80  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bull. 86. 
the  dawn  of  organized  existence.  The  general  section  shows  the  rocks 
of  Keweenaw  point  to  be  the  counterpart  of  those  of  isle  Eoyale,  except 
that  the  dip  of  the  sedimentary  rocks  is  reversed,  thus  rendering  it 
highly  probable  that  between  these  two  points  is  a  great  curvature 
in  the  strata  caused  by  the  elevation  along  the  line  of  two  volcanic 
fissures.  The  sandstone  on  the  southern  slope  of  the  axis,  equivalent 
to  the  Potsdam,  is  seen  dipping  away  from  the  crystalline  trap  at  a 
high  angle,  but  at  a  short  distance  from  the  line  of  igneous  outburst 
it  verges  toward  horizontally ;  and  along  the  coast  at  the  head  of 
Keweenaw  bay  it  is  seen  reposing  unconforma^Iy  on  the  slates  of  the 
Azoic  system.  From  L'Anse  to  Chippewa  island  in  the  Menominee 
river,  a  direct  distance  of  more  than*  80  miles,  the  country  is  occupied 
by  rocks  of  the  Azoic  system,  which  include  immense  deposits  of  specu- 
lar and  magnetic  oxide  of  iron,  and  are  invaded  at  many  points  by 
igneous  rocks,  both  granitic  and  trappean.  At  Chippewa  island  the 
Potsdam  sandstone  reposes  upon  the  upturned  edges  of  the  slates. 
The  Azoic  rocks  have  been  so  transformed  by  direct  and  transmitted 
heat  as  to  exhibit  few  traces  of  their  original  character.  Sandstone  has 
been  converted  into  massive  quartz,  limestone  into  saccharoidal  marble, 
and  shales  into  hard  crystalline  schists.  These  rocks  are  destitute  of 
life,  are  a  system  of  obscurely  stratified  rocks  interposed  between  the 
Potsdam  and  the  granite,  and  are  unconformable  to  the  former  in  dip. 
The  Azoic  series  of  the  southern  shore  have  not  been  capable  of  divi- 
sion into  two  groups  as  done  by  Logan  on  the  north  shore.  The 
rocks  are  highly  inclined  and  much  contorted,  and  nowhere  exhibit  the 
characters  of  a  purely  sedimentary  rock ;  but  the  evidences  of  nieta- 
morphism  are  more  striking  in  approaching  the  lines  of  igneous  out- 
burst. Gneiss  generally  flanks  the  granite,  succeeded  by  dark  masses 
of  hornblende,  with  numerous  joints,  but  obscure  lines  of  bedding, 
which  often  graduate  into  hornblende- slate  or  chlorite- si  ate  in  receding 
from  the  igneous  products.  The  greenstones  often  form  broad  sheets, 
bearing  the  same  relation  to  the  slates  that  the  trappean  bands  of  Ke- 
weenaw point  do  to  the  conglomerates.  Many  of  the  slates  appear  to 
be  composed  of  pulverulent  greenstone,  as  though  they  might  origin- 
ally have  been  ejected  as  an  ash  and  subsequently  deposited  as  a  sedi- 
ment. They  pass  by  imperceptible  gradations  from  a  highly  fissile  to  a 
highly  compact  slate. 
In  Sec.  19,  T.  49  K".,  E.  27  W.,  is  found  a  talcose  and  chlorite  slate, 
and  quartzose  rocks  enveloping  pebbles  and  displaying*  obscure  lines  of 
stratification.  In  Sec.  32,  T.  48  N.,  E.  26  W.,  and  near  Jackson  Com- 
pany's forge  are  found  quartzose  conglomerates.  On  the  line  between 
Sees.  29  and  32,  T.  47  K,  E.  27  W.,  is  a  conglomerate  forming  an  isolated 
rounded  elevation,  which  is  made  up  of  coarse  blocks  of  various  sorts 
belonging  to  the  neighboring  trappean  and  slaty  beds.  Among  them 
are  found  fragments  of  the  rock  associated  with  iron,  and  masses  of  the 
iron  itself,  and  of  the  banded  and  jaspery  varieties.    Most  of  the  frag- . 
