■vanhisb.]  DISCUSSIONS    OF    PRINCIPLES:  515 
That  it  will  not  do  to  regard  slaty  cleavage  or  foliation  as  bedding 
has  long  been  recognized,  although  it  has  often  occurred  that  in  re- 
gions in  which  this  has  been  distinctly  stated  it  has  also  been  said  that 
bedding  and  cleavage  do  correspond  without  any  evidence  of  such  cor- 
respondence being  given.  If  tangential  thrust  be  assumed  as  the  cause 
of  foliation,  unless  the  resultant  folding  be  close,  bedding  and  cleavage 
will  usually  not  correspond  for  any  considerable  area ;  for  cleavage  and 
foliation  have  a  tendency  to  develop  transverse  to  the  lines  of  pressure,, 
while  bedding  is  initially  in  the  lines  of  pressure.  In  the  closely  folded 
series  in  which  it  may  be  said  that  resultant  foliation  and  sedimentation 
correspond  throughout,  the  use  of  the  term  bedding  ought  to  be  dropped, 
as  there  is  no  longer  a  basis  upon  which  to  estimate  thickness,  because 
in  this  case  there  must  have  been  such  an  amount  of  movement  of  the 
particles  within  the  beds  over  each  other  as  to  render  it  doubtful  if 
bedding  does  still  exist. 
By  the  foregoing  it  is  not  meant  to  imply  that  the  foliation  of  thor- 
oughly crystalline  rocks  may  not  widely  correspond  with  bedding,  but 
only  that  this  is  not  usually  the  case  when  tangential  thrust  is  the  cause 
of  the  foliation.  However,  Smyth  (H.  L.)  has  discovered  when  there  is 
an  alternation  of  beds  of  different  degrees  of  massiveness  that  these 
often  control  the  movements  of  accommodation  during  the  folding  of 
the  series,  and  that  the  slipping  of  the  particles  over  each  other  par- 
allel to  the  bedding  develops  schistose  structure  along  the  same 
planes.  It  is  not  impossible  that  deeply  buried  beds  may  become 
thoroughly  crystalline,  with  foliation  and  bedding  parallel  when  super- 
incumbent pressure  and  metasomatic  changes  are  the  predominant 
forces.  A  sufficient  degree  of  heat  for  recrystallization  may  be  engen- 
dered by  very  deep  burying  or  by  laccolith'  intrusions.  In  such  cases 
the  structure  of  the  recrystallized  rock  will  naturally  conform  to  the 
bedding,  and  it  is  probable  that  differences  in  the  original  characters 
of  the  layers  will  be  preserved  in  the  metamorphosed  rock.  Arnica- 
schist  or  gneiss  thus  derived  from  a  shale  or  an  arkose,  now  showing 
in  its  interior  structure  no  evidence  of  clastic  character,  might  be 
underlain  by  a  quartzite  which  was  produced  by  the  cementation 
of  a  quartzose  sandstone.  The  quartzite  at  the  present  time  would 
reveal  its  detrital  origin,  while  the  mica -schist  or  gneiss  might  not. 
Some  such  explanation  seems  to  fit  the  thick  beds  of  mica-schist 
(the  structures  of  which  unmistakably  correspond  with  bedding) 
which  overlie  the  quartzites  of  the  Penokee  series  of  Michigan  and 
Wisconsin.  A  crystalline  series  of  the  origin  suggested,  Avhich  subse- 
quently reached  the  surface  by  denudation,  might  be  folded,  but  not 
sufficiently  to  produce  a  new  secondary  structure,  when  the  different 
bands  of  different  characters  would  truly  represent  sedimentary  beds. 
Some  such  explanation  seems  to  fit  the  gently  folded  thoroughly  crys- 
talline mica-schists  and  gneisses  of  the  Blue  ridge  west  of  Old  fort, 
liNorth  Carolina.    In  the  Adirondacks,  where  the  schistose  structure 
