



This entire zone of metamorphic rocks strikes in a northwesterly- 

 southeasterly direction. The dip of the foliation and of the bedding in 

 tlie few places where it could be determined is prevailingly to the north. 

 There are two general systems of jointing which roughly parallel these 

 two directions. To such an extent have the rocks been deformed that it 

 is impossible to determine the structure, though it has been suggested 

 that the gneisses represent the eroded core of an anticlinal axis. At a 

 locality on the lower White river a concentric arrangement of the strata 

 was noted encircling in part, at least, one of the massive granite stocks 

 and at variance with the general structural lines. This suggests that 

 this granite was intruded after the crustal movements which determined 

 the general structures. 



There is practically no clew to the thiclaiess of the sediments here de- 

 scribed, but it probably includes man}^ thousand feet of strata. The lime- 

 stone member, so far as determined, is at least 1,000 feet thick. 



Summing up the evidence from this section, it would appear that a 

 great thickness of strata had here been deposited which was deeply buried, 

 metamorphosed, and intruded by both basic and acid igneous rocks. The 

 whole series was then intensely deformed by crustal movements, and later 

 a second intrusion of granitic rocks took place. 



The geological section exposed between the Yiikon river at Circle and 

 Fairbanks, on the Tanana, includes what is probably a t}q3ical example of 

 the metamorphic rocks of the western end of the field. Here the entire 

 series is made of sediments, with the exception of a few stocks of intru- 

 sive granites. Two rocks are typical of the entire belt — a schistose 

 quartzite and a mica schist, often graphitic. The quartzite is more typi- 



