﻿J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  93 
  

  

  height, 
  descends 
  at 
  the 
  angle 
  mentioned. 
  Since 
  the 
  surface 
  

   there 
  exposed 
  became 
  solidified 
  against 
  the 
  northern 
  sandstone 
  

   wall 
  of 
  the 
  fissure, 
  the 
  rock 
  is 
  of 
  fine-grained 
  texture 
  and 
  has 
  

   an 
  irregularly 
  rifted 
  aspect. 
  The 
  foot 
  of 
  the 
  wall 
  is 
  about 
  200 
  

   feet 
  above 
  high 
  tide, 
  and 
  from 
  it 
  the 
  land, 
  underlaid 
  by 
  

   sandstone, 
  slopes 
  off 
  gently 
  to 
  the 
  eastward. 
  Since 
  the 
  direc- 
  

   tion 
  of 
  this 
  wall 
  of 
  trap 
  is 
  S. 
  15° 
  W., 
  or 
  that 
  of 
  the 
  movement 
  

   of 
  the 
  ice 
  over 
  this 
  region 
  in 
  the 
  Glacial 
  era, 
  the 
  wall 
  escaped 
  

   the 
  tearing 
  action 
  of 
  the 
  glacier, 
  and 
  so 
  retains 
  its 
  original 
  

   surface. 
  

  

  Farther 
  south, 
  along 
  a 
  line 
  from 
  d 
  to 
  0, 
  there 
  is 
  a 
  similarly 
  

   steep 
  slope, 
  but 
  it 
  is 
  made 
  of 
  displaced 
  blocks 
  of 
  trap. 
  At 
  its 
  

   base 
  there 
  is 
  a 
  flat, 
  terrace-like 
  surface, 
  which 
  is 
  near 
  200 
  feet 
  

   above 
  tide 
  level. 
  This 
  steep 
  slope 
  appears 
  hence 
  to 
  have 
  been 
  

   the 
  course 
  of 
  the 
  wall 
  of 
  another 
  part 
  of 
  the 
  supply-fissure. 
  

   The 
  flat 
  terrace, 
  although 
  nearly 
  100 
  feet 
  wide, 
  is 
  without 
  

   stones 
  over 
  its 
  surface 
  of 
  either 
  trap 
  or 
  sandstone 
  except 
  in 
  its 
  

   southern 
  portion, 
  and 
  there 
  occur 
  sandstone 
  in 
  fragments 
  along 
  

   with 
  trap, 
  and 
  an 
  outcrop 
  of 
  sandstone 
  over 
  trap 
  at 
  S. 
  This 
  

   fact 
  and 
  the 
  occurrence 
  of 
  a 
  perennial 
  spring 
  in 
  this 
  southern 
  

   part 
  (at 
  the 
  point 
  toward 
  which 
  the 
  two 
  paths 
  on 
  the 
  map, 
  

   Plate 
  III, 
  descend) 
  make 
  it 
  probable 
  that 
  the 
  terrace 
  rests 
  on 
  

   sandstone, 
  and 
  that 
  this 
  sandstone 
  was 
  that 
  bounding 
  on 
  the 
  

   east, 
  the 
  supply-fissure 
  above 
  referred 
  to. 
  

  

  But 
  there 
  is 
  trap 
  again 
  to 
  the 
  east 
  of 
  this 
  terrace, 
  showing 
  

   that 
  the 
  lower 
  eastern 
  slopes 
  were 
  supplied 
  from 
  a 
  more 
  

   eastern 
  fissure. 
  Along 
  from 
  c 
  to 
  d, 
  the 
  trap 
  of 
  the 
  outer 
  

   fissure 
  appears 
  to 
  have 
  flowed 
  over 
  and 
  coalesced 
  with 
  that 
  of 
  

   the 
  inner. 
  Again 
  south 
  of 
  e, 
  the 
  distinction 
  of 
  the 
  two 
  fissures 
  

   cannot 
  be 
  made 
  out. 
  But 
  the 
  fact 
  that 
  the 
  supply 
  fissures, 
  

   one 
  or 
  both 
  had 
  a 
  large 
  inclination 
  — 
  not 
  far 
  from 
  45° 
  — 
  is 
  

   evident 
  from 
  the 
  very 
  steep 
  slope 
  of 
  the 
  surface. 
  

  

  Sections 
  of 
  the 
  dikes 
  of 
  trap 
  are 
  nowhere 
  exposed, 
  and 
  hence 
  

   we 
  are 
  ignorant 
  of 
  the 
  width 
  of 
  the 
  supply-fissures. 
  Judging 
  

   from 
  those 
  of 
  Mill 
  Rock 
  and 
  Fine 
  Rock, 
  it 
  may 
  have 
  been 
  

   150, 
  200 
  or 
  300 
  feet 
  ; 
  but 
  it 
  was 
  possibly 
  much 
  less. 
  

  

  The 
  Outflows. 
  — 
  In 
  East 
  Rock, 
  the 
  trap 
  which 
  overlies 
  the 
  

   sandstone 
  along 
  the 
  front, 
  was 
  that 
  of 
  outflows 
  from 
  the 
  

   fissures 
  westward 
  between 
  layers 
  of 
  the 
  tilted 
  sandstone. 
  The 
  

   fact 
  that 
  the 
  columns 
  of 
  trap 
  have 
  a 
  position 
  at 
  right 
  angles 
  

   nearly 
  to 
  the 
  inclined 
  layers 
  of 
  sandstone 
  is 
  believed 
  to 
  be 
  

   good 
  evidence 
  of 
  this 
  intrusion 
  of 
  the 
  melted 
  trap. 
  

  

  Fig. 
  10 
  represents 
  the 
  view 
  that 
  has 
  ordinarily 
  been 
  held 
  

   with 
  regard 
  to 
  the 
  relative 
  positions 
  of 
  the 
  trap 
  and 
  sandstone. 
  

   According 
  to 
  it 
  the 
  trap 
  left 
  the 
  dike 
  to 
  flow 
  westward 
  

   between 
  sandstone 
  layers 
  having 
  a 
  dip 
  of 
  20° 
  to 
  25°. 
  A 
  space 
  

   was 
  opened 
  between 
  the 
  layers 
  of 
  sandstone 
  which 
  the 
  liquid 
  

  

  