116  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
are  anterior  to  the  purely  sedimentary  deposits.  Contemporaneous 
with  the  lower  portion  of  the  Silurian  system  are  the  bedded  traps 
and  amygdaloids  of  Keweenaw  Point,  Isle  Royal,  and  the  Ontonagon 
region,  which  are  composed  of  nearly  the  same  constituents  as  many 
of  the  older  igneous  rocks,  although  there  is  no  difficulty  from  the 
diversity  in  external  characters  in  drawing  the  line  of  demarcation 
between  them. 
Below  all  the  fossiliferous  groups  of  the  region  is  a  class  of  rocks 
consisting  of  crystalline  schists,  beds  of  quartz,  and  saccharoidal 
marble,  which  is  denominated  the  Azoic  system,  a  term  first  applied 
by  Murchison  and  De  Verneuil  to  designate  the  crystalline  masses 
which  preceded  the  Paleozoic  strata.  This  term  as  here  used  is  limited 
to  rocks  which  are  detrital  in  origin  and  which  have  been  formed  be- 
fore the  dawn  of  organized  existence.  The  general  section  shows  the 
rocks  of  Keweenaw  Point  to  be  the  counterpart  of  those  of  Isle  Royal, 
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  horizontality ;  and  along  the  coast  at  the  head  of 
Keweenaw  Bay  it  is  seen  reposing  unconformably  on  the  slates  of  the 
Azoic  system.  From  L'Anse  to  Chippewa  Island  in  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 
specular  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  trans- 
mitted heat  as  to  exhibit  few  traces  of  their  original  character.  Sand- 
stone has  been  converted  into  massive  quartz,  limestone  into  saccha- 
roidal 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  division  into  two  groups,  as  Logan  divided  the  series  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  metamorphism  are  more  striking  in  approaching  the  lines 
of  igneous  outburst.  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 
slate  in  receding  from  the  igneous  products.  The  greenstones  often 
form  broad  sheets,  bearing  the  same  relation  to  the  slates  that  the 
