﻿I04 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  into 
  two 
  pairs, 
  each 
  pair 
  consisting 
  of 
  two 
  joint 
  sets 
  at 
  right 
  

   angles 
  to 
  one 
  another, 
  and 
  the 
  joints 
  of 
  one 
  pair 
  bisecting 
  the 
  

   angles 
  between 
  the 
  joints 
  of 
  the 
  other 
  pair. 
  In 
  such 
  districts 
  it 
  

   is 
  seldom 
  the 
  case 
  that 
  all 
  four 
  joint 
  sets 
  are 
  exhibited 
  in 
  a 
  

   single 
  rock 
  exposure, 
  two 
  or 
  perhaps 
  three 
  of 
  the 
  four 
  showing, 
  

   rather 
  than 
  the 
  whole 
  number. 
  In 
  many, 
  if 
  not 
  in 
  most, 
  regions 
  

   where 
  four 
  or 
  more 
  joint 
  sets 
  occur, 
  it 
  is 
  found 
  that 
  one 
  pair 
  

   tends 
  to 
  north-south 
  and 
  east-west 
  directions, 
  with 
  another 
  pair 
  

   showing 
  northeast 
  and 
  northwest 
  trends. 
  The 
  joint 
  planes 
  often 
  

   curve 
  somewhat, 
  so 
  that 
  the 
  compass 
  direction 
  of 
  a 
  given 
  set 
  may 
  

   var}' 
  through 
  a 
  considerable 
  number 
  of 
  degrees. 
  This 
  tendency 
  

   much 
  increases 
  the 
  difficulty 
  of 
  discrimination 
  between 
  the 
  dif- 
  

   ferent 
  sets 
  in 
  districts 
  where 
  more 
  than 
  four 
  are 
  present, 
  as 
  is 
  

   quite 
  frequently 
  the 
  case. 
  

  

  In 
  folded 
  rocks 
  the 
  character 
  of 
  the 
  jointing 
  differs 
  considerably 
  

   from 
  that. 
  found 
  in 
  rocks 
  not 
  folded. 
  Since 
  in 
  our 
  region 
  here 
  we 
  

   have 
  rock 
  masses 
  of 
  each 
  sort, 
  Precambric 
  rocks 
  which 
  have 
  been 
  

   greatly 
  compressed 
  and 
  folded, 
  and 
  overlying 
  Paleozoic 
  rocks 
  which 
  

   are 
  comparatively 
  undisturbed, 
  it 
  will 
  be 
  convenient 
  to 
  consider 
  

   them 
  separately. 
  

  

  In 
  the 
  Precambric 
  rocks. 
  The 
  diagram 
  [fig. 
  5] 
  presents 
  a 
  

   summation 
  of 
  the 
  readings 
  taken 
  on 
  the 
  joints 
  of 
  the 
  Precambric 
  

   rocks 
  of 
  the 
  district 
  included 
  in 
  the 
  maps. 
  They 
  are 
  comparatively 
  

   few 
  in 
  number, 
  partly 
  because 
  of 
  the 
  comparatively 
  small 
  area 
  

   which 
  presents 
  these 
  rocks 
  at 
  the 
  surface, 
  and 
  partly 
  because 
  the 
  

   joints 
  were 
  found 
  to 
  be 
  so 
  irregular 
  that 
  no 
  satisfactory 
  readings 
  

   could 
  be 
  obtained 
  in 
  many 
  exposures. 
  The 
  rocks 
  are 
  not 
  as 
  abun- 
  

   dantly 
  jointed, 
  nor 
  are 
  the 
  joints 
  as 
  clear-cut 
  as 
  usual 
  in 
  the 
  Adiron- 
  

   dack 
  region. 
  

  

  In 
  closely 
  folded 
  sediments, 
  such 
  as 
  the 
  Grenville, 
  joints 
  are 
  apt 
  

   to 
  be 
  present 
  as 
  a 
  result 
  of 
  compression, 
  and 
  to 
  have 
  their 
  direc- 
  

   tions 
  controlled 
  to 
  a 
  considerable 
  extent 
  by 
  the 
  folds, 
  or 
  in 
  other 
  

   words 
  by 
  the 
  strike 
  and 
  dip 
  of 
  the 
  folded 
  sediments. 
  These 
  have 
  

   been 
  shown 
  to 
  have 
  a 
  general 
  northeast 
  strike 
  throughout 
  the 
  dis- 
  

   trict, 
  though 
  locally 
  varying 
  in 
  direction 
  through 
  more 
  than 
  90°. 
  

   The 
  more 
  usual 
  direction 
  however 
  is 
  n. 
  40° 
  e.-n. 
  60° 
  e. 
  Two 
  sets 
  of 
  

   joints 
  are 
  present 
  which 
  have 
  the 
  same 
  surface 
  trend, 
  that 
  of 
  the 
  

   rock 
  strike, 
  the 
  one 
  set 
  controlled 
  by 
  the 
  dip 
  and 
  having 
  approxi- 
  

   matelv 
  the 
  same 
  inclination, 
  the 
  other 
  inclined 
  in 
  the 
  opposite 
  direc- 
  

   tion, 
  or 
  to 
  the 
  southwest, 
  and 
  closely 
  at 
  right 
  angles 
  to 
  the 
  first 
  

  

  