SUMMARY    OF    GENERAL   LITERATURE.  99 
Throughout  the  area  the  Belt  terrane  is  overlain  unconformably  by 
middle  Cambrian  rocks  (Flathead).  The  Cambrian  rocks  rest  on 
various  members  of  the  Belt  series,  and  in  places  the  Belt  terrane  is 
entirely  wanting,  the  Cambrian  resting  directly  on  the  Archean 
schists.  In  such  cases,  moreover,  the  character  of  the  Belt  beds  indi- 
cates that  the  Cambrian  overlaps  the  Belt  series.  The  base  of  the 
Cambrian  is  not  markedly  conglomeratic.  At  most  of  the  outcrops 
where  the  lower  beds  of  the  Flathead  (Cambrian)  sandstone  come  in 
contact  with  the  Belt  rocks  the  dip  and  strike  of  the  two  are  usually 
conformable,  so  far  as  can  be  determined  by  measurement.  This 
holds  good  all  around  the  great  Belt  Mountain  uplift.  It  is  only 
when  contacts  are  examined  in  detail,  as  near  Helena,  that  the  minor 
unconformities  are  discovered,  and  only  when  comparisons  are  made 
between  sections  at  some  distance  from  one  another  that  the  extent  of 
the  unconformity  becomes  apparent.  In  general  from  3,000  to  4,000 
feet  of  the  upper  strata  of  the  Belt  terrane  were  removed  by  erosion 
in  late  Algonkian  time  before  the  middle  Cambrian  was  deposited. 
It  is  believed  that  an  unconformity  of  this  extent  is  sufficient  to  ex- 
plain the  absence  of  lower  Cambrian  rocks  and  fossils  and  to  warrant 
the  placing  of  the  Belt  terrane  in  the  Algonkian  system. 
The  fossils  thus  far  found  in  the  Belt  terrane  occur  in  the  Greyson 
shales  at  a  horizon  approximately  7,700  feet  beneath  the  summit  of 
the  Belt  terrane  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  thousands  of  fragments  of  one  or  more  genera  of  crus- 
taceans. 
The  fossils  of  the  Grand  Canyon  upper  Cambrian  series  consist  of 
specimens  of  a  small  discinoid  shell  found  in  the  upper  division  of 
the  Chuar  terrane,  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  identifica- 
tion is  doubtful. 
In  New  Brunswick  certain  rocks  below  the  middle  Cambrian,  ac- 
cording to  Matthew,  contain  fossils  which  may  be  pre-Cambrian. 
The  Llano  series  of  Texas  is  a  series  of  alternating  sandy  shales, 
sandstone,  and  limestone,  very  similar  to  those  of  the  Grand  Canyon 
pre-Cambrian  series  and  overlain  by  a  middle  Cambrian  sandstone 
.similar  to  the  Tonto  sandstone  of  the  Grand  Canyon  district.  Xc 
fossils  have  been  found  in  these  rocks,  although  no  systematic  search 
for  them  has  been  made. 
The  Avalon  series  of  Newfoundland  includes  all  the  pre-Cambrian 
sedimentary  rocks  of  that  area.  Overlying  them  are  Cambrian  strata 
carrying  Olcncllus  fauna. 
