'an  inss.]  THE    CORDILLERAS.  285 
ron  extends  across  to  the  Gallatin  canyon,  a  distance  of  24  miles.  This 
atter  belt  is  about  12  miles  in  width.  There  are  also  two  smaller 
jneissic  areas  bordering  the  southern  edge  of  the  Gallatin  valley.  The 
)eds  of  Lower  Cambrian  age  at  a  number  of  localities  in  the  range  include 
ingular  fragments  and  masses  of  gray  and  red  gneiss,  evidently  derived 
Voni  these  Archean  beds. 
In  the  vicinity  of  the  Three  forks  and  in  the  northern  portion  of  the 
3 ridger  range,  lying  between  the  basal  quartzite  of  Lower  Cambrian 
ige  and  the  Archean  gneisses  referred  to  above,  is  another  series  of  beds 
hat  are  considered  pre-Cambrian.  This  group,  referred  to  the  AJgon- 
;i;m,  lias  a,  thickness  of  5,000  or  0,000  feet  in  the  Bridger  range  and  is 
correlated  with  the  Lower  Cambrian  barren  slates  of  much  greater 
hickness  found  by  Davis  farther  to  the  north.  The  beds  are  made  up 
>f  alternations  of  coarse  micaceous  sandstones  and  fine  conglomerates 
nth  beds  of  hard  argillaceous  slates  and  bands  of  very  hard  thin- 
xnlded  dark  blue  limestones.  These  are  the  beds  seen  by  Hayden  in 
861  which  were  mistaken  for  eruptives,  and  which  were  described  by 
Vale  in  187-').  Where  the  lowest  exposures  were  noted,  pebbles  and 
bngular  masses  of  the  Archean  gneisses  are  numerous  in  these  sand- 
tones,  indicating  that  Archean  land  masses  existed  not  far  to  the  soul  h- 
rard.  In  fact  the  ancient  shore  line  crossed  the  area  from  12  to  15 
idles  south  of  the  northern  limit  of  the  Three  forks  sheet.  As  to  age, 
his  series  is  probably  x>re-Cambrian  as  it  lies  below  beds  containing 
jower  Cambrian  fossils,  being  nonfossiliferous  themselves  so  far  as 
xamiiied.  So  far  as  observed  no  evident  unconformity  exists  between 
lie  series  and  the  overlying  Cambrian  beds,  but  there  is  certainly  an 
Liiconformity  by  subsidence,  for  after  the  series  was  deposited  there 
fas  an  orographic  movement  by  which  the  Archean  area  of  nearly  the 
ntire  district  of  the  sheet  south  of  the  Three  forks  was  submerged  just 
>rior  to  the  beginning  of  the  Cambrian,  as  is  shown  by  the  great  south- 
raid  extent  of  the  lower  or  basal  quartzite  over  this  area  which  was 
tot  before  submerged  and  which  therefore  shows  no  rocks  of  this  Algon- 
:ian  series.  Whether  the  movement  occurred  immediately  after  the  la  \  - 
ag  down  of  the  Algonkian  beds  just  described  or  after  an  interval  can 
Lot  be  decided  with  the  meager  data  now  at  hand. 
South  of  the  old  shore  line  the  Algonkian  group  is  absent,  the  lower 
[uartzite  of  the  undoubted  Cambrian  everywhere  lying  uncon  forma  I  >iy 
Lpon  the  Archean  gneisses,  with  the  exception  of  two  localities,  where 
p  rests  upon  a  series  of  beds  considered  to  be  a  lower  division  of  the 
Ugonkian.  The  main  area  is  on  the  west  side  of  the  Madison  river, 
bout  20  miles  south  of  Meadow  creek,  and  is  about  8  miles  in  length 
y  about  4  in  width.  A  limited  area  of  these  beds  occur  on  the  east 
ide  of  tin'  Madison  valley  at  the  west  edge  of  the  Madison  range  be 
ween  Bear  and  Indian  creeks.  The  series  consists  of  crystalline  lime- 
hones,  mica- schists,  quartzites,  and  gneisses,  very  highly  inclined  and 
mforniable  so  far  as  seen   with   the   stratification  or  bedding  of  the 
