LAKE    SUPERIOR   REGION. 
the  Gogebic  range  they  are  apparently  4,000  feet  thick.  The  United 
States  Geological  Survey  figures  have  been  12,000  or  15,000  feet  for 
the  Gogebic  section. 
The  Keweenawan  series  is  regarded  as  representing  the  Lower  and 
Middle  Cambrian  and  not  the  upper  part  of  the  pre-Cambrian  sec- 
tion. They  are  described  as  Cambrian  beds  disturbed  by  coeval  vol- 
canic activity  and  faulting,  and  seem  comparable  with  inter-Kewee- 
nawan  phenomena,  while  the  Lake  Superior  sandstone  and  the  Upper 
Keweenawan  appear  closely  associated  not  merely  lithologically,  but 
stratigraphically.  The  thickness  of  the  Keweenawan  is  not  less  than 
15,000  feet  and  may  be  as  much  as  42,500  feet,  a  figure  which  may  be 
reduced  by  initial  dip  and  repetition  by  faulting  to  29,000  feet.  Sedi- 
ments constitute  not  more  than  15  per  cent  of  the  section.  The 
Keweenawan  is  divided  into  groups  as  follows,  with  names  and  char- 
acteristics varying  somewhat  from  those  previously  used : 
(1)  The  Bohemian  group,  corresponding  to  Irving's  groups  1  and  2. 
(2)  The  Central  Mine  group,  including  the  "Greenstone  group,"  the  "Phoe- 
nix Mine  group,"  etc.,  but  only  a  part  of  Puinpelly's  "  Portage  Lake  series,"  and 
just  about  that  part  included  and  well  exposed  in  the  workings  of  the  Central 
mine  on  a  cross  fissure,  exposing  a  good  section,  examined  by  Pumpelly  and 
Hubbard,  and  more  recently  supplemented  by  diamond-drill  cores  on  the  same 
property  (sees.  24,  25,  36,  T.  58  N.,  R.  31  W.).  This  is  a  new  name  we  would 
introduce  and  define  as  extending  from  the  Bohemia  conglomerate,  Marvine's 
conglomerate  3  or  8,  to  the  "  St.  Mary's  epidote,"  a  sediment,  volcanic  ash, 
just  above  the  "  Greenstone  "  and  Marvine's  conglomerate. 
(3)  The  Ashbed  group,  including  Marvine's  a  and  b  and  Irving's  5  and  6. 
(4)  The  Eagle  River  group,  Marvine's  c. 
(5)  The  Copper  Harbor  conglomerates,  including  the  Great  and  Outer  con- 
glomerates, which  have  been  subdivided  by  Hubbard  into  three  heavy  conglom- 
erates. 
(6)  The  Nonesuch  formation,  same  term  as  previously  used. 
The  Neo-Cambrian  or  Potsdam  or  Lake  Superior  sandstone  in- 
cludes (1)  the  sandstones  west  of  the  Copper  range,  to  which  the  new 
term  Freda  sandstone  is  applied,  (2)  the  sandstones  east  of  the  Cop- 
per range,  to  which  the  term  Jacobsville  is  applied,  and  (3)  the  sand- 
stones in  the  bluffs  back  of  Munising,  to  which  the  term  "Munising 
sandstone  is  applied.  The  relations  of  the  Freda  sandstone  to  the 
Lake  Superior  sandstone  are  uncertain,  but  they  are  regarded  as 
probably  one  formation.  The  Freda  sandstone  had  previously  been 
called  by  Irving  the  Western  sandstone,  correlated  with  the  Kewee- 
nawan, and  regarded  as  unconformably  below  the  Lake  Superior 
sandstone.  The  Jacobsville  and  Munising  sandstones  were  regarded 
by  Irving  as  true  Lake  Superior  or  Potsdam  sandstones. 
The  United  States  Geological  Survey  term  Algonkian  for  the  Hu- 
ronian  and  Keweenawan  series  is  not  used. 
Gordon  and  Lane,97  in  1007,  describe  a  geological  section  from 
Bessemer  down  Black  River  in  Michigan.  Keweenawan  rocks  are 
55721— Bull.  360—09 12 
