GENERAL  ACCOUNT    OF    THE    PRE-CAMBRIAN    ROCKS.  31 
a  hydrosphere  in  which  sedimentary  rocks  could  be  deposited  at  a 
period  long  antedating  that  of  the  formation  of  the  Archean  rocks 
which  now  appear  at  the  surface.  If  erosion  were  to  cut  much  deeper 
into  the  Archean  it  might  still  show  shreds  of  sedimentary  rocks, 
which  might  decrease  in  quantity  the  further  erosion  continued.  On 
this  hypothesis  also  the  earlier  rocks  should  be  less  well  assorted  than 
the  later  ones.  As  already  noted,  Chamberlin  and  Salisbury  contrast 
the  little-assorted  "  wacke  "  type  of  the  Archean  and  later  systems. 
Cleavage  .and  gneissose  structure  in  the  Archean  and  crustal 
shortening. — One  of  the  most  conspicuous  features  of  the  Archean 
rocks  is  the  prevalence  of  banded,  gneissose,  and  schistose  structures 
over  very  large  areas.  That  these  structures  are  in  some  part  de- 
veloped by  a  secondary  mechanical  action  admits  of  no  doubt,  It 
seems  to  be  likely,  however,  that  certain  of  the  gneissose  and  banded 
structures  were  developed  before  the  masses  had  finally  cooled.  Seder- 
holm  thinks  he  has  shown  this  conclusively  for  certain  of  the  Finnish 
rocks.  If  the  structures  are  secondary  it  is  certain  that  some  of  the 
rocks  have  been  under  such  extreme  conditions  of  pressure  and  tem- 
perature as  practically  to  be  brought  again  into  the  plastic  state. 
That  the  Archean  rocks  should  show  more  secondary  deformation 
than  the  later  ones  is  to  be  expected  from  their  greater  age.  Most 
theories  of  crustal  deformation  postulate  a  shearing  or  tangential 
movement  of  a  rigid  outer  crust  over  a  liquid,  plastic,  or  semiplastic 
interior.  The  observed  facts  in  the  field  suggest  the  existence  of 
such  a  slip  zone.  Van  Hise  has  suggested  that  the  slipping  below 
a  rigid  crust  is  due  to  the  fact  that  the  movement  here  is  by  rock 
flowage,  Avhich  seems  to  require  less  energy  than  deformation  by  rock 
fracture,  which  the  upper  rigid  zone  must  suffer.  Van  Hise  has 
further  suggested  that  the  eastward  dip  characteristic  of  many  of  the 
Archean  areas  of  North  America  may  have  been  developed  during  a 
westward  slipping  of  the  crust  under  tidal  stresses.  Chamberlin  has 
suggested  that  if  the  sea  beds  represent  areas  of  sunken  wedges  or 
blocks  of  the  earth,  and  the  continent  the  projecting  wedges  or  blocks, 
there  should  be  a  thrust  developed  near  their  contact  such  as  to 
produce  a  cleavage  dipping  toward  the  continental  areas  and  away 
from  the  sea. 
The  large  amount  of  folding  and  cleavage  in  the  Archean  has 
been  taken  to  imply  a  much  larger  amount  of  crustal  shortening  for 
this  period  than  has  occurred  in  any  subsequent  period.  It  is  diffi- 
cult to  make  any  quantitative  estimate  because  all  later  deformations 
have  affected  the  Archean,  because  the  cleavage  may  indicate  in  part 
tangential  shearing  of  an  outer  over  an  inner  shell,  as  described  above, 
rather  than  crustal  shortening,  and  because  the  vast  masses  of  igneous 
intrusions  of  the  pre-Cambrian  occupy  space  compensated  by  the 
folded  and  schistose  rocks  surrounding  them.     While  the  folding  and 
