276  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
of  white  quartz  and  granites  and  interstrati  tied  with  quartzite.  Schis- 
tose and  massive  basic  rocks,  much  altered",  in  dike-like  forms,  are 
found  in  the  clastic  scries  precisely  as  in  the  gueissic  series.  They 
oftentimes  strike  approximately  parallel  to  the  inclosing  rocks.  East 
of  Medicine  peak  the  rocks,  including  the  gneissoid  and  clastic  series, 
have  a  dip  of  about  60°  to  80°  to  the  southeast.  Therefore  the  Medi- 
cine peak  series  appears  to  underlie  the  gneissoid  series.  The  dip  of 
the  Medicine  peak  series  in  going  north  of  west  beyond  the  mountain 
becomes  flatter,  until  2  or  3  miles  beyond  the  crest  the  dips  are  not 
higher  than  30°,  which  observations  agree  with  Hague's  statement  that 
west  of  Medicine  peak  is  the  crown  of  an  anticline.  As  the  strike  of 
the  Medicine  peak  series  is  nearly  toward  Mill  peak,  and  as  on  the 
top  of  that  peak  there  are  cherts  (Hague's  amorphous  quartzite)  and 
cherty  limestones  very  like  those  found,  east  of  Medicine  peak,  it  seems 
probable  that  the  Mill  peak  series  represents  these  cherty  limestones. 
Though  the  original  sedimentary  character  of  the  Medicine  and  Mill 
peak  series  is  evident,  the  pressure  to  which  the  rocks  have  been  sub- 
jected is  so  great  in  places  that  the  slate-conglomerates  bearing  granite 
pebbles  take  on  an  appearance  closely  resembling  gneisses.  The  grains 
of  quartz  in  the  fragmental  quartzites  in  thin  section  also  sIioav  pro- 
found evidence  of  dynamic  action.  However,  as  the  layers  of  pebbles 
in  the  quartzites  and  the  fine  laminations  in  the  cherts  and  cherty 
limestones  correspond  with  the  schistose  structure,  there  can  be  no 
doubt  that  the  strikes  and  dips  are  those  of  bedding. 
The  foregoing  facts  seem  to  imply  that  in  passing  up  from  the  gneis- 
sic  series  to  west  of  Medicine  peak  we  have  passed  a  syncline  over- 
turned to  the  west,  and  2  or  3  miles  west  of  Medicine  peak  have  nearly 
reached  the  crown  of  the  next  anticline.  This  structure  makes  the 
slates  and  slate- conglomerates  bearing  granite  pebbles  the  base  of  the 
clastic  series,  above  which  are  the  quartzites,  and  occupying  the  high- 
est position  in  the  center  of  the  syncline  are  the  cherts  and  cherty  lime- 
stones of  Mill  peak  and  those  east  of  Medicine  peak.  The  elastics  thus 
rest  upon  the  granite-gneiss  series.  No  contacts  or  evidence  of  discord 
ance  in  strike  or  dip  were  found  between  them,  but  the  conglomerates 
bearing  granitic  detritus  show  the  presence  of  a  granite  earlier  than 
the  formation  of  these  beds,  and  presumably  the  present  apparent  ac- 
cordance and  transition  are  due  to  dynamic  action,  combined,  perliaj 
with  the  disintegration  of  the  earlier  series  before  the  elastics  were  do 
posited. 
SUMMARY   OF   RESULTS. 
It  is  plain  that  in  the  Medicine  bow  range  are  two  classes  of  rocks: 
those  which  are  thoroughly  crystalline  and  are  mostly  of  the  aci< 
type,  and  those  which  are  unmistakable  elastics,  such  as  quartzites, 
conglomerates,  marbles,  cherts,  etc.,  while  in  the  Laramie  and  Pari 
ranges  is  only  the  first  class,  if  the  white  quartzite,  which  seems  to  b( 
in  the  nature  of  a  vein   in   the  latter,  is  excluded.     The  granites  an( 
