THE   CORDILLERAS.  857 
The  series  has  thus  far  yielded  no  fossils.  It  attains  a  thickness  of 
8.000  feet.  Basic  and  acidic  intrusive  rocks  penetrate  the  Belt  forma- 
tion very  freely. 
At  many  localities  the  Belt  series  is  seen  to  be  in  conformable  rela- 
tions to  overlying  fossiliferous  rocks  of  Cambrian  age,  the  Flathead 
and  Gallatin  formations,  and,  while  assigned  to  the  Algonkian,  the 
series  is  spoken  of  as  forming  the  lower  part  of  the  Paleozoic  of  the 
area. 
Weed  and  Pirsson,145  in  1890,  briefly  describe  and  sketch  the 
geology  of  the  Little  Rocky  Mountains  of  central  Montana.  The 
core  of  the  mountains  is  formed  of  crystalline  schists,  of  which  the 
type  usually  seen  is  a  black  glistening  amphibole  schist,  or  amphibo- 
lite.  In  the  saddle  west  of  Shellrock  Mountain  the  series  consists  of 
amphibole  schists  and  mica  schists,  pink  gneiss,  and  white  quartz- 
ites,  the  various  rocks  occurring  in  rapidly  alternating  beds  but  a 
few  feet  thick. 
The  crystalline  schists  are  overlain  by  Cambrian  sedimentaries. 
Intruded  between  the  schists  and  sedimentaries  is  a  great  laccolithic 
body  of  granite  porphyry. 
The  presence  of  the  quartzite  is  taken  as  indicating  the  Algonkian 
age  of  the  crystalline  series.  However,  similar  schists  occurring  in 
Montana  have  been  generally  classed  as  Archean,  and  these  rocks  are 
metamorphosed  and  quite  unlike  the  slightly  altered  Belt  Mountain 
Algonkian  series.  The  crystallines  are,  therefore,  not  definitely  as- 
signed to  either  the  Archean  or  the  Algonkian. 
Grisavold,140  in  1898,  describes  the  geology  of  Helena,  Mont.,  and 
vicinity. 
Middle  Cambrian,  or  Flathead,  quartzite  forms  an  important  part 
of  the  ridge  stretching  from  Helena  southeast  to  Montana  City,  and 
northwest,  west,  and  south  around  Mount  Helena.  The  sedimentary 
rocks  underlying  most  of  the  area  of  the  city,  on  the  north  side  of 
this  Cambrian  quartzite,  are  classed  as  Algonkian.  The  Algonkian 
rocks  vary  from  clay  slates  to  micaceous,  sandy,  or  calcareous  slates, 
which  often  become  quartzites  or  limestones.  The  Algonkian  slates 
seem  to  conform  to  the  overlying  strata  in  the  dip  of  their  beds.  As 
there  are  many  small  folds,  it  is  difficult  to  determine  the  thickness: 
5,000  feet  does  not  seem  too  large  a  total. 
Walcott,  in  1899,  discusses  the  Belt  terrane  of  Montana.  See 
summary  in  Chapter  II,  pages  98-91). 
Weed,147  in  1899,  maps  and  describes  the  pre-Cambrian  rocks  in  the 
Fort  Benton  and  Little  Belt  Mountains  quadrangles,  in  Montana. 
The  Archean  rocks  are  found  only  in  the  Little  Bell  Range,  in  the 
southwestern  part  of  the  Fort  Benton  quadrangle  and  the  north- 
western part  of  the  Little  Belt  Mountains  quadrangle.  They  are 
gneisses   and   schists  of  various  kinds   and   of   somewhat    uncertain 
