THE   CORDILLERAS.  809 
Colorado  and  lying  between  the  meridians  105°  30'  and  105°  45'  west 
longitude  and  the  parallels  30°  30'  and  30°  45'  north  Latitude. 
Georgetown,  which  is  situated  in  the  northwest  corner  of  the  quad- 
rangle, is  40  miles  west  of  Denver.  The  area  lies  on  the  east  slope 
of  the  Front  Range. 
On  Trout  Creek,  30  miles  southeast  of  the  southeast  corner  of  the 
quadrangle,  little-metamorphosed  Cambrian  sediments  lie  unconform- 
ably  on  a  rock  complex  showing  approximately  the  same  amount  of 
metamorphism  as  the  rocks  of  the  Georgetown  quadrangle,  and  pre- 
sumably, in  a  general  way,  a  southward  extension  of  them.  Hence 
the  latter  rocks  are  provisionally  designated  pre-Cambrian. 
The  rocks  of  the  Georgetown  quadrangle  include  schists  and 
gneisses,  much  folded  and  often  crenulated,  which  are  intimately 
injected  by  dikes  and  irregular  masses  of  a  series  of  igneous  rocks. 
The  structure  of  these  plutonic  rocks  varies  from  gneissoid  to  mass- 
ive ;  and  the  rocks  belong  chiefly  to  the  granitic  and  dioritic  families. 
These  gneisses  and  igneous  rocks  are  cut  by  much  younger  dike  rocks. 
So  complex  is  the  injection  that  five  or  six  formations  often  outcrop 
in  a  single  ledge,  and  inclusions  within  inclusions  are  common. 
The  succession  of  different  pre-Cambrian  formations,  thus  far 
recognized,  is  as  follows: 
1.  Idaho  Springs  formation — schists  and  crystalline  rocks  (sedimentary). 
2.  Hornblende  gneiss  (mashed  diabase). 
3.  Quartz  monzonite  gneiss. 
4.  Gneissoid  granite. 
5.  Quartz  monzonite. 
6.  Quartz-bearing  diorite  and  hornblendites. 
7.  Rosalie  granite  (biotite  granite). 
8.  Silver  Plume  granite  (biotite  granite). 
9.  Pegmatite  and  associated  granite  and  granite  porphyry. 
The  oldest  of  the  gneisses,  and  indeed  the  oldest  formation  in  the 
Georgetown  quadrangle,  is  a  series  of  intergrading  biotitic  and  silli- 
manitic  schists  and  gneisses  and  quartz  gneisses.  This  series  has 
been  intensely  folded  and  shattered  by  later  intrusion.  An  excellent 
schistosity,  with  prevailing  northerly  dip,  has  been  developed.  Cer- 
tain phases  of  this  gneiss  scries  show  inclusions,  which  vary  in  shape, 
in  proportion  to  the  amount  of  stretching  they  have  undergone  during 
the  process  of  metamorphism,  from  a  normal  pebble-like  form  to 
those  which  are  more  and  more  elongated  till  they  appear  a-  mere 
streaks  or  pencils.  The  material  of  those  inclusions  is  uniform  and 
appears  in  the  field  to  be  quartz,  although  the  microscope  shows  also 
the  presence  of  sillimanite.  It  is  regarded  as  probable  that  such 
rocks  represent  metamorphosed  conglomerates;  and  from  this  it  i>  a 
natural  step  to  regard  other  phases  of  (lie  gneisses  of  very  similar 
composition  to  the  conglomeratic  phases  and  intcrbcddcd  with  them. 
