258  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
probably  coincident  with  the  elevation  of  the  mountains.  It  is  impossi- 
ble to  estimate  with  any  degree  of  accuracy  the  thickness  of  the  Archean 
schists,  as  they  are  highly  inclined  and  distorted,  and  in  their  present 
metamorphosed  and  denuded  condition  it  can  not  be  determined  whether 
they  are  the  remnants  of  several  great  folds  or  whether  they  are  the 
broken  strata  of  one  vast  fold,  though  the  latter  seems  to  be  the  more 
probable  structure 5  in  which  case  the  total  thickness  of  the  Archean 
strata  must  be  more  than  100,000  feet,  about  25  miles.  The  examina- 
tion showed  no  evidence  of  the  duplication  of  any  parts  of  the  Archean 
strata,  and  it  is  presumed,  if  a  repeated  folding  has  taken  place,  that  it 
did  not  occur  within  the  area  exposed  in  the  hills. 
The  Archean  sedimentaries  are  divided  into  two  groups,  schists  and 
slates.  The  schists  include  quartzose,  garnetiferous,  ferruginous  and 
micaceous  varieties,  together  with  some  gneiss,  chloritic,  talcose,  and 
hornblendic  schists,  and  quartzite.  The  schists  are  occasionally  stau- 
rolitic.  The  whole  series  is  coarse  in  texture,  highly  crystalline,  and 
contains  seams  or  veins  of  quartz  conformable  with  the  stratification 
and  having  a  lenticular  form.  The  slates  are  distinguished  from  the 
schists  mainly  by  their  fine  and  compact  texture,  although,  as  shown 
by  Caswell,  their  ultimate  mineral  composition  is  similar.  They  are 
mainly  micaceous  clay-slate,  siliceous  slate  and  quartzite,  which  are 
sometimes  associated  with  specular  oxide  of  iron.  On  Box  Elder  creek 
is  a  ridge  400  feet  in  height,  which  is  a  vast  deposit  of  siliceous  hema- 
tite and  resembles  the  siliceous  hematites  of  the  lake  Superior  region- 
The  quartzites  of  the  two  classes  are  similar.  The  mica-schist  passes 
into  chlorite-schist,  siliceous  schist  and  quartzite.  The  schists  of  the 
southern  part  of  the  hills  are  associated  with  an  area  of  highly  field! 
spathic  granite  which  culminates  in  the  region  of  Harney  peak.  On 
the  outskirts  of  this  district  are  many  smaller  masses  of  granite.  So 
far  as  the  structure  was  made  out,  each  of  the  bodies  has  a  lenticular 
shape  and  is  intercalated  among  the  strata  of  the  schists.  No  granite 
is  found  associated  with  the  slate.  The  general  strike  of  the  rocks  is 
northwest  and  southeast.  The  topography  shows  that  there  is  a  series; 
of  ridges  in  this  direction  which  mark  the  position  of  the  particularly 
hard  layers  such  as  quartzites.  The  schists  are  found  in  the  western 
and  southwestern  parts  of  the  area  and  the  slates  in  the  east  and  north 
east. 
Between  these  two  groups  Jenney  noted  a  distinct  discordance  of  dip 
on  Castle  creek,  but  in  the  absence  of  corroborative  observations  thi 
unconformity  of  the  two  series  can  not  be  insisted  upon.  The  divisio] 
of  the  system  into  two  series  is  then  based  on  lithological  difference 
purely  and  is  warranted  on  this  ground.  The  lithological  difference  is 
however,  more  a  mineral ogical  one  than  chemical,  being  probably  du 
to  a  difference  in  metamorphism.  The  apparent  discordance  discovers 
by  Jenney,  and  this  lithological  difference,  gives  strong  support  to  th 
view  that  the  slate  and  schist  periods  were  separated  by  an  interval 
