486  PRE-CAMBRIAN    ROCKS    OF    NORTH   AMERICA.  [bull. 86: 
hiatus  between  the  ancient  sedimentables  and  the  basal  complex  might 
be  urged  as  evidence  of  the  truthfulness  of  this  hypothesis.  It  is  in- 
teresting here  to  remember  that  Emmons  denned  pyrocrystalline  rocks 
as  those  due  to  the  consolidation  of  the  earth's  crust,  which  rocks  were 
said  to  increase  in  thickness  by  additions  below. 
Sedimentation  must  have  begun  in  the  earliest  seas,  while  upon 
parts  of  possible  continental  areas  volcanic  materials  alone  were  still 
accumulating.  These  latter,  in  accordance  with  the  definition,  would 
belong  to  the  Archean.  There  would  be  in  this  case  no  positive  equat- 
ing one  with  the  other.  When  later,  upon  these  Archean  rocks  con- 
temporaneous with  the  earlier  Algonkian  rocks,  sedimentaries  began  to 
form,  this  would  be  for  this  region  the  opening  of  the  Algonkian. 
However,  it  is  not  impossible  that  all  such  supposed  contemporaneous 
Archean  materials  may  have  been  carried  away  by  erosion.  Certainly 
this  would  have  been  the  case  with  a  large  portion  of  them,  and  it  fol- 
lows that  this  difficulty  may  be  rather  theoretical  than  practical. 
The  banded  and  contorted  granite-gneiss  which  serves  as  a  back- 
ground for  the  Archean  may  not  improbably  be  the  part  which  has  the 
origin  above  suggested,  while  the  other  parts  of  the  complex  may  be 
due  to  subsequent  intrusives;  the  whole  being  kneaded  into  their  pres- 
ent extraordinary  complex  relations  by  repeated  dynamic  movements 
and  other  metamorphic  influences.  This  igneous  theory  of  the  origin 
of  the  Archean,  modified  so  as  to  include  the  pre-sedimentary  original 
crust,  if  any  remains,  and  the  deeper  crust  which  has  reached  the  sur- 
face by  denudation,  perhaps  more  nearly  covers  the  facts  than  any 
other  as  to  the  relations  of  the  Archean  to  subsequent  rocks,  its  com- 
plex lithological  character,  the  relations  of  the  rock  phases  to  each 
other,  and  the  long  history  written  in  the  strained,  altered,  and  broken 
mineral  constituents.  It  accords  with  the  idea  held  by  Irving,  Bou- 
ncy, and  others,  that  this  earliest  crystalline  complex  was  produced 
under  conditions  differing  from  those  of  the  rocks  of  any  subsequent 
period. 
But  the  difficulties  in  the  theoretical  delimitation  of  the  top  of  the 
Archean  are  so  great  that  I  prefer  to  confine  myself  to  a  statement  of 
some  possible  solutions  rather  than  to  commit  myself  to  any  theory, 
although  now  inclining  toward  the  third  theory  modified  as  suggested. 
Although  the  obstacles  are  not -nearly  so  great  in  delimiting  later  peri-' 
ods,  the  difficulties  of  making  an  exact  definition  for  the  Silurian,  De- 
vonian, or  Carboniferous  are  so  considerable  that  almost  any  of  those 
given  have  been  found  to  controvert  the  facts  of  some  locality.  If 
this  is  the  case  with  reference  to  these  later  periods  in  which  so  much 
more  is  known,  it  should  not  be  surprising  that  the  obstacles  to  an 
accurate  delimitation  of  the  Archean  are  at  present  apparently  insu- : 
perable. 
But  while  it  is  impossible  to  make  a  wholly  satisfactory  theoretical 
definition  of  the  Archean,  it  is  frequently  easy  in  the  field  to  say  with 
a  great  degree  of  probability  what  rocks  are  Archean  and  what  post- 
