860  pre-cambria:\t  geology  of  north  America. 
Davis  and  the  other  geologists  of  the  Northern  Transcontinental 
Survey  distinguished  the  great  series  of  undoubted  sedimentary 
rocks  forming  the  main  mass  of  the  Little  Belt  and  Big  Belt  moun- 
tains, classifying  this  great  thickness  of  10,000  feet  or  more  of  rocks 
as  Lower  Cambrian.  In  1888  Iddings  and  Weed  published  Folio  1 
of  the  Geologic  Atlas  of  the  United  States.  In  this  publication  the 
Archean  complex  constituting  the  nucleus  of  the  mountain  ranges 
lying  east  of  Yellowstone  River  is  described,  but  no  discrimination 
was  made  of  the  thinly  banded  series  of  marbles,  quartzites,  and 
schists  constituting  the  upper  portion  of  this  series  of  crystalline 
rocks  and  lying  unconformably  beneath  the  Flathead  quartzite,  the 
lowest  part  of  the  Paleozoic  series.  Similar  conditions  prevail  east- 
ward, and  have  been  recognized  in  the  vicinity  of  Red  Lodge,  Mont., 
and  extending  southward  into  Wyoming.  In  the  Three  Forks  quad- 
rangle, which  immediately  adjoins  the  Livingston  quadrangle  on  the 
west,  Peale  distinguished  a  series  of  semicrystalline  rocks  as  the 
Cherry  Creek  formation,  which  he  assigns  to  the  Algonkian.  This 
series  of  rocks  occurs  on  the  west  side  of  the  Madison  River,  forming 
an  area  4  miles  wide  and  8  miles  long.  The  formation  consists  of 
crystalline  limestones,  mica  schists,  quartz  schists,  and  gneisses,  in 
steeply  inclined  bands.  So  far  as  recognizable,  the  structure  of  this 
series,  like  that  observed  south  of  Livingston,  Mont.,  is  conformable 
with  the  lamination  of  the  gneisses  and  granite  to  the  south.  These 
rocks  are  unconformably  overlain  by  the  little-disturbed  Cambrian. 
Similar  rocks  occur  in  the  Three  Forks  quadrangle  north  of  Virginia 
City,  but  were  not  discriminated  by  Peale  on  the  map. 
The  great  series  of  barren  slates  found  in  the  Little  Belt  and  Big 
Belt  mountains,  and  mapped  by  Davis,  are  clearly  recognizable  as 
sedimentary  rocks,  and  differ  very  markedly  in  character  from  the 
rocks  last  described.  In  mapping  the  regions  lying  about  the  Mis- 
souri River  it  became  necessary  to  name  this  formation,  and  after 
conference  Weed  and  Peale  adopted  the  name  Belt  formation,  or 
Belt  series,  for  the  entire  thickness  of  rocks.  These  rocks  have  not 
been  found  in  contact  with  the  Cherry  Creek  Algonkian,  and  in  the 
Little  Belt  Mountains  the  old  shore  line  of  the  Belt  formation  is 
clearly  recognizable  in  Archean  schists  and  gneisses  wholly  unlike 
the  Cherry  Creek  series,  or  the  unnamed  series  found  south  of  Living- 
ston. As  the  Belt  series  is  nowhere  in  contact  with  the  Cherry 
Creek,  its  relatively  later  age  is  assigned  to  it  because  the  former 
Cherry  Creek  rocks  are  extremely  metamorphosed  and  have  a  struc- 
ture similar  to  that  of  the  Archean  gneisses.  The  age  of  the  Belt 
series  is  determined  by  unconformity  at  the  base  and  the  fact  that 
fragments  of  Archean  rocks  occur  in  its -basal  conglomerate,  by  the 
occurrence  of  middle  Cambrian  sediments  immediately  above  it  with 
a  profound  unconformity  between,  and  by  a  vast  difference  in  the 
