854  PRE-CAMBRT  AN    GEOLOGY    OF    NORTH    AMERICA. 
are  either  quartz  or  micaceous  gneiss,  showing  that  the  sediments  were 
derived  directly  from  the  metamorphic  rocks.  The  lowest  strata  of 
unchanged  rocks  are  here  regarded  as  Silurian,  and  probably  Pots- 
dam, although  no  organic  remains  were  found.  The  Carboniferous 
limestones  higher  up  are  filled  with  characteristic  fossils.  In  the 
Middle  Canyon  of  the  Madison  the  stratified  rocks  are  also  believed 
to  belong  to  the  Potsdam  epoch,  although  no  fossils  were  found  lower 
than  the  Carboniferous*,  and  here  the  unconformable  relations  of  the 
limestones  to  the  metamorphic  rocks  are  clearly  shown.  On  both 
sides  of  the  Madison  there  is,  in  restricted  localities,  an  enormous 
development  of  very  hard,  gray  quartzitic  sandstone,  apparently  par- 
tially metamorphosed,  which  evidently  forms  the  underlying  rocks 
of  the  sedimentary  strata  resting  on  the  strictly  metamorphic  gneiss. 
No  organic  life  has  been  found,  yet  it  undoubtedly  belongs  to  the 
oldest  Silurian.  Along  the  valley  of  the  Madison,  below  the  mouth 
of  Cherry  Creek,  for  several  miles  there  are  successions  of  gneissic 
beds,  thousands  of  feet  in  thickness,  which  show  great  variety  of 
composition  and  flexures  in  the  bedding.  In  this  gneiss  are  layers 
of  black  hornblende  gneiss,  4  to  6  feet  thick,  which  appear  as  though 
they  were  intrusions  of  trap.  Near  Helena  the  sedimentary  beds 
overlying  the  granite  are  tilted  from  20°  to  45°  past  vertical.  The 
work  of  reducing  the  metamorphic  strata  which  underlie  the  entire 
country  to  a  system  and  connecting  them  over  extended  areas  has  not 
been  attempted,  and  it  seems  to  the  author  an  almost  hopeless  as  well 
as  a  fruitless  task. 
Peale,46  in  1873,  describes  at  many  localities  crystalline  rocks  in 
southwestern  Montana  and  adjacent  regions.  Gneissic  and  granitic 
rocks  are  mentioned  in  the  Cinnabar  Mountains,  in  the  rocks  of  the 
Third  Canyon  of  the  Yellowstone,  at  Elk  Creek,  at  the  junction  of  the 
two  forks  of  the  Yellowstone,  at  West  Gallatin  Canyon,  Bozeman 
Creek,  and  other  localities.  On  one  of  the  headwaters  of  the  Madison 
are  found  quartz  schists  and  chlorite  schists,  below  which,  in  apparent 
conformity,  are  layers  of  limestone.  Still  below  these  are  Carbonifer- 
ous limestones.  The  whole  is  believed  to  be  an  overturn.  Between 
Red  Rock  Lake  and  Henry  Lake  is  an  exposure  of  quartz  schist  dip- 
ping to  the  southwest  at  an  agle  of  20°,  estimated  to  be  2,000  feet  in 
thickness,  which  is  believed  to  rest  directly  upon  the  granite.  On 
Cherry  Creek  the  gneissic  rocks  are  succeeded  by  beds  of  massive 
quartzite,  shale,  limestones,  etc.,  resting  unconformably  upon  them, 
the  latter  being  probably  Lower  Silurian. 
Hayden,139  in  1876,  describes  some  geological  sections  about  the 
headwaters  of  Missouri  and  Yellowstone  rivers.  There  is  an  anti- 
clinal axis  between  the  Madison  and  the  Jefferson  which  has  a  gra- 
nitic nucleus,  and  on  the  east  side  of  the  Gallatin  the  Silurian  rocks 
rest  upon  granitic  hills. 
