374  PRE-CAMBRIAN    ROCKS    OF    NORTH    AMERICA.  [bui  r. se, 
of  Greylock  suggests  that  the  inetamorphisin  is  in  general  in  accord- 
ance with  this  idea  eastward,  and  also  perhaps  downward. 
As  a  result  of  the  work  it  is  concluded  that  the  gneisses  of  the  Green 
mountains  are  just  as  capable  of  stratigraphical  investigation  as  the 
unaltered  sediments  of  the  Appalachians,  only  the  problem  is  much 
more  difficult,  owing  to  the  secondary  structures  produced  by  metamor- 
phism. 
Dale,68  in  1889,  gives  a  systematic  treatment  of  the  structural  and 
areal  geology  of  Greylock.  The  rocks  are  all  metamorphic  and  of  few 
kinds,  crystalline  limestone  and  various  schists.  The  key  to  the  struc- 
ture is  in  the  distinction  between  cleavage  foliation  and  stratification 
foliation.  The  phenomena  of  cleavage  and  stratification  as  studied  on 
Greylock  lead  to  the  adoption  of  the  following  structural  principles: 
1.  Lamination  in  schist  or  limestone  may  be  either  stratification  folia- 
tion or  cleavage  foliation,  or  both,  or  sometimes,  in  limestone  at  least, 
"  false  bedding."  To  establish  conforinability  the  conformability  of  the 
stratification  foliations  must  be  shown.  II.  Stratification  foliation  is 
indicated  by:  (a)  The  course  of  minute  but  visible  plications;  (b)  the 
course  of  the  microscopic  plications ;  (c)  the  general  course  of  the  quartz 
laminae  whenever  they  can  be  clearly  distinguished  from  those  which 
lie  in  the  cleavage  planes.  III.  Cleavage  foliation  may  consist  of:  (a) 
Planes  produced  by  or  coincident  with  the  faulted  limbs  of  the  minute 
plications;  (b)  planes  of  fracture,  resembling  joints  on  a  very  minute 
scale,  with  or  without  faulting  of  the  plications;  (c)  a  cleavage  ap- 
proaching 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,  and  where  stratification  foliation  is  no  longer  visible.  IV. 
A  secondary  cleavage,  resembling  a  minute  jointing,  occurs  in  scattered 
localities.  V.  As  ascertained  by  Pnmpelly,  the  degree  and  direction 
of  the  pitch  of  a  fold  are  indicated  by  those  of  the  axes  of  the  minor 
plications.  VI.  The  strike  of  the  stratification  foliation  and  cleavage 
foliation  often  differ  in  the  same  rock,  and,  as  suggested  by  Pnmpelly, 
are  then  regarded  as  indicating  a  pitching  fold.  When  the  fold  is 
horizontal  the  two  are  parallel.  VII.  Such  a  correspondence  exists 
between  the  stratification  foliations  and  cleavage  foliations  of  the  great 
iolds  and  those  of  the  minute  plications  that  a  very  small  specimen 
properly  oriented  gives;  in  many  cases,  the  key  to  the  structure  over  a 
large  portion  of  the  side  of  a  fold. 
There  are  huge  areas,  sometimes  half  a  mile  square,  where  the  only 
foliation  presented  by  the  outcrops  is  of  secondary  character  and  where 
no  trace  of  stratification  can  be  detected.  As  the  cleavage  foliation  in 
some  places  coincides  with  the  stratification  foliation  both  in  strike  and 
dip,  in  others  agrees  in  strike  while  differing  in  angle  of  dip,  and  in  still 
others  differs  both  in  strike  and  dip,  and,  furthermore,  as  the  marks 
of  stratification  are  not  infrequently  subject  to  purely  local  changes, 
the  whole  matter  is  attended  with  much  difficulty.     As  a  rule  the 
