prindli:.]        GOLD  PLACERS  OF  FAIRBANKS  DISTRICT,    ALASKA.  67 
The  country  is  composed  of  ridges  and  valleys.  The  broad  backed 
ridges  have  an  altitude  of  about  2,500  feet  above  sea  level,  and  to  an 
observer  at  this  height  all  seem  to  attain  about  the  same  general  level 
with  the  exception  of  isolated  dome-like  elevations  of  somewhat  greater 
height.  They  slope  gradually  toward  the  Tanana  and  break  off  more 
or  less  abruptly  to  the  extensive  lowland  of  the  latter  stream.  This 
southern  edge  of  the  Yukon-Tanana  block  of  the  plateau  is  much 
dissected  by  numerous  minor  streams,  whose  valleys  are  sunk  to  a 
depth  of  1,200  to  1,000  feet  below  the  comparatively  even  sky  line  of 
the  ridges.  The  narrow  V-shaped  gulches  of  their  headwaters  widen 
to  open  valleys,  often  bounded  on  the  one  side  by  precipitous  slopes 
and  on  the  other  by  long,  gently  sloping  spurs  from  the  main  ridge 
to  the  stream  valleys. 
The  moss-covered  surface  of  the  highest  divides  gives  place  to  that 
of  the  dwarf  lurch  and  alder  on  the  lower  ridges,  and  the  growth  of 
small  spruce  on  the  slopes  and  stream  bottoms  is  abruptly  terminated 
by  the  willow-covered  strip  which  follows  the  waterway.  Along  the 
upper  slopes  and  spurs  are  scattering  poplar  and  birch,  which  lower 
down  cover  the  hillsides  bounding  the  Tanana  Valley. 
GENERAL  GEOLOGY. 
As  this  region  in  its  surface  features  closely  resembles  that  on  the 
Yukon  side  of  the  divide,  so  the  rocks  are  similar  in  character  and 
form  a  part  of  the  series  already  proved  to  be  of  economic  importance 
there. 
The  bed  rock  of  the  gold-placer  diggings  of  the  Birch  Creek  mining 
district  is  composed  of  an  essential^  schistose  series  of  metamorphosed 
sedimentary  rocks,  varying  from  a  comparatively  massive  quartzite  to 
a  quartzite-sehist  and  mica-schist.  These  have  been  cut  by  intrusive 
rocks,  most  of  which  are  of  a  granitic  character.  This  same  series  of 
more  or  less  completely  schistose  quartzites  and  mica-schists  was  found 
to  extend  southward  from  the  Birch  Creek  region  to  the  diggings  of 
the  Fairbanks  mining  district.  Associated  with  these  schists  in  the 
Fairbanks  region  are  hornblende-schists,  gneiss,  and  granitic  rocks  of 
probably  intrusive  character.  The  distribution  of  the  igneous  rocks 
has  not  yet  been  determined. 
The  area  from  the  Yukon  to  the  Tanana  is  closely  folded  and  the 
rocks  often  exhibit  beautiful  illustrations  of  overthrust  folds  in  a 
nearly  horizontal  position.  The  apparent  dips  are  generally  low  and 
give  the  rocks  the  appearance  of  gently  folded  strata. 
The  strike  is  variable,  but  the  general  structure  seems  to  run  north- 
east-southwest. Quartz  stringers  occur  in  the  schists,  but  the  pro- 
portion of  quartz  in  the  gravels,  as  in  the  Birch  Creek  district,  is 
small. 
