THE   CORDILLERAS.  799 
Wheeler  Peak,  having  an  elevation  of  12,000  feet,  is  formed  in  part 
of  pre-Cambrian  quartzite  standing  nearly  perpendicular.  The 
dip  is  to  the  south-southwest,  gradually  decreasing  to  15°,  and  Lower, 
Middle,  and  Upper  Cambrian,  Ordovician,  and  Lower  Carboniferous 
strata  are  exposed  before  the  range  dies  out  to  the  south.  On  the 
northern  and  western  slopes  of  Wheeler  Peak  there  is  exposed  a  small 
area  of  granite  and  schistose  rocks  which  may  represent  the  basement 
complex.  The  granite  is  intruded  by  several  coarse-grained  dikes 
of  quartz  porphyry.  It  varies  from  fine  to  coarse  grain  and  on  the 
western  slope  in  the  vicinity  of  the  hubnerite  mine  has  a  rudely 
bedded  structure  parallel  to  the  dip  of  the  sedimentary  beds. 
On  the  northern  slope  of  the  peak  there  is  exposed  from  100  to  200 
feet  of  conglomerate  composed  of  subangular  masses  of  granite  and 
quartz  porphyry.  At  the  base  the  material  varies  from  the  size  of 
an  egg  up  to  6  or  8  inches  in  diameter.  The  material  rapidly  becomes 
smaller  and  more  rounded  and  grades  into  quartzite.  This  is  suc- 
ceeded by  soft,  argillaceous,  and  siliceous  schists,  well  exposed  along 
the  Placer  Company's  ditch.  These  are  followed  by  other  quartzites, 
which  underlie  shales  in  which  was  found  Olenellus. 
The  determination  of  the  Archean  age  of  these  granites  and  schists 
rests  on  the  occurrence  of  what  certainly  appears  to  be  a  basal  con- 
glomerate composed  of  granitic  material.  This  is  the  only  occur- 
rence of  the  kind  in  the  Great  Basin  region  known  to  Weeks.  There 
are  many  other  localities  in  which  sedimentary  rocks  varying  from 
Cambrian  to  Carboniferous  are  in  contact  with  granitic  masses.  In 
the  vicinity  of  Wheeler  Peak,  however,'  no  rocks  except  the  pre- 
Cambrian  quartzite  was  seen  in  contact  with  this  granite. 
SUMMARY   OF  PRESENT  KNOWLEDGE. 
In  the  basin  ranges  of  the  northern  part  of  Nevada  the  geologists 
of  the  Fortieth  Parallel  Survey  mapped  and  described  a  number  of 
granitic  areas  as  pre-Cambrian.  It  is  probable  that  some  at  least 
of  these  supposed  pre-Cambrian  granites  are  later  than  the  pre- 
Cambrian  and  are  intrusive  into  the  sediments.  In  the  Star  Peak 
Range,  for  example,"  granite  which  the  Fortieth  Parallel  geologists 
considered  Archean  has  been  lately  shown  to  be  post-Triassic  and 
probably  post-Jurassic.  The  state  of  knowledge  in  regard  to  the 
whole  area  of  the  Fortieth  Parallel  maps  is  therefore  not  such  as 
to  allow  one  to  accept  their  conclusions  without  further  examina- 
tion. In  the  areas  to  the  south  of  that  mapped  by  the  Fortieth 
Parallel  Survey  Spurr  maps  a  considerable  number  of  granitic  areas, 
but  concludes  that  all  of  them  are  later  than  the  pre-Cambrian,  with 
tt  Louderback,  G.  i>..  Basin  range  structure  of  the  Humboldl  region:  Bull.  Geol.  Soc. 
America,   vol.    15,    L904,   p.    318. 
