van  hise.1  DISCUSSIONS   QF   PRINCIPLES.  487 
Archean.  For  instance,  in  the  Arizona)  region,  as  has  been  seen,  above 
the  typical  Archean  complex  there  is  the  most  profound  unconformity, 
upon  the  upper  side  of  which  the  rocks  are  readily  recognized  elastics. 
From  the  writer's  point  of  view  the  same  thing  is  true  for  a  large  part 
of  the  lake  Superior  region.  In  the  Uinta  and  Wasatch  mountains, 
again,  below  the  quartzites  of  probable  Algonkian  age,  is  a  great  un- 
conformity, and  then  appears  the  implicated  Archean.  In  certain  other 
regions  the  separation  of  the  Algonkian  and  the  Archean  is  a  matter 
of  exceeding  difficulty.  As  representative  of  this  class  of  cases  may 
be  taken  the  Front  range  of  Colorado,  along  the  east  side  of  which  are 
unmistakable  elastics,  with  an  apparent  gradation  between  them  and 
the  crystalline  complex.  In  the  Appalachians,  again,  where  for  the 
most  part  the  oldest  clastic  rocks  recognizable  are  Cambrian,  it  can 
not  be  said  whether  the  crystalline  complex  below  is  Algonkian  or  Ar- 
chean. Here  the  separation  of  the  Cambrian  from  the  pre- Cambrian 
has  been  accomplished  only  by  a  minute  and  laborious  study.  The  two 
appeared  to  be  in  conformity  and  to  grade  into  each  other.  It  is  only 
recently  that  this  gradation  has  been  shown  by JPumpelly  to  be  consist- 
ent with  a  great  unconformity  between  the  two.  The  causes  produc- 
ing this  gradation  between  the  Cambrian  and  pre-Cambrian  in  Massa- 
chusetts (post- Cambrian  dynamic  action  and  pre-Cambrian  disintegra- 
tion) may  also  be  found  to  explain  the  conformities  and  gradations  be- 
tween the  Algonkian  and  Archean. 
While  it  is  then  not  easy  to  define  the  Archean,  it  is  plain  that  the 
discrimination  in  the  field  between  Archean  and  Algonkian  is  a  real 
one  and  should  continue  to  be  applied  even  if  its  exact  theoretical 
meaning  can  not  be  said  to  be  certainly  known.  It  has  been  the  cus- 
tom in  the  past  to  refer  to  the  Archean  practically  all  crystalline  rocks, 
with  many  semicrystalline  rocks  which  seem  to  be  old,  or  the  age  of 
which  was  not  determined.  Under  this  practice  vast  areas  in  the  Ap- 
palachians have  been  referred  to  the  Archean,  which  are  now  being 
placed  in  the  Cambrian,  Devonian,  and  Carboniferous.  Doubtless  in 
the  same  way  many  other  areas  which  have  been  placed  in  the  Archean 
upon  closer  study  will  be  removed  from  it  and  the  rocks  distributed 
from  the  Algonkian  upward.  At  this  point  a  reform  in  geology  is 
needed.  If,  for  instance,  the  oldest  rocks  of  clearly  recognizable  age 
are  Triassic,  and  these  Triassic  rocks  rest  upon  a  complex,  the  struc- 
tural relations  of  which  are  not  studied  in  detail,  this  complex  should 
be  denominated  pre-Triassic  rather  than  Archean.  A  large  part  of  the 
difficulty  in  getting  to  understand  from  the  literature  the  actual  facts 
as  to  the  occurrences  and  relations  of  the  crystalline  rocks  has  arisen 
from  this  practice  of  using  the  Archean  as  the  dumping  ground  for 
everything  of  unknown  age. 
STRATIGRAPHY    OF   ARCHEAN. 
In  characterizing  the  Archean  the  methods  applicable  to  its  subdi- 
vision are  clearly  pointed  out.     If  no  part  of  it  is  demonstrably  sedi- 
