﻿I06 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  The 
  foliation 
  of 
  the 
  Laurentian 
  granite 
  gneiss, 
  and 
  of 
  the 
  gneis- 
  

   soid 
  portion 
  of 
  the 
  Alexandria 
  syenite 
  is 
  concordant 
  with 
  that 
  of 
  

   the 
  Grenville 
  rocks, 
  and 
  in 
  them 
  these 
  same 
  joint 
  sets 
  are 
  developed, 
  

   though 
  in 
  a 
  much 
  less 
  prominent 
  way. 
  In 
  the 
  more 
  massive 
  igne- 
  

   ous 
  rocks 
  they 
  are 
  replaced 
  by 
  a 
  set 
  of 
  vertical, 
  northeast 
  joints. 
  

  

  At 
  right 
  angles 
  to 
  the 
  set, 
  or 
  sets, 
  of 
  northeast 
  joints 
  is 
  a 
  set 
  

   with 
  northwest 
  trend, 
  with 
  planes 
  nearly 
  or 
  quite 
  vertical, 
  and 
  rang- 
  

   ing 
  from 
  n. 
  40° 
  w. 
  to 
  n. 
  55° 
  w. 
  in 
  direction, 
  27 
  of 
  the 
  readings 
  

   falling 
  within 
  those 
  limits. 
  A 
  less 
  conspicuous 
  east-west 
  set 
  is 
  also 
  

   indicated 
  by 
  the 
  12 
  readings 
  between 
  n. 
  70° 
  w. 
  and 
  n. 
  80° 
  w., 
  to- 
  

   gether 
  with 
  the 
  14 
  between 
  n. 
  80° 
  e. 
  and 
  e. 
  As 
  seen 
  in 
  the 
  field 
  

   also 
  this 
  set 
  is 
  more 
  variable 
  and 
  less 
  prominent 
  than 
  the 
  north- 
  

   west 
  set. 
  The 
  number 
  of 
  northerly 
  readings 
  is 
  not 
  great, 
  and 
  is 
  

   spread 
  rather 
  uniformly 
  over 
  50° 
  of 
  compass 
  range, 
  coinciding 
  with 
  

   the 
  impression 
  given 
  in 
  the 
  field 
  as 
  to 
  the 
  comparative 
  scarcity 
  and 
  

   great 
  irregularity 
  of 
  that 
  joint 
  set. 
  

  

  Notwithstanding 
  the 
  rather 
  small 
  number 
  of 
  total 
  readings 
  the 
  

   diagram 
  shows 
  that 
  30 
  out 
  of 
  the 
  possible 
  number 
  of 
  36 
  different 
  

   5° 
  directions 
  are 
  represented. 
  Nowhere 
  in 
  the 
  field 
  were 
  more 
  than 
  

   four 
  sets 
  of 
  joints 
  noted 
  in 
  a 
  given 
  rock 
  exposure, 
  and 
  all 
  the 
  

   joints 
  showed 
  considerable 
  tendency 
  to 
  curve 
  and 
  vary 
  in 
  direction, 
  

   leading 
  to 
  the 
  belief 
  that 
  this 
  spreading 
  of 
  the 
  readings 
  is 
  owing 
  

   to 
  this 
  variability 
  and 
  in 
  no 
  wise 
  indicative 
  of 
  a 
  great 
  number 
  of 
  

   joint 
  sets. 
  

  

  Locally 
  the 
  more 
  rigid 
  of 
  the 
  Precambric 
  rocks, 
  the 
  quartzites 
  

   and 
  granites, 
  are 
  excessively 
  jointed, 
  the 
  joints 
  being 
  very 
  close 
  

   spaced, 
  chopping 
  up 
  the 
  rock 
  into 
  small, 
  angular 
  blocks 
  [see 
  pi. 
  

   3]. 
  In 
  such 
  places 
  signs 
  of 
  slipping 
  are 
  usually 
  to 
  be 
  made 
  out. 
  

   These 
  so 
  called 
  " 
  shear 
  zones 
  " 
  result 
  from 
  readjustment 
  under 
  com- 
  

   pression 
  under 
  conditions 
  such 
  that 
  these 
  rigid 
  rocks 
  fractured 
  and 
  

   slipped 
  along 
  the 
  fractures, 
  while 
  those 
  less 
  rigid, 
  the 
  limestones 
  for 
  

   example, 
  effected 
  readjustment 
  in 
  other 
  manner. 
  

  

  That 
  the 
  Precambric 
  rocks 
  were 
  jointed 
  prior 
  to 
  the 
  deposition 
  of 
  

   the 
  Potsdam 
  sandstone 
  is 
  conclusively 
  shown, 
  firstly 
  by 
  the 
  absence 
  

   in 
  the 
  Paleozoic 
  rocks 
  of 
  compression 
  joints 
  and 
  shear 
  zones, 
  and 
  

   secondly 
  by 
  the 
  occurrence 
  of 
  joint 
  cracks 
  in 
  the 
  Grenville 
  lime- 
  

   stones 
  which 
  became 
  widened 
  by 
  solution 
  and 
  in 
  that 
  condition 
  were 
  

   filled 
  with 
  sand 
  as 
  the 
  Potsdam 
  sands 
  commenced 
  to 
  be 
  deposited. 
  

   In 
  the 
  few 
  cases 
  in 
  the 
  district 
  where 
  contacts 
  between 
  Potsdam 
  

   and 
  Grenville 
  limestone 
  are 
  exposed 
  these 
  features 
  appear 
  [see 
  

   fi^- 
  I 
  J 
  P- 
  5^] 
  ^^^ 
  ^^^ 
  apparently 
  widespread. 
  

  

  