﻿124 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  past 
  one 
  another 
  except 
  under 
  the 
  weight 
  of 
  a 
  considerable 
  thick- 
  

   ness 
  of 
  overlying 
  rock. 
  

  

  In 
  addition 
  to 
  these 
  two 
  great 
  zones 
  there 
  are 
  a 
  host 
  of 
  minor 
  

   shearing 
  planes 
  all 
  through 
  the 
  knob, 
  all 
  showing 
  slickensides 
  on 
  

   their 
  faces, 
  with 
  usually 
  horizontal 
  striae. 
  The 
  lava 
  balls 
  always 
  

   show 
  exterior 
  slickensides, 
  and 
  the 
  intervening 
  matter 
  is 
  every- 
  

   where 
  greatly 
  sheared. 
  

  

  During 
  1910 
  great 
  sheared 
  surfaces 
  became 
  exposed 
  at 
  the 
  base 
  

   of 
  the 
  excavations 
  in 
  the 
  central 
  part 
  of 
  the 
  knob. 
  The 
  sheared 
  

   material 
  had 
  considerable 
  thickness 
  and 
  consisted 
  of 
  a 
  mass 
  of 
  

   shaly, 
  thin 
  plates, 
  with 
  beautifully 
  polished 
  slickensides 
  on 
  all 
  sur- 
  

   faces. 
  The 
  sheared 
  zone 
  had 
  an 
  easterly 
  dip 
  of 
  about 
  40 
  and 
  

   seemed 
  to 
  pass 
  with 
  that 
  inclination 
  entirely 
  through 
  the 
  knob, 
  

   from 
  bottom 
  to 
  top. 
  It 
  was 
  actually 
  disclosed 
  only 
  for 
  about 
  one- 
  

   third 
  of 
  the 
  whole 
  height. 
  Plate 
  19 
  is 
  a 
  photograph 
  of 
  a 
  portion 
  

   of 
  this 
  sheared 
  mass, 
  showing 
  the 
  shaly 
  material 
  in 
  considerable 
  

   thickness. 
  At 
  the 
  time 
  it 
  did 
  not 
  occur 
  to 
  us 
  that 
  this 
  was 
  the 
  

   actual 
  base 
  of 
  the 
  trap, 
  and 
  we 
  interpreted 
  it 
  as 
  a 
  shear 
  zone 
  

   within 
  the 
  mass. 
  At 
  subsequent 
  visits 
  it 
  was 
  found 
  covered 
  largely 
  

   by 
  quarry 
  debris, 
  so 
  that 
  it 
  showed 
  but 
  poorly. 
  We 
  learned, 
  how- 
  

   ever, 
  that 
  the 
  material 
  had 
  been 
  drilled 
  into 
  for 
  a 
  depth 
  of 
  12 
  feet, 
  

   in 
  search 
  for 
  additional 
  trap 
  underneath, 
  but 
  that 
  none 
  had 
  been 
  

   found 
  within 
  that 
  distance. 
  This 
  suggests 
  that 
  we 
  have 
  here 
  the 
  

   actual 
  base 
  of 
  the 
  lava. 
  The 
  material 
  does 
  not 
  exactly 
  resemble 
  

   the 
  ordinary 
  shale, 
  however, 
  and 
  there 
  is 
  some 
  reason 
  for 
  the 
  

   belief 
  that 
  it 
  represents 
  a 
  sheared 
  mixture 
  of 
  shale 
  and 
  lava. 
  Here 
  

   again 
  the 
  striae 
  on 
  the 
  slickensides 
  approach 
  the 
  horizontal. 
  

  

  There 
  is 
  one 
  more 
  indication 
  of 
  dislocation 
  in 
  the 
  knob 
  ; 
  its 
  back 
  

   seems 
  to 
  be 
  broken. 
  This 
  becomes 
  quite 
  evident 
  to 
  the 
  observer 
  

   on 
  the 
  summit 
  of 
  the 
  knob, 
  looking 
  down 
  into 
  the 
  excavation. 
  

   The 
  main 
  shear 
  zone 
  in 
  the 
  northern 
  half 
  of 
  the 
  knob 
  trends 
  

   N. 
  io° 
  E., 
  which 
  is 
  also 
  the 
  trend 
  of 
  the 
  lava 
  mass 
  there. 
  The 
  

   southern 
  half, 
  however, 
  trends 
  N. 
  45 
  ° 
  W. 
  and 
  its 
  main 
  shear 
  zone 
  

   does 
  likewise. 
  The 
  knob 
  appears 
  as 
  if 
  it 
  had 
  been 
  cracked 
  in 
  two 
  

   midway 
  by 
  a 
  vertical 
  rent, 
  and 
  the 
  two 
  halves 
  twisted 
  out 
  of 
  aline- 
  

   ment 
  through 
  an 
  angle 
  of 
  55 
  °. 
  

  

  All 
  the 
  evidence 
  indicates 
  that 
  the 
  lava 
  of 
  the 
  knob 
  has 
  experi- 
  

   enced 
  compressive 
  dislocation 
  of 
  the 
  same 
  type 
  as 
  have 
  the 
  over- 
  

   thrust 
  shales 
  in 
  whose 
  midst 
  it 
  lies; 
  of 
  the 
  same 
  type 
  and 
  quite 
  

   comparable 
  in 
  amount. 
  

  

  