﻿GEOLOGY 
  OF 
  SARATOGA 
  SPRINGS 
  AND 
  VICINITY 
  

  

  12 
  

  

  If 
  the 
  great 
  diagonal 
  shear 
  zone 
  just 
  described 
  lies 
  at 
  the 
  actual 
  

   base 
  of 
  the 
  lava, 
  as 
  we 
  suspect, 
  then 
  the 
  lava 
  forms 
  a 
  short, 
  sheet- 
  

   like 
  mass 
  inclosed 
  in 
  the 
  shales 
  and 
  dipping 
  in 
  conformity 
  with 
  

   them, 
  as 
  shown 
  in 
  figure 
  14. 
  

  

  This 
  corresponds 
  with 
  Woodworth's 
  original 
  conception 
  as 
  may 
  

   be 
  seen 
  by 
  reference 
  to 
  his 
  figure 
  (figure 
  9, 
  page 
  117). 
  The 
  

   present 
  relief 
  of 
  the 
  knob 
  is 
  due 
  to 
  erosion, 
  the 
  trap 
  being 
  more 
  

   resistant 
  than 
  the 
  adjacent 
  shales. 
  The 
  shale 
  wedges 
  in 
  the 
  lava 
  

   may 
  be 
  due 
  to 
  lava 
  tongues 
  running 
  out 
  into 
  the 
  shales 
  from 
  the 
  

   main 
  mass, 
  or 
  they 
  may 
  be 
  the 
  result 
  of 
  dislocation. 
  There 
  is 
  

   shearing 
  and 
  faulting 
  in 
  plenty 
  to 
  account 
  for 
  all 
  the 
  observed 
  

   phenomena. 
  

  

  The 
  lava 
  then 
  seems 
  to 
  lie 
  within 
  the 
  shales 
  after 
  the 
  manner 
  

   of 
  an 
  intrusive 
  sheet; 
  yet 
  it 
  can 
  not 
  possibly 
  be 
  a 
  sheet. 
  In 
  the 
  

   first 
  place 
  -it 
  is 
  altogether 
  too 
  short. 
  At 
  the 
  north 
  it 
  ends 
  very 
  

  

  Fig. 
  14 
  Diagram 
  of 
  inferred 
  relationship 
  of 
  lava 
  and 
  shales 
  in 
  Stark's 
  knob 
  

  

  abruptly; 
  a 
  trench 
  dug 
  in 
  the 
  rock 
  on 
  the 
  north 
  slope 
  of 
  the 
  knob 
  

   is 
  entirely 
  in 
  shale. 
  At 
  the 
  south 
  it 
  becomes 
  involved 
  with 
  shale 
  

   wedges, 
  and 
  just 
  south 
  of 
  the 
  knob 
  a 
  poor 
  exposure 
  shows 
  a 
  thick- 
  

   ness 
  of 
  about 
  1 
  foot 
  of 
  trap 
  interbedded 
  with 
  the 
  shales 
  never- 
  

   theless 
  here 
  also 
  the 
  termination 
  is 
  fairly 
  abrupt. 
  The 
  entire 
  length 
  

   along 
  the 
  strike 
  from 
  north 
  to 
  south 
  is 
  not 
  over 
  200 
  yards. 
  More- 
  

   over 
  the 
  nature 
  of 
  the 
  lava 
  itself, 
  the 
  ball 
  structure 
  and 
  intervening 
  

   glass, 
  decisively 
  negative 
  any 
  notion 
  that 
  we 
  may 
  be 
  dealing 
  with 
  

   a 
  sheet. 
  The 
  structure 
  indicates 
  surface 
  lava, 
  either 
  a 
  flow 
  or 
  a 
  

   volcanic 
  neck, 
  a 
  deposit 
  in 
  the 
  throat 
  of 
  a 
  volcano. 
  

  

  Microscopic 
  characters 
  of 
  the 
  lava. 
  There 
  are 
  two 
  chief 
  varie- 
  

   ties 
  of 
  the 
  rock 
  of 
  the 
  knob, 
  the 
  finely 
  crystalline, 
  dull, 
  black 
  rock 
  

   of 
  the 
  balls, 
  and 
  the 
  black 
  glass 
  of 
  the 
  intervening 
  material. 
  Both 
  

   are 
  locally 
  amygdaloidal, 
  but 
  we 
  saw 
  no 
  such 
  material 
  in 
  place, 
  

  

  