448  PRJX'AMBKIAN    ROCKS    OF    NORTH     AMERICA.  [bull. '*}. 
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  ridge  of  Penn- 
sylvania 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,  Texas,  and  the  Black 
hills ;  the  Bocky  mountains  series,  embracing  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  30,000  feet,  and  of 
the  latter  from  10,000  to  20,000  feet.  The  Laurentian  rocks  are  meta- 
morphic  or  crystalline,  with  few  exceptions,  and  include  granite,  gneiss, 
mica-schist,  hornblende,  and  pyroxenic  rocks,  beds  of  crystalline  lime- 
stone, quartzite,  conglomerate,  and  labradorite.  The  Laurentian  beds 
are  altered  sedimentary  rocks  of  the  ordinary  character,  as  the  schists 
grade  into  true  slates,  the  quartzites  into  sandstones,  and  conglomer- 
ates 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  Bhizopods  and  Protozoans,  occur. 
This  is  shown  by  the  occurrence  of  the  fossil  Eozoon  canadense.  The 
Huronian  includes  the  series  on  the  north  shore  of  lake  Huron  com- 
posed of  slates,  conglomerates,  quartzites,  layers  of  jasper  and  chert, 
with  quartz  and  jasper  conglomerates,  limestones,  beds  of  diorite  which 
graduate  into  syenite  or  epidote,  and  also  other  areas  which  have  been 
placed  as  the  equivalent  of  this  series  on  lithological  grounds. 
King,10  in  1878,  states  that  in  the  Archean  outcrops  of  the  fortieth 
parallel  one  can  not  fail  to  notice  the  widespread  simplicity  of  peno- 
logical forms,  the  prevalence  of  granites,  granitoid  gneisses,  and  dio- 
ritic  metamorphic  rocks,  the  paucity  of  argillites,  quartzites,  limestones, 
and  zirconiferous  and  staurolitic  schists,  the  infrequence  of  large  bodies 
of  magnetic,  specular,  or  spathic  iron,  and  the  complete  absence  of 
corundum,  chrysolite,  serpentine,  steatite,  pyroxene  rocks,  the  true- 
nacreous  schists,  and  other  minor  forms  observed  in  the  Appalachian 
system. 
Without  doubt,  the  most  interesting  laws  which  come  out  of  the 
comparison  of  these  exposures  are,  that,  when  considered  in  depth,  from 
the  uppermost  limits  of  our  so-called  Huronian  to  the  lowest  Lauren- 
tian exposure,  there  is,  first,  a  regular,  steady  increase  of  the  inten- 
sity of  metamorphism,  and,  secondly,  a  pretty  regular  increase  m  the 
thickness  of  individual  members  of  the  series.  The  lowest  Lauren  fcian 
aplitic  granitoid  bodies  of  the  Laramie  hills  are  the  heaviest  beds  and 
the  most  changed  from  their  original  sedimentary  condition.  The 
higher  Huronian  group  of  gneisses,  quartzites,  conglomerates,  dolo- 
mites, and  argillites  are  at  once  the  most  thinly  bedded  and  least  meta- 
morphosed. Individual  beds  remain  as  specialized  as  the  day  they 
were  deposited.    At  the  lower  exposures  of  the  whole  Archean  forma- 
