DESCRIPTIVE GEOLOGY. 17 
STRATIGRAPHY. 
METAMORPHICS ( PALEOZOIC; OR PRE-PALEOZOIC) . 
The rocks characterized as metamorphics were, for the most part, 
originally sediments, including sandstone, shale, and a small propor- 
tion of limestone. This material, through various processes included 
under the term metamorphism, has all been more or less changed 
until it now exhibits characteristics of structure and composition 
that differ greatly from those of the original rocks. Of the new 
minerals formed mica is the most generally distributed, and its ex- 
tensive development in nearly all the rocks under consideration has 
imparted a schistose character to them. They split readily along 
the planes parallel to the cleavage of the micas, and are easily recog- 
nized as schists by the miners Avho are working in the areas where 
this older group forms the bed rock. These rocks in places have 
been but partly metamorphosed and retain many characteristics of 
their sedimentary origin. Some of the quartzitic rocks, for example, 
contain but a small proportion of mica and are but slightly schistose. 
In a more advanced stage of metamorphism, however, there is a more 
or less complete rearrangement of material by solution and recrys- 
tallization, until the rock that owed its characteristics originally to 
sedimentation becomes one with most of its characteristics clue to 
entirely different processes. In the same way igneous rock, orig- 
inally formed by consolidation of molten material, may lose the 
structure, texture, and mineral composition characteristic of its 
mode of origin and take on those of metamorphism. 
In a region of metamorphic rocks, therefore, rocks of both sedi- 
mentary and igneous origin may be present as schists. In the eastern 
part of the Yukon-Tanana region, metamorphosed igneous intrusives 
form an essential part of the metamorphic assemblage. In the 
western part of the area, however, including the Birch Creek and 
Fairbanks regions, the schists are predominantly of sedimentary 
origin and include quartz schists, quartz-mica schists, feldspathic, 
carbonaceous, garnetiferous, and hornblendic schists, and crystalline 
limestone. 
The structure of the rocks is complex. The trend or strike, where 
the bedding is Inclined at a considerable angle or is vertical, is in 
most places northeast and southwest. The rocks are so closely folded 
in some places that the sides of the folds are approximately parallel, 
and the planes of their axes are commonly so nearly horizontal as 
to make the whole assemblage appear level or horizontal. Quartz 
veins are common throughout the schist areas. They are generally 
small, occurring as stringers from a fraction of an inch up to a few 
24304— Bull, 337— OS 2 
