'anhise.]  THE    CORDILLERAS.  273 
ire  classed  under  granites,  gneiss,  mica  schist,  and  hornblende  schist, 
;he  first  covering  much  the  largest  area.  The  central  body  consists  of 
loarse  grained  granite.  Above  this,  and  forming  the  outer  edges,  dip- 
ring  east  and  west  away  from  the  main  mass,  occur  heavily  bedded  gran- 
toid  rocks.  At  the  north  and  south  ends  of  the  range  the  granites  grad- 
ually pass  into  well  defined  gneisses  and  schists,  there  being  the  most, 
gradual  transitions  from  the  massive  granites  to  the  thinly  laminated 
Whists.  Among  the  crystalline  rocks  is  a  variety  of  gabbro  in  the  re- 
gion of  Iron  mountain,  Chugwater,  and  Horse  creeks,  where  it  forms 
cnobs  and  knolls  protruding  through  the  granitoid  rocks.  At  Iron 
Mountain,  northiof  Chugwater  creek,  are  masses  of  titaniferous  iron 
ires  incased  in  the  granite.  No  large  bodies  of  eruptive  granites  were 
een  nor  eruptive  rocks  younger  than  the  Archean.  In  structure  the 
iaramie  hills  are  regarded  as  a  broad  anticline,  accompanied  by  many 
econdary  folds.  There  is  no  case  of  decided  nonconformity  in  the 
nt ire  series  of  beds,  and  their  uniform  character  indicates  that  they 
11  belong  to  one  division  of  the  Archean,  which  without  doubt  is  the 
|iaurentia.n.  The  sedimentary  rocks  of  the  eastern  foothills  every* 
here  rest  unconformably  upon  the  Archean  crystallines.  East  of 
able  mountain  is  the  only  outlying  mass  of  Archean  granite  occurring 
istward  of  the  sedimentary  foothills. 
The  second  great  range  of  the  Rocky  mountains — the  Medicine  bow— 
lc  the  Laramie  range,  is  made  up  almost  exclusively  of  Archean  crys- 
lline  rocks.     In   their  general  habit  they  resemble  the  formation  of 
le  eastern   range,   but  additional  varieties   are  found.     The  rocks 
iclude  granite,  gneiss,  hornblende-schist,  mica-schist,  dioritic  schist, 
ate,  argillite,  quartzite,  chert,  horn  stone,  conglomerate,  and  limestone, 
he  larger  bodies  of  true  granite  are   confined  to  the  southern  end  of 
ic  range,  where  it  is  closely   connected  with  the  Front  range  of  Colo- 
do.     Even  this  granite  shows  more  or  less  tendency  to  bedding,  the 
nstituent  minerals  being  arranged  in  parallel  layers.     From  Brush 
eek  northward  15  or  20  miles  are  light  colored  mica-gneisses  and  dark 
►rnblende-schists,  with  occasional  beds  of  vitreous  quartzite.     Medi- 
ae peak  is  a  mass  of  pure  white  quartzite  rising  2,000  feet  above  the 
Grounding  country.    The  main  ridge  has  a  trend  approximately  north 
°  east,  which  appears  to  be  the  strike  of  the  rocks.    The  dip  is  to  the 
&ward  at  a  high  angle.    While  no  accurate  measurements  could  be 
ide,  the  thickness  of  the  formation  is  certainly  not  under  2,000  feet. 
ie  quartzite  is  white,  compact,  and  brittle,  with  a  uniform  texture,  and 
traversed  by  thin  iron   seams.     Near  the  base  of  the  formation  the 
artzite  is  mterstratified  with  beds  of  conglomerate,  the  pebbles  being 
quartz  and  many  of  them  having  been  pressed  and  elongated  in  the 
eetion  of  the  strata.     The  formation  is  cut  by  dikes  of  dark  intru- 
e  rocks  which  are  probably  diorites.     At  the  head  of  the  northern 
inches  of  French  creek,  conformably  under  a  quartzite,  is  a  series  of 
lly  laminated  dark  argillaceous  slates  and  schists,  which  dip  east- 
Bull.  80 18 
