30  PRE-CAMBRIAN    GEOLOGY   OF    NORTH    AMERICA. 
rocks  when  buried  under  great  thickness  of  later  rocks,  and  that,  after 
having  undergone  subcrustal  fusion,  they  have  invaded  the  overlying 
rocks  and  thus  may  have  served  as  the  basement  upon  which  the  over- 
lying rocks  were  deposited,  notwithstanding  the  fact  that  they  now 
show  intrusive  relations  to  the  overlying  rocks.  Such  intrusive  rocks, 
whatever  their  ultimate  source,  are  igneous  at  the  point  of  intrusion, 
and  thus  the  subject  of  subcrustal  fusion  becomes  one  of  the  origin 
of  igneous  rocks,  which  need  for  our  purposes  be  only  incidentally  re- 
ferred to.  The  particular  evidence  brought  forward  that  some  of  the 
Archean  igneous  rocks  have  resulted  from  fusion  of  sediments  is: 
(1)  Observed  gradation  between  igneous  rocks  and  sediments;  (2) 
composition  of  the  igneous  rocks  influenced  by  the  digestion  of  the 
adjacent  sedimentary  rocks;  (3)  density  stratification  of  the  digested 
rock  resulting  in  differentiation,  thus  perhaps  causing  the  part  of 
the  igneous  mass  in  contact  with  the  given  sedimentary  to  have  a  dif- 
ferent composition  from  that  required  by  the  digestion  of  the  adja- 
cent sediments;  and  finally,  (4)  broad  field  relations,  as  for  instance 
where  great  oval  areas  of  granite  bulge  up  through  limestone  or  other 
formations,  as  in  the  Hastings  and  original  Laurentian  districts.  In 
limited  areas  evidence  of  subcrustal  fusion  seems  to  be  reasonably  con- 
clusive, but  to  the  present  time  the  quantity  of  material  which  has 
been  proved  to  be  of  this  origin  is  insignificant  as  compared  with  the 
masses  of  igneous  rocks  in  the  Archean.  In  the  nature  of  things,  con- 
clusive evidence  could  scarcely  be  expected  throughout  these  masses. 
That  subcrustal  fusion  has  taken  place  upon  such  a  scale  as  to  account 
for  any  large  part  of  the  Laurentian  is  yet  unproved. 
Departure  from  vniformitarianism. — When  we  consider  the  relative 
proportions  of  rocks  of  igneous  and  sedimentary  origin  the  Archean 
does  represent  a  departure  from  the  uniformity  of  conditions  of  later 
geological  time.  Igneous  rocks  make  up  by  far  the  greater  part  of 
the  Archean.  The  Archean  was  essentially  a  period  of  world-wide 
volcanism.  Sedimentary  rocks  are  relatively  insignificant.  The 
sediments  may  have  been  more  abundant  in  upper  horizons  of  the 
Archean  than  in-  lower,  and  have  been  largely  removed  by  erosion. 
Sedimentary  rocks  may  have  been  metamorphosed  beyond  recogni- 
tion into  gneisses  and  schists.  To  some  extent  this  is  doubtless  true, 
but  even  if  a  considerable  part  of  the  schists  and  gneisses  of  unknown 
origin  were  found  ultimately  to  be  sedimentary  the  sedimentary  part 
of  the  Archean  would  still  be  subordinate  to  the  igneous  part. 
So  far  as  present  evidence  of  the  relative  importance  of  sediments 
and  igneous  rocks  in  the  Archean  is  concerned,  the  known  facts  do 
not  contradict  Chamberlin's  hypothesis  of  the  development  of  the 
earth  by  solid  meteoric  accretion.  On  this  hypothesis  the  earth 
must  have  reached  a  sufficient  mass  to  hold  at  least  the  beginnings  of 
