2D4  PRE-CAMBRIAN    ROCKS    OF    NORTH   AMERICA.  [bull. 86. 
sedimentary  rocks  abuts  against  its  northern  side,  the  principal  mem- 
bers sweeping  around  and  in  part  covering  its  eastern  portion  and  con- 
tinuing southward  in  an  almost  horizontal  position.  There  is  no  especial 
disturbance  of  these  beds  in  contact  with  the  granite  so  far  as  observed. 
Neither  are  any  masses  or  fragments  of  sedimentary  rock  included  in 
the  granite.  Regional  metamorphism  exists  in  changing  the  sandstone 
to  quartzite  and  limestone  to  marble,  and  porphyry  dikes  cross  the 
sedimentary  strata,  but  these  have  no  necessary"  connection  with  the 
granite.  If  the  granite  is  an  intrusive  mass  cutting  the  Carboniferous 
strata,  it  is  necessary  to  believe  that  it  Las  assimilated  or  eaten  np  over 
500  cubic  miles  of  sedimentary  rocks.  If  it  has  done  this  it  has  left  no 
trace  of  fusion  in  the  adjoining  rock,  and  without  showing  in  its  own 
structure  and  position  any  marked  variation  from  that  of  a  normal 
rock.  It  is  farther  difficult  to  understand  where  the  great  supply  of 
heat  is  to  be  obtained  to  do  this  work. 
Walcott,35  in  1889,  places  the  Olenellus  horizon  at  the  base  of  the 
Cambrian  and  the  great  series  of  conformable  siliceous  rocks,  11,000 
feet  thick,  below  this  zone  as  pre-Cambrian  or  Algonkian  rocks. 
Van  Hise,9  in  1889,  made  an  examination  of  several  canyons  of  the 
Wasatch.  While  the  Cottonwood  granite  mass  has  a  regular  struc- 
ture which  is  seen  in  the  great  cliffs,  it  is  apparently  completely  mass- 
ive, even  in  huge  blocks.  The  apparent  lamination  is  due  to  the 
parallel  arrangement  of  the  minerals,  which  have  crystallized  with 
their  longer  axis  in  the  same  direction.  The  lamination  of  the  granite 
is  not  more  marked  than  is  the  case  with  some  of  the  unmistakable 
gabbros  of  the  Keweenaw  series.  An  examination  of  the  granite  in 
thin  section  shows  that  the  feldspars  have  universally  a  well  marked 
and  beautiful  zonal  structure,  such  as  is  known  only  in  eruptive  rocks. 
Sections  of  some  of  the  black  bowlder-like  areas  so  common  in  the  gran- 
ite differ  from  the  mass  of  granite  only  in  that  hornblende  is  more  plen- 
tiful. The  main  Cottonwood  mass  and  Clayton  peak  granite  are 
identical  in  character.  In  places  the  sedimentary  rocks,  and  especially 
the  limestones,  are  exceedingly  metamorphosed.  In  one  place,  near 
the  head  of  Little  Cottonwood  canyon,  at  a  contact  with  the  granite  it 
was  exceedingly  difficult  in  the  field  to  tell  where  the  white  granite 
ended  and  the  crystalline  marble  began.  In  thin  section  there  is  no 
difficulty  in  separating  the  marble  and  granite,  so  that  there  is  no 
transition  here.  However,  the  granite  became  a  true  porphyry  in 
places,  a  fact  difficult  of  explanation  uidess  it  is  regarded  as  a  later 
intrusive. 
In  the  Cambrian  of  the  Little  Cottonwood  was  found  a  conglomerate 
which  carries  unmistakable  granite  pebbles  and  black  fragments  which 
were  thought  in  the  field  to  resemble  the  black  hornblendic  areas  so 
often  mentioned  in  the  granite.  These,  however,  when  examined  in 
thin  section  were  found  to  be  entirely  unlike  those  contained  in  the 
granite.  The  granite  fragments  are  small  and  sparse  and  do  not  ap- 
pear to  be  litholog'ically  like  the  massive  granite  of  the  Cottonwood. 
