van  hise.]  DISCUSSIONS    OF    PRINCIPLES.  443 
stone,  which  sometimes  pass  into  a  state  of  conglomerate,  inclosing 
globes  of  quartz  and  jasper.  Beds  of  a  reddish  argillaceous  limestone 
are  found  interspersed  with  these  sandstones,  which  are  cut  through 
and  covered  by  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  are  found 
sandstones  having  a  more  vitreous  aspect,  and  conglomerates  more 
abundant  than  on  lake  Superior,  associated,  however,  with  schists  and 
schistose  conglomerates,  resembling  those  which  we  have  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  two  latter.  The  formation  of  the  metalliferous 
veins  belongs  to  an  epoch  still  more  recent. 
This  Huronian  formation  is  observed  over  a  distance  of  nearly  150 
leagues  on  lakes  Huron  and  Superior. 
Dana,5  in  1863,  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  that 
the  area  of  tke  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  northwest 
to  the  Arctic  ocean,  the  Adirondacks  of  northern  New  York,  a  similar 
area  south  of  lake  Superior,  west  of  the  Mississippi  a  small  area  in 
Missouri,  the  Black  hills  in  Dakota,  the  Laramie  range  in  Nebraska, 
part  of  the  Ozark  mountains  in  Arkansas ;  and  in  northern  New  Jersey 
Azoic  gneiss7  limestone,  and  other  crystalline  rocks  containing  beds  of 
iron  ore.  The  rocks  of  the  Azoic  are  mostly  of  the  metamorphic  series, 
related  to  granite,  gneiss,  syenite,  and  the  like,  but  they  embrace  only 
the  most  ancient  of  these  rocks.  The  Azoic  rocks  are  nearly  all  crys- 
talline, a  few  sandstones,  slates,  and  conglomerates  being  the  only 
exceptions.  They  are  remarkable  for  the  small  amount  of  silica  they 
contain,  as  shown  in  the  diorites  and  labradorite  rocks.  Prevalence  of 
iron  ore  is  another  characteristic,  and  none  of  the  minerals  are  simple 
silicates  of  aluminum.  While  the  Azoic  rocks  are  crystalline  they 
follow  one  another  in  variations  and  alternations  like  sedimentary  beds 
of  later  date.    Granite  or  gneiss  may  lie  between  layers  of  slate  or 
