GENERAL   ACCOUNT    OF    THE    PRE-CAMBRIAN    ROCKS.  47 
The  correspondence  of  Algonkian  rocks  in  various  parts  of  America 
with  reference  to  lithology  and  succession  is,  thus,  far  from  close,  but 
it  may  be  pointed  out  that  close  correspondence  is  scarcely  to  be  ex- 
pected, even  if  the  rocks  are  of  the  same  age.    To  quote  Chamberlin 
and  Salisbury:" 
The  phases  of  sedimentation  taking  place  about  any  land  mass  at  any  time 
are  largely  dependent  upon  the  height  of  the  land,  the  freeness  of  the  exposure 
of  its  coasts,  the  amount  of  precipitation,  the  climate,  and  the  character  of  the 
formations  suffering  erosion.  Those  various  factors  were  as  likely  to  be  dis- 
similar about  the  various  centers  of  sedimentation  as  to  correspond.  Igneous 
rocks  form  a  not  inconsiderable  part  of  the  Proterozoic  systems,  and  there  is  no 
law  requiring  igneous  activities  in  different  regions  to  correspond  either  in  time 
or  in  the  nature  of  their  products.  Even  body  deformations,  which  are  much 
the  most  general,  and  which  are  the  basis  for  the  subdivision  of  the  systems, 
need  not  have  corresponded  in  their  particular  phases  in  different  regions.  It 
follows  (1)  that  the  number  of  series  with  the  Proterozoic  in  one  place  may  not 
be  the  same  as  in  another;  (2)  that  the  thickness  of  the  various  systems  may 
vary  within  wide  limits;  (3)  that  there  need  have  been  no  close  correspondence 
in  the  sorts  of  rock  in  different  regions  at  the  outset,  and  (4)  that  they  may 
have  been  metamorphosed  unequally  since  their  deposition. 
Even  where  there  is  some  correspondence  between  two  pre-Cam- 
brian  successions,  this  may  not  be  apparent  because  of  difference  in 
amount  of  study  which  has  been  given  to  the  two  regions,  or  because 
the  characters  of  one  or  both  successions  have  been  obscured  by  struc- 
tural and  metamorphic  changes  or  in  part  removed  by  erosion. 
PRE-CAMBRIAN  ROCKS  OF  OTHER  COUNTRIES. 
Little  has  been  clone  toward  the  correlation  of  the  pre-Cambrian 
rocks  of  North  America  with  those  of  other  countries.  Similarities  in 
the  broader  features,  however,  are  beginning  to  be  apparent.  The 
pre-Cambrian  of  Europe,  as  in  the  United  States,  may  be  divided  into 
a  basal  complex  characterized  by  gneiss,  schist,  and  granite,  uncon- 
formably  overlain  by  a  series  dominantly  sedimentary,  for  the  most 
part  lacking  fossils  but  showing  a  few  slight  traces  of  them.  The 
former  corresponds  to  the  Archean,  the  latter  to  the  Algonkian.  It 
is  significant  that  the  terms  Archean  and  Algonkian  are  beginning  to 
appear  in  this  sense  on  some  of  the  European  maps. 
BRITISH    ISLES. 
The  Northwest  Highlands  of  Scotland  afford  one  of  the  most  con- 
tinuously exposed  sections  of  pre-Cambrian  rocks  known.  The  mem- 
bers of  the  Geological  Survey  of  Great  Britain  have  mapped  the  area 
with  great  care,  using  as  a  base  the  British  6-ineh  Ordnance  maps. 
"Text-book  of  geology,  vol.  2,  190C>.  pp.  -1  1  -215. 
