THE   CORDILLERAS.  837 
throw  which  has  left  only  traces  of  the  easterly  dips.  The  rocks  are 
granite,  gneiss,  hornblende  schists,  and  dioritic  rocks,  with  a  limited 
quantity  of  quartzites,  there  being  no  eruptive  rocks,  unless  some 
obscure  dioritic  bodies  are  intrusive.  At  Jacks  Creek  is  a  bed  of  pure 
white  quartzite  50  feet  thick.  The  upper  members  of  the  Medicine 
Bow  and  Park  ranges,  somewhat  less  than  12,000  or  14,000  feet  thick, 
are  referred  to  the  Huronian  and  the  remaining  formations  to  the 
Laurentian. 
Endlich,109  in  1879,  describes  Rawlings  Peak  as  consisting  of  a 
metamorphic  granite  nucleus  about  which  the  sedimentary  strata  are 
quaquaversally  arranged. 
Van  Hise,3S  in  1889,  made  observations  on  the  Laramie  and  Medi- 
cine Bow  ranges. 
The  Laramie  Hills  at  Sherman,  where  most  structureless,  are  found 
to  have  alternate  bands  of  coarse  and  fine  material.  The  latter  are 
more  resistant  to  weathering  and  stand  out  as  ridges.  This  stratifica- 
.tion  or  flowage  or  foliation  structure  is  at  a  Hat  angle — 15°  or  20°. 
The  country  granite  is  cut  by  very  numerous  dikes  of  granite,  which 
project  above  the  ground  in  intersecting  ridges. 
The  course  of  travel  in  the  Medicine  Bow  Range  was  up  one  of  the 
branches  of  the  Laramie  River  to  Medicine  Peak,  and  over  this  range 
in  a  course  north  of  west,  across  the  strike  of  the  rocks,  down  Brush 
Creek.     Mill  Peak  was  visited. 
The  pre-Cambrian  rocks  first  found  are  banded  and  contorted 
gneisses,  varying  from  fine-grained  to  granitoid  varieties,  which  are 
cut  by  hornblendic  and  granitic  veins  or  dikes,  with  here  and  there 
considerable  areas  of  massive  granite.  Toward  the  interior  of  the 
range  the  granite  becomes  less  plentiful  and  the  gneiss  more  lami- 
nated, passing  into  regularly  banded  gneiss,  which  appears  to  grade 
by  imperceptible  stages  into  fine-grained  green  schist,  and  finally 
into  black  slate.  Farther  west  quartzites  are  found,  then  a  broad 
belt  of  yellowish  white,  finely  granular  chert,  with  layers  of  cherty 
limestone  sometimes  ferruginous.  About  a  mile  before  Medicine 
Peak  is  reached  the  quartzites  appear.  These  continue  (often  con- 
glomeratic) to  beyond  Medicine  Peak.  AYest  of  Medicine  Peak  are 
again  found  slates,  slate  conglomerates  carrying  abundant  pebbles 
of  white  quartz,  and  granites,  inters! ratified  with  quartzite.  Schistose 
and  massive  basic  rocks,  much  altered,  in  dikelike  forms,  arc  found 
in  the  clastic  series  precisely  as  in  the  gneissic  series.  In  many 
places  they  strike  approximately  parallel  to  the  inclosing  rock-. 
East  of  Medicine  Peak  the  rocks,  including  the  gneissoid  ami  clastic 
series,  have  a  dip  of  about  60°  to  80°  SE. ;  therefore  the  Medicine 
Peak  series  appears  to  underlie  the  gneissoid  series.  North  of  west, 
beyond  the  mountain,  the  dip  of  the   Medicine   Peak   -cries  becomes 
