GENERAL   ACCOUNT    OF    THE    PRE-CAMBRIAN    ROCKS.  33 
of  northern  Montana,  Idaho,  and  southern  British  Columbia.  The 
Belt  series  is  subdivided  lithologically  into  a  number  of  formations, 
but  represents  practically  continuous  deposition.  To  the  south  in 
Montana  the  Cherry  Creek  sedimentary  rocks  are  assigned  to  the 
Algonkian.  While  their  relations  to  the  Belt  series  are  not  shown 
by  direct  contact,  the  relatively  greater  metamorphism  and  deforma- 
tion of  the  Cherry  Creek  group  indicate  its  earlier  age  and  prob- 
able separation  by  unconformity.  The  quartzites  of  the  Uinta  and 
Wasatch  mountains,  correlated  with  the  Belt  series,  are  essentially 
structural  units.  The  same  is  true  of  the  quartzites  and  slates  of 
Needle  Mountains,  Colorado.  In  the  Grand  Canyon  of  the  Colorado 
there  are  two  groups  classed  in  the  Algonkian,  separated  from  each 
other  by  a  minor  unconformity. 
The  vast  quantities  of  intrusives  and  igneous  rocks  in  the  Algon- 
kian are  given  the  names  of  the  series  which  they  intrude  or  are 
interbedded  with,  provided  it  can  not  be  shown  that  they  are  of  later 
age.  Thus  granites  intrusive  into  the  Algonkian  but  not  into  later 
systems  are  Algonkian  granites.  This  differs  from  the  method  of 
some  geologists  of  the  Canadian  Survey,  who  have  commonly  called 
such  granites  "  Laurentian,"  but  this  method  is  now  discontinued  by 
the  Canadian  Survey  so  far  as  practicable. 
FOSSILS  IN   THE  ALGONKIAN. 
Fossils  are  very  rare  in  the  Algonkian  system.  From  several 
localities  discoveries  of  fossil  forms  have  been  reported,  but,  with 
two  exceptions,  these  are  held  by  Walcott  to  be  inorganic  or  doubt- 
fully organic.  The  two  exceptions  are  the  fossils  in  the  Belt  series 
of  Montana  and  those  in  the  Chuar  group  of  the  Grand  Canyon 
series,  both  of  them  referred  to  the  Algonkian.  In  the  Belt  series  the 
fossils  thus  far  found  occur  in  the  Greyson  shale,  at  a  horizon  ap- 
proximately 7,700  feet  beneath  the  summit  of  the  Belt  series  at  its 
maximum  development.  The  fauna  includes  four  species  of  annelid 
trails  and  a  variety  that  appears  to  have  been  made  by  a  minute 
mollusk  or  crustacean.  There  also  occur  in  the  same  shales  thou- 
sands of  fragments  of  one  or  more  genera  of  crustaceans,  some  of 
them  of  considerable  size.  Crustaceans  were  found  also  by  Weller 
in  the  same  series  in  the  lowest  limestone  exposed  in  the  Rocky 
Mountain  front,  near  the  forty-ninth  parallel.  The  fossils  of  the 
pre-Cambrian  Grand  Canyon  series  are  a  small  discirioid  shell 
found  in  the  upper  division  of  the  Chuar  group  and  a  Stromatopora- 
like  form  from  the  upper  portion  of  the  lower  division  and  the 
central  portion  of  the  upper  division  of  the  Chuar.  Other  obscure 
forms  appear  whose  identification  is  doubtful. 
55721— Bull.  360—09 3 
