van  hiss.]  DISCUSSIONS    OF    PRINCIPLES.  447 
as  the  chloritic  schists,  often  inclose  rolled  pebbles,  many  of  which  are 
derived  from  the  Lanrentian  gneiss.  This  Hnronian  terrain  comprises, 
moreover,  a  band  of  about  300  feet  of  granular  limestone,  which  is  im- 
pure and  often  very  siliceous.  The  Hnronian  terrain  on  lake  Huron 
has  a  thickness  of  about  18,000  feet.  It  is  also  found  on  the  Ottawa, 
and  from  there  it  extends  to  the  west  of  the  Mississippi,  though  cov- 
ered in  large  part  by  Paleozoic  terrains.  It  does  not  seem  to  exist  in 
the  eastern  region  of  Canada,  but  recent  observations  made  on  the 
island  of  Newfoundland  and  in  Nova  Scotia  have  demonstrated  there 
the  existence  of  rocks  that  have  been  referred  to  this  old  terrain,  which 
for  the  rest  seems  to  correspond  to  the  primitive  schists  (Urschiefer)  of 
Scandinavia.  No  fossils  have  yet  been  found  in  this  terrain.  Consid- 
erable masses  of  schistose  hematitic  iron  ore  are  inclosed  in  this  Hnro- 
nian terrain  on  the  northeast  shore  of  lake  Superior,  and  in  still 
greater  abundance  at  the  south,  where  the  famous  iron  mines  of  Mar- 
quette are  found.  This  terrain  is  more  or  less  affected  by  undulations 
anterior  to  the  Silurian  epoch. 
Dana,8  in  1872,  states  that  lithological  evidence  for  the  chrono- 
logical arrangement  of  the  crystalline  rocks  of  New  England  means 
nothing  until  tested  by  thorough  strati  graphical  investigation.  This 
evidence  means  something,  or  probably  so,  with  respect  to  Laurentian 
rocks,  but  it  did  not  until  the  age  of  the  rocks,  in  their  relations  to 
others,  was  first  strati  graphically  ascertained.  It  may  turn  out  to  be 
worth  something  as  regards  later  rocks  wrhen  the  facts  have  been  care- 
fully tested  by  stratigraphy.  A  fossil  is  proved,  by  careful  observa- 
tion, to  be  restricted  to  the  rocks  of  a  certain  period  before  it  is  used— 
and  then  cautiously — for  identifying  equivalent  beds.  Has  any  one 
proved  by  careful  observation  that  crystals  of  staurolite,  kyanite,  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  distinguished 
in  the  rock.  The  Azoic  age  is  the  era  in  which  the  physical  conditions 
wTere  incompatible  with  the  existence  of  life.  But  this  era,  so  far  as 
now  known,  is  without  recognizable  records 5  for  no  rocks  have  yet 
been  shown  to  be  earlier  in  date  than  those  which  are  now  supposed  to 
have  been  formed  since  the  first  life  began  to  exist.  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 
