268 BROOKS AND KINDLE PALEOZOIC ROCKS OF UPPER YUKON 



out their probable genesis. In addition to these, massive granites are not 

 imcommon. There are also some rocks having a primary gneissic struc- 

 ture whose genesis is uncertain. Many greenstones and greenstone schists 

 are associated with the metamorphic sediments, and are probably for the 

 most part derived from intrusives. 



There is little clew to the age of the granitic rocks. Prindle has de- 

 scribed a granite stock cutting Cretaceous sediments in the Eampart re- 

 gion. This probably represents a later phase of injection than that repre- 

 sented by the gneisses. In the foothills of the Alaska range south of the 

 Tanana there are extensive exposures of a gneissoid rock which were first 

 regarded as basal, but which has been proved by Prindle*" to be an altered 

 rh3rolite associated with cherts and slates, probably of Silurian or Devo- 

 nian age. These facts suggest that the other gneisses of similar character 

 may be of post-Ordovician age. 



The character of the metamorphic and crystalline rocks of the eastern 

 part of the area is well illustrated in the highlands traversed by the lower 

 White river. Here a belt of gneissoid and granitic rocks is flanked on 

 the north and south by metamorphic sediments — a relation which was 

 interpreted as indicating that the former were a part of the Basal Com- 

 plex, but which later investigations have shown to be for the most part " 

 altered igneous rocks intrusive in the sediments. It is not to be denied, 

 however, that there are some gneisses whose structure is primary, and it 

 will remain for more detailed investigations to determine what the genesis 

 of these rocks is and their relation to the sediments and known intrusives. 



The section extends from the Yukon river to the White-Tanana flat, a 

 distance of about 50 miles. The rocks lying north of the gneisses and 

 forming the type section of the "Nasina series," as originally defined, are 

 dominatingly made up of qiiartz and mica schists, often finely laminated. 

 One belt of white crystalline limestones, with intercalated phyllites carry- 

 ing some graphite, is exposed about 10 miles from the mouth of White 

 river, and this appears to be an integral part of the metamorphic series. 

 Greenstone schists, largely made up of secondary minerals, and whose 

 color is due to the presence of actinolite and chlorite, which are probably 

 altered igneous rocks, occur widely distributed, but do not form any con- 

 siderable part of the mass of the metamorphic rocks. 



The southern belt of metamorphic sediments of this section are not as 

 highly crystalline as those above described. They are dominatingly made 

 up of micaceous and graphitic schists and phyllites. These rocks locally 

 become calcareous, grading into impure schistose limestones. 



'" L. M. Prindle : The Fairbanks and Kampai-t quadrangles. Bulletin no. 3.37, U. S. 

 Geological Survey, p. 28. 



