14  OUTLINE    OF    THIS    PAPER. 
quence  of  inward  crystallization  and  subsequent  deep  denudation.  The  Archean 
rocks  have  no  limit  below,  but  are  limited  above  by  the  Algonkian.  While  structural 
methods  have  been  applied  to  the  Archean  rocks,  they  have  not  thus  far  been  suc- 
cessful, and  the  only  subdivisions  which  are  at  present  applicable  are  those  of  a 
lithological  character. 
In  various  parts  of  North  America  are  one  or  more  series  of  clastic  rocks  between 
the  Archean  and  Cambrian.  These  occur  in  the  Original  Laurentian  area,  the  Orig- 
inal Huronian  area,  in  the  lake  Superior  region,  in  the  region  about  Hudson  bay,  in 
the  Eastern  townships,  in  southern  New  Brunswick,  in  Nova  Scotia  and  cape  Breton, 
in  Newfoundland,  in  the  Black  hills,  in  Missouri,  in  Texas,  in  many  ranges  of  the 
Cordilleras,  in  the  Grand  canyon  of  the  Colorado,  in  British  Columbia,  in  the  Adi- 
rondacks,  and  in  other  areas.  Within  these  rocks,  in  many  localities,  are  evidences 
of  abundant  life,  and  in  a  few  places  are  definitely  recognized  fossils.  The  U.  S. 
Geological  Survey,  recognizing  that  it  is  too  early  to  classify  the  great  thicknesses 
of  rocks  between  the  Archean  and  the  Cambrian  in  North  America  into  systems  co- 
ordinate in  value  with  those  of  the  subdivisions  of  the  Paleozoic,  has  proposed  the 
term  Algonkian  for  the  whole.  The  Algonkian  system  is  then  delimited  below  by 
the  Archean  and  above  by  the  Cambrian. 
In  many  regions  it  is  easy  to  differentiate  the  Algonkian  from  other  rocks.  This 
is  especially  true  where  they  are  separated  from  them  by  unconformities.  In  other 
regions  it  is  difficult  to  separate  them  from  the  Archean  and  Cambrian  rocks,  as 
there  sometimes  appear  to  be  gradations  between  them.  In  this  respect  the  Algon- 
kian system  is  in  no  way  different  from  others. 
Algonkian  stratigraphy  is  more  difficult  than  post- Algonkian  stratigraphy;  be- 
cause the  further  back  we  go  in  the  history  of  the  world  for  any  given  region  the 
more  numerous  and  frequent  have  been  the  changes  through  which  any  given  rock 
stratum  has  passed;  and  because  as  yet  fossils  have  not  been  found  in  sufficient 
quantity  to  be  of  any  assistance  in  stratigraphy.  However,  in  many  regions  it  has 
been  possible  to  subdivide  the  Algonkian  rocks  into  series  and  these  series  into  for- 
mations. In  different  regions  these  formations  and  series  differ  widely  in  the  degree 
of  crystallization,  in  their  lithological  character,  and  in  their  order  of  succession. 
In  this  respect  Algonkian  rocks  are  not  different  from  those  of  post-Algonkian  age. 
Considering  the  continent  as  a  whole,  age  is  no  guide  to  the  chemical  or  mineral  com- 
position, texture,  color,  degree  of  crystallization,  or  any  other  property  of  a  forma- 
tion, or  vice  versa,  although  in  a  given  district  or  region  any  one  of  them  may  be- 
come important  guides  in  stratigraphy. 
In  certain  regions  it  has  been  found  possible  to  correlate  series  with  a  great  de- 
gree of  probability  and  in  some  cases  formations  which  occur  in  different  districts. 
In  regions  separated  from  each  other  by  great  distances,  and  which  therefore  may 
have  had  different  physical  conditions  at  the  same  period,  it  is  not  yet  possible  to 
make  safe  correlations  of  subdivisions  of  the  Algonkian  of  America.  If  this  con- 
clusion be  correct,  it  is  evident  that  the  application  of  such  American  terms  as 
Keweenawan,  Huronian,  etc.,  to  European  rocks  is  not  warranted.  In  working  out 
the  geology  of  a  region,  local  names  should  be  applied  to  formations  and  series. 
Later,  with  fuller  knowledge,  these  may  perhaps  be  correlated  with  series  to  which 
classical  names  have  been  applied. 
Physical  methods  of  correlation  being  the  only  ones  at  present  available,  it  becomes 
necessary  to  closely  scrutinize  these  methods,  estimate  their  relative  value,  and  point 
out  the  services  that  each  may  be  expected  to  perform.  Of  these  methods  uncon- 
formity must  be  given  the  foremost  place  for  major  divisions  within  a  region. 
Unconformity  may  be  established  by  any  one  of  the  following  phenomena,  or  by  a 
combination  of  two  or  more  of  them :  (1)  Ordinary  discordance  of  bedding ;  (2)  differ- 
ence in  the' number  of  dynamic  movements  to  which  the  series  have  been  subjected; 
(3)  discordance  of  bedding  of  upper  series  and  foliation  of  lower;  (4)  relations  with 
eruptives;  (5)  difference  in  degree  of  crystallization;  (6)  basal  conglomerates;  (7) 
general  field  relations. 
