THE   CORDILLERAS.  827 
dominant,  but  in  some  places  along  the  eastern  border  this  gneissoid 
granite  varies  into  a  great  series  of  completely  crystalline  schists. 
Taking  schistose  structure  for  bedding,  King  estimated  the  thickness 
of  the  series  at  Clear  Creek  to  be  25,000  feet.  However,  the  structure 
is,  without  doubt,  foliation,  and  the  figure  given  only  serves  to  show 
the  .extent  of  the  schistose  area. 
In  the  district  of  Coal,  Boulder,  and  Thompson  creeks,  on  the 
east  side  of  the  range,  is  a  narrow  belt  of  mica  schists,  quartzites, 
quartz  schists,  and  schist  conglomerates,  containing  abundant  pebbles 
of  white  quartz  and  fewer  of  gneissoid  granite,  the  thickness  being 
unknown;  but  if  foliation  corresponds  to  bedding  the  maximum 
thickness  is  at  least  1,000  feet.  The  clastic  character  of  this  series  is 
unmistakable.  The  higher  members  of  the  series  are  quartz  schists 
alone,  but  lower  down  mica  schists  and  quartz  schists  are  interstrati- 
fied  with  beds  of  conglomerate.  At  the  base  the  mica  schists  become 
interlaminated  with  gneiss,  which  rock  becomes  more  and  more  abun- 
dant, and  upon  Coal  Creek  pegmatitic  granite  veins  cut  them. 
Nowhere  between  the  clastic  and  the  gneissoid  series  has  any  struc- 
tural discordance  been  discovered,  there  appearing  to  be  a  somewhat 
rapid  gradation  between  them.  The  series  has  been  subjected  to  such 
intense  dynamic  action  as  to  almost  entirely  obliterate  the  clastic  char- 
acters, with  the  exception  of  the  schist  conglomerates,  and  even  in 
these  the  matrix  is  a  completely  crystalline  schist,  the  only  unmis- 
takable evidence  of  the  sedimentary  character  being  the  greatly 
mashed  and  flattened  pebbles.  By  careful  observation,  however,  the 
sedimentary  schists  may  be  sharply  delimited  from  the  gneisses.  Be- 
cause of  the  lithological  separation  which  is  possible,  the  presence  of 
schist  conglomerate  near  the  base  of  the  clastic  series,  and  the  fact 
that  the  clastic  series  is  not  cut  by  pegmatite  intrusives  to  the  same 
extent  as  the  lower  gneissoid  series,  it  is  believed  that  the  sediments 
rest  unconformably  upon  the  gneisses  and  granites.  In  this  area  the 
next  overlying  series  is  Triassic,  but  elsewhere  in  the  Front  Range 
little-metamorphosed  upper  Cambrian  sediments  overlie  the  crys- 
talline rocks.  For  this  reason  the  crystallines  may  be  either  pre- 
Cambrian  or  early  Cambrian,  but  from  their  lithological  character  it 
is  believed  that  they  are  pre-Cambrian.  The  gneisses  and  granites 
are  further  classified  as  Archean  and  the  quartzite  series  unconform- 
ably above  as  Algonkian. 
In  the  Georgetown  quadrangle,  west  of  Denver,  Spun-  and  Ball 
find  schists  and  gneisses  cut  by  seven  different  intrusions.  The  oldest 
schist,  named  the  Idaho  Springs  formation,  is  believed  to  represent 
a  much  altered  sediment.  The  associated  hornblende  gneiss  represents 
a  metamorphosed  impure  limestone.  The  rocks  are  provisionally 
assigned  to  the  pre-Cambrian  because  Cambrian  sediments  are  found 
to  overlie  them  to  the  southeast  of  the  quadrangle.    The  oldest  gneiss 
