SUMMAKY   OF    GENERAL   LITERATURE.  67 
To  the  Cambrian  and  Huronian  system  belong  the  rocks  on  the 
north  shores  of  Lakes  Huron  and  Superior— a  series  of  schists,  sand- 
stones, limestones,  and  conglomerates,  interspersed  with  heavy  layers 
of  diorite  and  resting  uncomformably  on  the  Laurentian  system.  As 
these  rocks  are  lower  than  the  Silurian  terrane,  and  as  they  have  thus 
far  not  yielded  a  single  fossil,  they  may  well  be  referred  to  the  Cam- 
brian system  (the  Lower  Cambrian  of  Sedgwick).  The  schists  of 
this  system  on  Lake  Superior  are  bluish  and  inclose  layers  of  horn 
flint  having  calcareous  bands  and  cracks  often  filled  with  anthracite. 
These  rocks  are  covered  with  a  considerable  thickness  of  trap,  on 
which  are  superposed  heavy  beds  of  white  and  red  sandstone,  which 
sometimes  pass  into  a  state  of  conglomerate  inclosing  globes  of  quart/ 
and  jasper.  Beds  of  a  reddish  argillaceous  limestone  are  found  inter- 
spersed with  these  sandstones,  which  are  cut  through  and  covered  b/y 
a  second  formation  of  diorite  of  great  thickness,  offering  a  columnar 
structure.  This  formation,  which  has  a  total  thickness  of  nearly 
4,000  meters,  is  traversed  by  a  great  number  of  trap  dikes.  In  the 
corresponding  formation  of  the  north  shore  of  Lake  Huron  arc 
found  sandstones  having  a  more  vitreous  aspect,  and  conglomerates 
more  abundant  than  on  Lake  Superior,  associated,  however,  with 
schists  and  schistose  conglomerates  resembling  those  just  described, 
the  whole  presenting  great  masses  intercalated  with  diorite.  A  layer 
of  limestone  having  a  thickness  of  1G  meters  forms  part  of  this  series. 
After  the  eruption  of  the  interstratified  diorites  have  appeared  two 
systems  of  dikes  of  diorite,  and  a  third  of  granite,  of  an  epoch  inter- 
mediate between  the  other  two.  The  formation  of  the  metalliferous 
veins  belongs  to  an  epoch  still  more  recent. 
This  Huronian  formation  is  observed  over  a  distance  of  nearly  L50 
leagues  on  Lakes  Huron  and  Superior. 
Dana,5  in  18G3,  gives  an  account  of  the  Azoic  age.  This  age  is  de- 
fined as  the  age  in  the  earth's  history  preceding  the  appearance  of 
animal  life.  Among  the  Azoic  rocks  are  included  all  the  rocks  that 
are  older  than  the  Potsdam  sandstone  of  New  York,  between  which 
and  the  Azoic  general  unconformable  relations  obtain.  The  Azoic 
rocks  constitute  the  only  universal  formation.  They  cover  the  whole 
globe,  and  were  the  floor  of  the  oceans  and  the  rocks  of  all  emerged 
land  when  animal  life  was  first  created.  But  subsequent  operations 
over  the  sphere  have  buried  the  larger  part  of  the  ancient  surface,  and 
to  a  great  extent  worn  away  and  worked  up  anew  its  material,  so  thai 
the  area  of  the  old  floor  now  exposed  to  view  is  small.  The  Azoic 
regions  include  Canada  north  of  the  St.  Lawrence,  reaching  northeast 
from  Lakes  Huron  and  Superior  to  Labrador,  and  continuing  north- 
west to  the  Arctic  Ocean,  the  Adirondacks  of  northern  New  York,  a 
similar  area  south  of  Lake  Superior,  west  of  the  Mississippi  a  -mall 
area  in  Missouri,  the  Black  Hills  in  Dakota,  the  Laramie  Rang 
