vanhtse]  DTSCtJSSIONP    OF    PRINCIPLED.  48J) 
like  relations  with  elastics  of  Algonkian  and  post  Algonkian  time. 
Upon  either  of  these  theories,  if  sedimentary  rocks  are  only  buried  deep 
enough,  they  will  pass  into  crystallines  by  progressive  metamorphism 
or  by  subcrustal  fusion,  just  as  do  rocks  of  Cambrian  and  post-Cambrian 
age.  This  the  elder  Hitchcock  so  clearly  saw  that  he  distinctly  said 
that  the  so-called  Laurentian  granites  and  gneisses  of  Vermontare  prob- 
ably, in  part  at  least,  not  older  than  the  fossiliferons  series.  If  the 
Archean  be  made  to  include  all  the  thoroughly  crystalline  rocks  below 
pre  Cambrian  elastics,  it  includes  rocks  the  age  of  which  varies  from 
Algonkian  to  pre- Algonkian.  This  anomaly  is  perhaps  best  met  by 
making  a  more  or  less  arbitrary  division  between  Archean  and  post- 
Archean  crystallines.  The  natural  theoretical  plane  to  choose  is  the 
beginning  of  life;  that  is,  to  include  in  the  Archean  all  truly  azoic 
rocks.  While  this  suggestion  has  a  plausible  sound,  we  must  believe 
that  the  dawn  of  life  was  very  gradual  and  that  its  traces  in  its  early 
stages  are  exceedingly  sparse,  so  that  there  would  be  great,  if  not  in- 
superable difficulties  in  its  practical  application. 
If  the  third  theory,  that  the  Archean  includes  only  pre-sedimentary 
rocks,  be  correct,  its  upward  limit  is  easy  to  define;  the  Algonkian 
begins  for  each  region  at  the  time  of  the  deposition  of  the  first  sedi- 
mentaries. But  there  are  those  who  deny  the  existence  at  the  present 
time  of  any  such  ancient  rocks,  although  they  concede  their  existence  at 
one  time,  and  they  believe  that  the  Archean  as  thus  defined  represents  a 
vast  lapse  of  time  in  the  history  of  the  earth.  This  denial  of  the  present 
existence  of  any  rocks  of  greater  age  than  the  oldest  sedimentaries  is  of 
course  a  pure  unverified  assumption  defended  on  the  ground  of  probabil- 
ity. If  the  original  crust  of  the  earth  be  defined  as  including  more  than 
the  first  outer  skin,  it  is  a  question  whether  the  converse  proposition  is 
not  eo, u ally  probable.  Even  if  the  position  be  true,  the  school  that  be- 
lieves in  the  igneous  origin  of  the  Archean  would  still  have  a  large  mass 
of  rocks  for  the  Archean  by  shifting  their  ground  so  as  to  include  in  it 
all  the  material  which  in  the  slow  process  of  inward  crystallization  has 
now  reached  the  surface  of  the  earth,  not  by  intrusion  in  the  rocks 
above,  but  by  erosion.  This  position  would,  however,  be  controverted 
by  those  who  regard  such  rocks  as  plutonic  and  belonging  to  the  age 
of  their  equivalent  sedimentaries.  But  in  the  nature  of  the  case  it  is 
not  possible  to  designate  the  particular  age  to  which  these  rocks  belong. 
That  there  exists  upon  the  surface  of  the  earth  a,  part  of  the  original 
crust  of  the  earth,  or  its  downward  continuation  by  later  cooling,  can 
hardly  be  doubted;  and  since  these  can  never  be  assigned  to  any  defi- 
nite period  of  sedimentaton  they  might  well  be  considered  as  Archean. 
At  any  rate  they  are  a  class  by  themselves  which,  if  not  here  placed, 
can  not  be  referred  to  any  of  the  geological  periods.  Further,  this  class 
of  rocks  when  in  contact  with  detritals  of  whatever  age,  by  the  very 
hypothesis  of  their  origin  mustiest  ui  icon  formal)  ly  below  them.  The 
coincidence  that  so  frequently,  it'  not    always,  there  is  really   a  great 
