72  PRE-CAMBRIAN    GEOLOGY    OF    NORTH   AMERICA. 
regards  later  rocks  when  the  facts  have  been  carefully  tested  by 
stratigraphy.  Before  a  fossil  is  used  for  identifying  equivalent  beds 
it  is  by  careful  observation  proved  to  be  restricted  to  the  rocks  of  a 
certain  period,  and  even  then  it .  is  used  cautiously.  Has  anyone 
proved  by  careful  observations  that  crystals  of  staurolite:  cyahite,  or 
andalusite  are  restricted  to  rocks  of  a  certain  geological  period? 
Dana,9  in  1876,  gives  an  account  of  Archean  time.  The  Archean 
time  includes  an  Azoic  and  an  Eozoic  era,  though  not  yet  dis- 
tinguished in  the  rock.  The  Azoic  age  is  the  era  in  which  the  physical 
conditions  were  incompatible  with  the  existence  of  life.  But  this  era . 
so  far  as  now  known,  is  without  recognizable  records;  for  no  rocks 
have  yet  been  shown  to  be  earlier  in  date  than  those  which  are  now 
supposed  to  have  been  formed  since  life  began.  The  Archean  rocks 
of  North  America  are  mostly  crystalline  or  metamorphic  rocks,  and 
their  beds  stand  at  all  angles,  owing  to  the  uplifting  and  flexing 
which  they  have  undergone.  Where  the  Silurian  strata  overlie  them 
the  two  are  unconformable,  the  latter  being  often  spread  out  in  hori- 
zontal beds  over  the  upturned  edges  of  the  Archean  rocks. 
The  areas  of  the  Archean  include  those  which  have  always  remained 
uncovered,  those  which  have  been  covered  by  later  strata  but  from 
which  the  superimposed  beds  have  been  removed  by  erosion,  and  those 
like  the  last  which  in  the  course  of  mountain  making  have  been  pushed 
upward  among  the  displaced  strata.  The  principal  areas  are  the  great 
northern,  to  which  belong  the  Lake  Superior  region  and  properly  the 
Adirondack  area ;  the  area  along  the  Appalachian  line,  including  the 
Highlands  of  New  York  and  New  Jersey  and  the  Blue  Riclge  of 
Pennsylvania  and  Virginia ;  the  Atlantic  coast  range,  including  areas 
in  Newfoundland,  Nova  Scotia,  and  eastern  New  England ;  isolated 
areas  of  the  Mississippi  basin,  in  Missouri,  Arkansas,  Texasy  and  the 
Black  Hills;  the  Rocky  Mountain  series,  embracing  the  Wind  River, 
Laramie,  and  other  summit  ranges,  and  the  Pacific  coast  range  of 
Mexico. 
The  Archean  era  is  divided  into  two  periods,  the  Laurentian  and 
the  Huronian.  The  estimated  thickness  of  the  former  is  80,000  feet, 
and  of  the  latter  from  10,000  to  20,000  feet.  The  Laurentian  rocks 
are  metamorphic  or  crystalline,  with  few  exceptions,  and  include 
granite,  gneiss,  mica  schist,  hornblende,  and  pyroxenic  rocks,  beds  of 
crystalline  limestone,  quartzite,  conglomerate,  and  labradorite.  The 
Laurentian  beds  are  altered  sedimentary  rocks  of  the  ordinary  char- 
acter, as  the  schists  grade  into  true  slates,  the  quartzites  into  sand- 
stones, and  conglomerates  and  gneisses  into  gneissoid  granites.  No 
distinct  remains  of  plants  have  been  observed.  Graphite  is  very 
abundant.  Only  the  lowest  division  of  animal  life,  such  as  the 
rhizopods  and  protozoans,  occur.  This  is  shown  by  the  occurrence  of 
the  fossil  Eozoon  canadense.     The  Huronian  includes  the  series  on 
