﻿lOO 
  -NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  leaves 
  or 
  needles; 
  so 
  that, 
  in 
  so 
  far 
  as 
  the 
  mineral 
  particles 
  have 
  

   longer 
  diameters, 
  or 
  scalelike 
  shapes, 
  these 
  develop 
  in 
  the 
  planes 
  

   at 
  right 
  angles 
  to 
  the 
  direction 
  of 
  compression 
  and 
  give 
  the 
  rock 
  

   a 
  tendency 
  to 
  split 
  along 
  them. 
  Obviously 
  a 
  better 
  cleavage 
  

   will 
  usually 
  develop 
  in 
  rocks 
  which 
  consist 
  of 
  more 
  than 
  one 
  

   mineral 
  than 
  in 
  those 
  composed 
  chiefly 
  of 
  a 
  single 
  one, 
  and 
  in 
  

   the 
  former 
  case 
  a 
  better 
  cleavage 
  will 
  appear 
  w^here 
  there 
  is 
  

   large 
  difference 
  in 
  the 
  characters 
  of 
  the 
  different 
  mineral 
  species 
  

   than 
  where 
  this 
  difference 
  is 
  small. 
  Thus 
  a 
  quartz-mica 
  rock, 
  or 
  

   a 
  feldspar-hornblende 
  rock, 
  will 
  be 
  apt 
  to 
  have 
  a 
  much 
  better 
  

   foliation 
  than 
  a 
  quartz-feldspar 
  rock. 
  

  

  A 
  rock 
  in 
  which 
  a 
  good 
  foliation 
  cleavage 
  is 
  developed, 
  so 
  that 
  

   it 
  tends 
  to 
  split 
  rather 
  evenly 
  and 
  readily 
  is 
  said 
  to 
  be 
  schistose, 
  

   or 
  called 
  a 
  schist. 
  When 
  the 
  foliation 
  is 
  less 
  even, 
  and 
  less 
  

   ready, 
  gneissoid 
  is 
  the 
  adjective, 
  and 
  gneiss 
  the 
  substantive 
  em- 
  

   ployed. 
  As 
  a 
  general 
  rule 
  certain 
  sediments, 
  such 
  as 
  shales 
  and 
  

   impure 
  (or 
  shaly) 
  limestones 
  and 
  sandstones, 
  recrystallize 
  into 
  

   schists, 
  while 
  pure 
  sandstones 
  and 
  limestones, 
  which 
  recrystallize 
  

   into 
  pure 
  quartz 
  or 
  pure 
  calcite 
  rocks, 
  and 
  consist 
  chiefly 
  of 
  the 
  

   one 
  mineral, 
  show 
  little 
  or 
  no 
  foliation. 
  Igneous 
  rocks 
  are 
  

   usually 
  already 
  crystalline, 
  and 
  in 
  general 
  do 
  not 
  recrystallize 
  

   with 
  as 
  prominent 
  a 
  foliation 
  as 
  do 
  many 
  of 
  the 
  sediments, 
  henc€ 
  

   are 
  more 
  prone 
  to 
  form 
  gneisses 
  than 
  schists. 
  

  

  Foliation 
  in 
  the 
  Granville 
  rocks. 
  The 
  pure 
  Grenville 
  quartzites 
  

   and 
  limestones 
  are 
  now 
  quite 
  massive 
  crystalline 
  rocks 
  wath 
  

   little 
  or 
  no 
  foliation, 
  though 
  there 
  is 
  some 
  development 
  of 
  frac- 
  

   ture 
  cleavage 
  in 
  the 
  resistant 
  quartzites, 
  which 
  is 
  lacking 
  in 
  the 
  

   more 
  plastic 
  limestones. 
  Even 
  the 
  quite 
  impure 
  limestones 
  show 
  

   usually 
  but 
  little 
  foliation. 
  The 
  impure 
  quartzites 
  have 
  de- 
  

   veloped 
  either 
  pyroxene 
  or 
  mica 
  on 
  recrystallizing, 
  usually 
  the 
  

   former, 
  and 
  this 
  rock 
  has 
  poor 
  cleavage 
  while 
  the 
  latter 
  become 
  

   quartz 
  schists. 
  In 
  the 
  mass 
  of 
  Grenville 
  rocks 
  of 
  varying 
  com- 
  

   position 
  to 
  which 
  the 
  general 
  name 
  of 
  the 
  " 
  schist 
  series 
  " 
  has 
  

   been 
  applied, 
  foliation 
  cleavage 
  is 
  in 
  general 
  prominent. 
  But 
  

   even 
  here 
  rocks 
  with 
  considerable 
  development 
  of 
  minerals 
  of 
  

   the 
  mica 
  type 
  are 
  relatively 
  rare, 
  and 
  since 
  such 
  constitute 
  the 
  

   most 
  prominently 
  foliated 
  rocks, 
  their 
  rarity 
  militates 
  against 
  the 
  

   prominence 
  of 
  foliation 
  in 
  the 
  series, 
  the 
  bulk 
  of 
  which 
  would 
  be 
  

   better 
  classed 
  as 
  gneissoid, 
  rather 
  than 
  as 
  schistose. 
  Some 
  varie- 
  

   ties 
  of 
  the 
  amphibolites 
  are 
  quite 
  micaceous 
  and 
  hence 
  possess 
  

   good 
  foliation 
  cleavage. 
  The 
  green 
  schists 
  and 
  ordinary 
  amphi- 
  

   bolites 
  usually 
  show 
  fair 
  foliation 
  only, 
  and 
  a 
  general 
  assemblage 
  

  

  