THE   CORDILLERAS.  835 
rocks  of  the  eastern  foothills  everywhere  rest  unconformably  upon 
the  Archean  crystallines.  East  of  Table  Mountain  is  the  only  out- 
lying mass  of  Archean  granite  occurring  eastward  of  the  sedimentary 
foothills. 
The  second  great  range  of  the  Rocky  Mountains — the  Medicine 
Bow — like  the  Laramie  Range,  is  made  up  almost  exclusively  of 
Archean  crystalline  rocks.  In  their  general  habit  they  resemble  the 
formation  of  the  eastern  range,  but  additional  varieties  are  found. 
The  rocks  include  granite,  gneiss,  hornblende  schist,  mica  schist,  dio- 
ritic  schist,  slate,  argillite,  quartzite,  chert,  hornstone,  conglomerate, 
and  limestone.  The  larger  bodies  of  true  granite  are  confined  to  the 
southern  end  of  the  range,  where  it  is  closely  connected  with  the 
Front  Range  of  Colorado.  Even  this  granite  shows  more  or  less 
tendency  to  bedding,  the  constituent  minerals  being  arranged  in  par- 
allel layers.  From  Brush  Creek  northward  15  or  20  miles  are  light- 
colored  mica  gneisses  and  dark  hornblende  schists,  with  occasional 
beds  of  vitreous  quartzite.  Medicine  Peak  is  a  mass  of  pure  white 
quartzite  rising  2,000  feet  above  the  surrounding  country.  The  main 
ridge  has  a  trend  approximately  N.  20°  E.,  which  appears  to  be  the 
strike  of  the  rocks.  The  dip  is  to  the  east  at  a  high  angle.  While  no 
accurate  measurements  could  be  made,  the  thickness  of  the  forma- 
tion is  certainly  not  under  2,000  feet.  The  quartzite  is  white,  com- 
pact, and  brittle,  with  a  uniform  texture,  and  is  traversed  by  thin 
iron  seams.  Near  the  base  of  the  formation  the  quartzite  is  inter- 
stratified  with  beds  of  conglomerate,  the  pebbles  being  of  quartz,  and 
many  of  them  having  been  pressed  and  elongated  in  the  direction  of 
the  strata.  The  formation  is  cut  by  dikes  of  dark  intrusive  rocks 
which  are  probably  diorites.  At  the  head  of  the  northern  branches 
of  French  Creek,  conformably  under  a  quartzite,  is  a  series  of  thinly 
laminated,  dark  argillaceous  slates  and  schists,  which  dip  eastward 
into  the  mountain.  Below  these  are  quartzose  argillites.  which 
are  again  underlain  by  crystalline  schists.  Mill  Peak,  north  of  east 
from  Medicine  Peak,  has  at  its  base  a  white  quartzite,  which  is  over- 
lain by  a  body  of  red  conglomerate  resembling  the  red  jasper  con- 
glomerate of  the  Huronian  series  of  Lake  Huron.  Above  this  is 
amorphous  quartzite,  and  the  peak  is  capped  by  white  and  gray  sili- 
ceous limestone.  The  prevailing  dips  at  Mill  Peak  are  to  the  west, 
while  those  at  Medicine  Peak  are  to  the  east,  indicating  that  there  is  a 
broad  synclinal  fold  between  the  two.  A  striking  characteristic  of 
the  entire  series  is  the  banded  and  laminated  appearance  of  the  con- 
stituent minerals.  The  Archean  series  of  the  Medicine  Bow  Range 
present  many  marked  features  analogous  to  the  Huronian  formation 
on  the  shores  of  Lake  Huron  and  in  Canada,  as  well  a-  to  various 
localities  throughout  the  Appalachian  chain;  and  they  are  -with 
considerable   hesitation,   however — recognized    as  of    Huronian    age, 
