298  PRE-CAMBRTAN    ROCKS    OF    NORTH    AMERICA.  Lbull.86. 
than  this,  but  the  steepness  increasing  to  more  than  this  amount  in 
the  upper  slopes  of  the  mountains.  If  the  strata  are  considered  as; 
originally  deposited  with  a  dip  of  10°  away  from  the  mountains,  this 
would  reduce  the  average  slope  of  the  mountain  to  35°.  Even  with 
these  allowances  it  is  to  be  said  that  a  buried  slope  of  this  degree  of  i 
steepness,  which  is  covered  by  the  debris  of  an  advancing  shore  line 
with  the  additional  altitude  of  the  mountains  required  in  order  to  fur- 
nish the  surrounding  debris,  seems  highly  improbable, 
So  far  as  facts  Avere  observed  by  the  writer  bearing  upon  the  ques- 
tion of  the  character  of  the  Cottonwood  granite,  they  tend  in  the  same 
direction  as  Geikie's  conclusion.  If  this  granite  is  supposed  to  be  a 
later  intrusive,  the  metamorphism  of  the  sedimentary  beds  noted  by 
the  Fortieth  Parallel  surveyors,  the  presence  in  them  of  the  granite- 
porphyry  dikes,  the  sharp  contacts  and  manner  in  which  this  eruptive 
mass  cuts  across  the  Archean  crystalline  schists  at  the  foot  of  Little 
Cottonwood  canyon,  are  all  explained.  In  this  connection  it  is  to  be] 
said  that  so  far  as  known  the  zonal  character  of  the  feldspars  so  well 
displayed  by  this  rock  is  nowhere  found  in  such  perfection  in  such, 
ancient  rocks  as  the  pre-Cambrian,  although  this  point  may  not  be  one 
of  much  weight.  If  the  granfte  is  an  eruptive,  the  contained  round 
black  areas  described  by  nearly  all  observers,  and  early  taken  as  evi- 
dence that  the  granite  is  metamorphic,  must  either  be  partially  absorbed 
fragments  caught  by  the  eruptive  mass,  or  else  segregations  of  horn- 
blende which  formed  at  the  time  of  the  crystallization  of  the  rocks.  If 
the  post-Cambrian  eruptive  origin  of  the  granite  be  accepted,  there  is 
no  necessity  for  considering  the  Wasatch  fault  of  such  great  magni- 
tude and  the  pre-Cambrian  mountains  of  almost  incredible  steepness 
and  height. 
Upon  the  other  side,  it  must  be  said  that  if  the  estimate  given  by 
Emmons  of  the  amount  of  sedimentary  material  of  necessity  absorbed 
by  the  eruptive  theory — 500  cubic  miles — be  correct,  it  is  almost  an 
equal  strain  upon  belief.  In  this  connection  the  question  arises  whether 
a  closer  study  of  the  region  will  not  show  that  the  Cambrian  strata 
have  a  quaquaversal  arrangement  about  the  Cottonwood  granite  as  do 
the  Silurian,  Devonian,  and  Carboniferous  strata,  'the  edges  of  the 
Cambrian  being  uptilted,  and,  like  them,  the  structure  as  a  whole  being 
batholitic.  While  it  may  be  found  that  the  entering  granite  has  ab- 
sorbed a  portion  of  the  Cambrian  qnartzite,  the  amount  thus  assimi- 
lated  would  be  comparatively  small.  On  this  hypothesis  the  present 
distribution  of  the  rocks  would  be  explained  by  intrusion  and  erosion. 
The  relations  of  the  elastics  to  the  much  larger  Fannin gton  and 
northern  Archean  areas,  unquestionably  older  than  the  sedimentaries, 
are  such  as  to  require  comparatively  low  and  relatively  gentle  slopes 
and  pre-Cambrian  mountains  not  higher  than  from  12,000  to  18,000 
feet;  that  is,  mountains  not  higher  than  those  at  present  existing  adja- 
cent to  the  ocean. 
