THE   CORDILLERAS.  803 
are  observed  in  layers.  In  many  instances  the  large  masses  of 
gneissoid  granite  string  out  like  veins  on  all  sides  from  the  center, 
and  these  veinlike  projections  break  up  into  these  nodules,  and  thus 
finally  disappear.  It  is  sufficiently  evident,  then,  that  these  are  not 
metamorphosed  pebbles,  but  concretions,  the  result  of  segregation, 
which  mark  the  formation  of  the  separate  layers  of  quartz,  feldspar, 
and  mica  in  gneiss  and  of  the  great  masses  of  coarse  granite  which 
occur  so  frequently  in  the  gneisses  and  schists.  Gneissoid  granite  is 
exceedingly  common.  It  often  occurs  in  the  gneiss  as  great  included 
masses  of  irregular  shape  or  in  elongate,  vein  form,  spreading  from 
a  center  and  throwing  out  seams  which  become  exceedingly  thin 
before  they  disappear.  In  each  instance  the  deposit  seems  to  bear  no 
relation  to  the  bedding  of  the  including  rock.  For  the  most  part, 
however,  it  is  found  entirely  displacing  the  gneiss  and  forming  the 
prevailing  rock  for  miles,  although  in  every  such  instance  it  occa- 
sionally changes  into  gneiss  for  short  distances.  Not  infrequently 
seams  of  granite  are  found  along  the  planes  of  cleavage.  This  gran- 
ite, which  may  be  termed  segregated  granite  to  distinguish  it  from  the 
granite  which  many  regard  as  eruptive,  is  coarsely  crystalline,  with  the 
feldspar  in  great  quantity,  while  the  proportion  of  mica  is  very  small. 
The  feldspar  varies  in  color  from  white  to  red,  and  the  rock  as  a 
whole  yields  readily  under  the  influence  of  the  weather.  The  gneis- 
soid granite  of  Taylor  River  exhibits  granite  of  both  the  eruptive 
and  the  metamorphic  varieties,  one  passing  into  the  other  with  no  line 
of  separation.  There  is,  then,  no  room  to  doubt  that  they  are  of 
common  origin  and  that  the  whole  is  metamorphic. 
The  gneiss  of  Tenmile  Creek  is  compact  and  might  be  mistaken  for 
a  quartzite.  Below  the  junction  is  an  immense  segregation  of  granite, 
thoroughly  veinlike,  interlacing  and  running  across  the  bedding  in 
every  conceivable  way,  but  not  persistent,  as  each  of  the  veins  tapers 
off  until  it  disappears.  In  the  canyon  of  the  Arkansas,  above  the 
junction  with  Tennessee  Creek,  is  a  gneiss  which  has  very  close  affin- 
ities to  the  granites  usually  called  eruptives.  It  passes  gradually  into 
a  micaceous  schist.  On  Trout  Creek  is  s}^enite,  succeeded  by  granite. 
which  gradually  assumes  a  gneissoid  structure  and  contains  frag- 
ments of  gneiss  from  6  to  20  inches  in  diameter  that  are  fragment al 
in  shape.  Their  presence  is  difficult  to  account  for.  If  the  granite  is 
eruptive,  these  might  be  included  fragments,  but  there  is  no  reason  to 
assign  any  such  origin  to  it,  for  its  gradual  passage  into  the  gneiss  is 
easily  traced.  On  Currant  ('reek,  at  the  west  side  of  South  Park. 
near  Mount  Lincoln,  at  Idaho  Springs,  and  on  the  east  side  of  South 
Park  massive  granite  or  syenite  is  seen  grading  into  gneiss  and  mica 
schist.  At  Chicago  Creek  coarsely  crystalline  granite  is  sharply 
separated  from  the  adjacent  gneiss,  the  junction  being  as  sharp  as 
between  a  trap  dike  and  the  adjoining  rock.    Some  of  the  porphyrii  lc 
