vanhise.]  THE    CORDILLERAS.  275 
12,000  to  18,000  leet.  From  the  lowest  exposure  to  the  highest  there 
is  a  gradual  passing  from  the  structureless  granite  to  the  dark  mica- 
gneisses.  Among  eruptive  rocks  are  granites,  gabbros,  and  fflsite- 
porphyries.  The  Clark's  peak  ridge  is  thought  to  be  another  and  later 
series  of  rocks  than  those  of  the  Colorado  range. 
In  the  Medicine  bow  range,  above  the  hornblendic  and  dioritic, 
gneisses  and  schists  are  quartzitic  schists,  argillites,  and  limestones. 
The  gneisses  and  hornblende-schist  are  older  and  underlie,  in  apparent 
conformity,  the  quartzites. 
In  the  Park  range  the  crystalline  rocks  all  dip  to  the  west,  being  but 
half  of  an  anticline,  the  other  half  having  suffered  a  deep  downthrow 
which  has  only  left  traces  of  the  easterly  dips,  The  rocks  are  granite, 
gneiss,  hornblende-schists,  and  dioritoid  rocks,  with  a  limited  quantity 
of  quartzites,  there  being  no  eruptive  rocks,  unless  some  obscure  dio- 
ritic 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,8  in  1879,  describes  Eawlings  peak  as  consisting  of  a  meta- 
morphic  granite  nucleus  about  which  the  sedimentary  strata  are  qua 
quaversaily  arranged. 
Van  Hise,9  in  1889,  made  observations  upon  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  fiat  angle — 15°  or  20°.  The 
country  granite  is  cut  by  very  numerous  dikes  of  granite,  which  pro- 
ject 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  consider- 
able areas  of  massive  granite.  In  passing  toward  the  interior  of  the 
range  the  granite  becomes  less  plentiful  and  the  gneiss  more  laminated, 
passing  into  regularly  banded  gneiss,  which  appears  to  grade  by  imper- 
ceptible stages,  into  fine  grained  green  schist,  and  finally  into  black 
slate.  In  continuing  to  pass  from  east  to  west  quartzites  are  found, 
then  a  broad  belt  of  yellowish  white,  finely  granular  chert,  with  layers 
)f  cherty  limestone  sometimes  ferruginous.  About  a  mile  before  Medi- 
cine peak  is  reached  the  quartzites  appear.  These  continue  (often  con- 
glomeratic) to  beyond  Medicine  peak.  West  of  Medicine  peak  are 
igain  found    slates,  slate -conglomerates  carrying  abundant  pebbles 
