516  PRK-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bulu86 
and  bedding  of  the  Algonkian  rocks  appear  to  correspond,  the  great 
laccolites  or  batholites  of  gabbro  are  probably  the  chief  cause  of  the 
nietamorphism. 
In  crystalline  series,  when  but  one  structure  can  be  found,  it  is  safe 
only  to  assume  that  it  is  foliation.  Not  only  will  it  not  do  to  use  cleavage 
for  working  out  structure,  but  an  actual  regular  alternation  of  mineral 
constituents  in  schistose  and  gneissoid  rocks  can  not  be  regarded  as  any 
evidence  of  sedimentation.  The  great  series  of  regularly  banded  gneisses 
in  which  alternate  zones  of  nearly  pure  quartz  and  feldspar  andi 
other  zones  in  which  the  bisilicates  are  concentrated,  if  taken  as  due 
to  sedimentation  would  result  in  the  conclusion  that  the  thickness  of. 
the  beds  in  which  these  structures  occur  is  incredibly  great.  In  thor- 
oughly schistose  rocks  it  is  manifest  that  the  best  and  most  reliable 
means  upon  which  to  base  a  conclusion  as  to  strikes  and  dips  is  to  find 
contacts  between  thick  beds  of  rocks  of  a  fundamentally  different  char- 
acter, as  a  layer  of  quartzite,  quartz- schist,  or  mica-schist,  with  lime- 
stone, or  either  of  these  with  gneiss. 
The  clearest,  briefest,  and  most  comprehensive  enunciation  of  the 
principles  applicable  to  a  formation  in  which  are  cleavage  foliation  and 
stratification  foliation  known  to  the  writer  is  that  of  Dale  and  Pum- 
pelly  (see  pp.  768-770),  which  is  here  quoted  in  substance,  slight  modifi 
cations  being  made  to  fit  the  change  of  setting : 
I.  Lamination  in  schist  or  limestone  may  be  either  stratification  foliation  or  cleav- 
age foliation,  or  both,  or  sometimes  "  false  bedding." .  To  establish  conformability 
the  conformability  of  the  stratification  foliations  must  be  shown. 
II.  Stratification  foliation  is  indicated  by :  (a)  The  course  of  minute  but  visible 
plications;  (&)  the  course  of  the  microscopic  plications;  (c)  the  general  course  o] 
the  quartz  laminaB  whenever  they  can  be  clearly  distinguished  from  those  which  lie 
in  the  cleavage  planes. 
This  statement  was  made  with  reference  to  a  particular  district.  It 
is  of  course  wholly  possible  that  some  other  substance  should  play  the 
same  role  as  quartz.  In  the  application  of  these  criteria  it  must  be 
premised  that  the  parting  is  not  a  second  or  third  cleavage.  If  an 
earlier  cleavage  existed  the  criteria  might  give  the  direction  of  this 
first  one  rather  than  the  bedding,  which  might  have  become  obliterated 
at  the  time  of  the  development  of  the  first  cleavage. 
III.  Cleavage  foliation  may  consist  of:  (a)  Planes  produced  by  or  coincident  with 
the  faulted  limbs  of  the  minute  jilications;  (&)  planes  of  fracture,  resembling 
joints  on  a  very  minute  scale,  with  or  without  faulting  of  the  plications;  (c)  a  cleav- 
age approaching  slaty  cleavage  in  which  the  axes  of  all  the  particles  have  assumed 
either  the  direction  of  the  cleavage  or  one  forming  a  very  acute  angle  to  it,  anc 
where  stratification  foliation  is  no  longer  visible ;  (d)  a  secondary  cleavage,  resem- 
bling a  minute  jointing  may  occur. 
IV.  The  degree  and  direction  of  the  pitch  of  a  fold  are  indicated  by  those  of  the; 
axes  of  the  minor  plications  on  its  sides. 
V.  The  strike  of  the  stratification  foliation  and  cleavage  foliation  often  differs  in 
the  same  rock,  and  are  then  regarded  as  indicating  a  pitching  fold. 
VI.  Such  a  correspondence  exists  between  the  stratification  and  cleavage  foliations  [ 
