838  PKE-CAMBKIAN    GEOLOGY   OF    NORTH    AMERICA. 
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  prob- 
able that  the  Mill  Peak  series  represents  these  cherty  limestones. 
Though  the  original  sedimentary  character  of  the  Medicine  Peak  and 
Mill  Peak  series  is  evident,  the  pressure  to  which  the  rocks  have  been 
subjected  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 
show  profound  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  going  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 
limestones  of  Mill  Peak  and  those  east  of  Medicine  Peak.  The  elas- 
tics thus  rest  upon  the  granite  gneiss  series.  No  contacts  or  evi- 
dence of  discordance  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  accordance  and  transition  are  due  to  dynamic  action, 
combined,  perhaps,  with  the  disintegration  of  the  earlier  series  before 
the  elastics  were  deposited. 
Black  welder,110  in  1908,  states  that  in  the  southern  part  of  the 
Laramie  uplift  a  complex  sequence  of  pre-Cambrian  igneous  and 
other  rocks  has  been  worked  out.  The  oldest  rocks  are  schistose 
basic  and  acidic  volcanics,  with  other  metamorphic  rocks  of  unknown 
origin,  and  a  few  which  seem  to  be  highly  altered  quartzites.  Into 
the  schists  have  been  intruded  two  varieties  of  granite,  both  of  which 
have  been  subsequently  metamorphosed  into  gneisses.  Into  both  the 
schistose  and  the  gneissic  series  a  variety  of  other  igneous  rocks  have 
been  intruded.  They  comprise  diorites,  gabbros,  granite  porphyries, 
and  anorthosite.  A  few  of  these  are  slightly  metamorphosed,  but 
the  majority  are  not  schistose.  All  of  the  foregoing  rocks  occur  in 
the  form  of  irregular  patches  embedded  in  and  surrounded  by  a  vast 
intrusive  mass  of  coarse-grained  granite,  believed  to  be  the  same  as 
that  which  has  a  wide  distribution  southward  in  Colorado.     No  dis- 
