786  PRE-CAMBRIAN    GEOLOGY   OF    NORTH   AMERICA. 
most  precipitous  face  of  30,000  feet  to  the  westward.  The  height  of 
the  range  was  then,  therefore,  from  17,000+  to  30,000+  feet. 
Passing  upward  from  the  Archean,  at  the  base  of  the  Paleozoic 
slates  are  Lower  Cambrian  slates  and  dark  argillites  and  interca- 
lated siliceous  schists  800  feet  thick;  above  this,  Cambrian  quartzite, 
an  immense  series  of  siliceous  and  arkose  rocks,  12,000  feet ;  and  above 
this,  Cambrian  calcareous  shales  of  variable  thickness  and  contain- 
ing Primordial  fossils,  75  to  600  feet.  This  great  thickness  is  found 
in  the  Cottonwood  area,  on  the  lower  half  of  Big  Cottonwood 
Canyon,  and  from  Big  Cottonwood  Canyon  in  a  northeasterly 
direction  across  the  spur  which  divides  the  waters  of  Cotton- 
wood Creek  from  Mill  Creek.  In  other  localities  the  Cambrian 
is  far  thinner  or  wholly  absent.  The  section  in  Big  Cotton- 
wood Canyon  comprises,  from  the  base  upward,  black  slates  and 
thinly  laminated  argillites  800  or  900  feet  in  thickness;  above 
these,  8,000  or  9,000  feet  of  mixed  siliceous  schists  and  argilla- 
ceous schists ;  and  above  these,  3,000  feet  of  true  quartzite  capped  by 
200  feet  of  schistose  rock,  quite  micaceous  toward  the  bottom,  and  at 
Twin  Peak  approaching  a  true  mica  schist.  At  the  second  section 
the  series  consists  of  four  members :  The  bottom  slates,  800  feet  thick ; 
varying  siliceous  and  argillaceous  schists,  containing  some  mica- 
bearing  zones,  8,000  or  9,000  feet  thick;  salmon-colored  and  white 
quartzites,  intercalated  with  dark  schists,  2,500  to  3,000  feet;  and 
the  capping  schists,  200  feet,  which  are  partly  argillaceous  and  cal- 
careous rocks  and  partly  mica-bearing  argillites.  Passing  up  the 
Little  Cottonwood,  the  successively  higher  members  of  the  Cambrian 
rest  against  the  granite  until  the  latter  rises  into  contact  with  the 
Silurian  limestone,  which  conformably  overlies  the  Cambrian.  Al- 
though a  careful  search  was  made'in  these  schists,  no  fossils  were 
found. 
Geikie,40  in  1880,  discusses  the  nature  of  the  pre-Cambrian  moun- 
tains of  the  Wasatch  and  the  eruptive  or  metamorphic  origin  of  the 
Cottonwood  granite.  That  this  granite  is  eruptive  is  maintained  on 
the  grounds  of  the  enormous  height  of  the  cliff  which  would  be  re- 
quired in  case  it  was  an  Archean  island;  that  if  it  were  an  old  shore 
line,  somewhere  granite  pebbles  would  be  found  to-day;  that  the 
granite  is  said  by  King  to  be  a  source  of  local  metamorphism ;  that 
there  are  porphyries  cutting  the  limestones,  probably  dependent  on 
the  granite;  and  that  it  is  exceedingly  improbable  that  there  was  a 
cliff  12  miles  high  which  has  been  turned  over  on  its  back,  as  required 
by  the  descriptions  and  sections  by  King.  All  these  difficulties  are 
overcome  by  regarding  the  granite  as  a  subsequent  intrusive  of  post- 
Carboniferous  age. 
Walcott,20  in  1886,  describes  the  Big  Cottonwood  Canyon  section 
of  Cambrian  rocks,  which  is  found  to  be  12,000  feet  thick.     It  consists 
