856  PRE-CAMBRXAN    GEOLOGY    OF    NORTH    AMERICA. 
area  on  the  east  side  of  the  Madison  Valley,  at  the  western  edge 
of  the  Madison  Range.  The  rocks  are  marbles  and  interlaminated 
mica  schists,  quartzites,  and  gneisses.  Between  Cherry  Creek  and 
Wigwam  Creek,  on  the  west  side  of  the  Madison  Valley,  Cambrian 
strata  rest  unconformably  upon  the  upturned  edges  of  the  Cherry 
Creek  beds.  Before  the  deposition  of  the  Belt  formation  the  Cherry 
Creek  beds  suffered  extensive  deformation. 
The  Belt  formation  occurs  in  the  northern  portion  of  the  district — 
in  the  foothills  of  the  northern  portion  of  the  Bridger  Range,  in 
the  hills  north  of  Gallatin  and  East  Gallatin  rivers,  and  in  the 
rugged  hills  of  Jefferson  Canyon.  In  the  lower  portion  of  the 
formation  are  coarse  sandstones  and  conglomerates,  in  the  central 
part  appear  argillites  and  siliceous  limestones,  and  in  the  upper 
part  sandstones  predominate.  The  Belt  formation  is  overlain  by 
the  Flathead  (Cambrian)  quartzite.  It  is  possible  that  further  in- 
vestigation may  result  in  the  reference  of  this  formation  to  the 
lower  part  of  the  Cambrian.  At  present,  however,  it  is  referred 
provisionally  to  the  Algonkian. 
The  Flathead  and  Gallatin  formations  (Cambrian)  rest  with 
marked  unconformity  upon  the  Archean  for  three-fourths  of  the  dis- 
trict ;  for  the  remainder  of  the  district  they  rest  upon  the  Algonkian, 
and  the  unconformity,  if  it  exists,  is  very  slight. 
Iddings,  Weed,  and  Hague,143  in  1894,  describe  and  map  the 
geology  of  the  Livingston  quadrangle,  Montana.  Archean  crystalline 
rocks  constitute  a  part  of  the  southern  half  of  the  region.  These  in- 
clude mica  schists,  phyllite,  gneiss,  and  granite.  Much  of  the  granite 
is  eruptive  and  carries  angular  blocks  of  other  rocks.  The  foregoing 
are  cut  by  veins  and  dikes  of  crystalline  rocks,  both  basic  and  acidic. 
Resting  unconformably  upon  the  Archean  rocks  is  the  Belt  forma- 
tion, which  is  supposed  to  belong  to  the  Algonkian  period.  This 
formation  is  found  on  the  western  flank  of  the  Bridger  Range.  The 
rocks  comprise  sandstones,  conglomerates,  slates,  and  arenaceous  lime- 
stones. The  series  is  about  2,500  feet  thick  within  the  area  mapped. 
The  Algonkian  rocks  are  overlain  conformably  by  the  Cambrian 
Flathead  quartzite. 
Weed  and  Pirsson,144  in  189G,  map  and  describe  the  geology  of  the 
Castle  Mountain  mining  district  of  Montana.  The  Belt  group  of 
rocks,  assigned  lo  the  Algonkian,  occupies  large  areas  in  the  district. 
The  series  presents  no  definite  lithological  horizons,  but  there  is  a 
general  sequence,  from  the  base  upward,  as  follows: 
Alternating  shales  and  sandy  beds. 
Dark-gray,  laminated,  thinly  bedded  limestone. 
Pearl-gray  sericitic  shales. 
Sandy  shales  with  thin  beds  of  ripple-marked  sandstone. 
Red  shales  and  slates. 
