﻿J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  99 
  

  

  a 
  catastrophe 
  that 
  closed 
  the 
  fissure 
  for 
  the 
  interval 
  between 
  

   them, 
  which 
  is 
  only 
  200 
  feet 
  wide, 
  yet 
  left 
  it 
  giving 
  out 
  heat, 
  

   and 
  generating 
  volumes 
  of 
  hot 
  vapors 
  for 
  the 
  consolidating 
  work. 
  

   The 
  East 
  Rock 
  masses 
  of 
  trap 
  may 
  therefore 
  be 
  traced 
  to 
  

   two 
  ranges 
  of 
  fissures. 
  The 
  western 
  was 
  the 
  probable 
  source 
  

   of 
  the 
  most 
  northern 
  area, 
  AA 
  , 
  and 
  of 
  the 
  summit 
  portion 
  of 
  

   that 
  of 
  BB' 
  on 
  East 
  Rock. 
  The 
  eastern, 
  contributed 
  to 
  the 
  

   lower 
  slopes 
  of 
  East 
  Rock 
  ; 
  and 
  also 
  through 
  its 
  continuation 
  

   southward 
  gave 
  origin 
  to 
  the 
  trap 
  of 
  Indian 
  Head 
  and 
  Snake 
  

   Rock. 
  But 
  for 
  the 
  accident 
  to 
  the 
  hanging 
  wall 
  of 
  the 
  great 
  

   fissure, 
  the 
  trap 
  of 
  Indian 
  Head 
  and 
  Snake 
  Rock 
  would 
  

   have 
  made 
  one 
  continuous 
  mass, 
  and 
  the 
  columnar 
  front 
  of 
  the 
  

   former 
  might 
  have 
  been 
  continued 
  over 
  part 
  of 
  the 
  present 
  

   Snake 
  Rock 
  area. 
  The 
  areas 
  of 
  trap 
  in 
  the 
  East 
  Rock 
  series 
  

   narrow 
  both 
  to 
  the 
  south 
  and 
  the 
  north. 
  

  

  4. 
  WEST 
  ROCK. 
  

  

  The 
  facts 
  and 
  conclusions 
  relating 
  to 
  the 
  West 
  Rock 
  region 
  

   derive 
  prominent 
  interest 
  from 
  their 
  pertaining 
  to 
  one 
  of 
  the 
  

   long 
  trap-ranges 
  of 
  the 
  Connecticut 
  "Valley 
  region. 
  The 
  area 
  

   is 
  represented 
  on 
  the 
  accompanying 
  map, 
  Plate 
  VI, 
  from 
  a 
  

   survey 
  made 
  by 
  the 
  author 
  with 
  chain 
  and 
  hand-level 
  in 
  1879 
  

   and 
  1880. 
  The 
  20-foot 
  contour-lines 
  of 
  the 
  steep 
  western 
  and 
  

   southern 
  fronts 
  of 
  the 
  Rock 
  and 
  the 
  geographical 
  positions 
  

   are 
  from 
  Bache's 
  Coast 
  Survey 
  map 
  ; 
  but 
  the 
  other 
  contour- 
  

   lines 
  exhibiting 
  the 
  surface 
  features, 
  which 
  required 
  for 
  map- 
  

   ping 
  detailed 
  measurements, 
  are 
  those 
  of 
  the 
  author.* 
  

  

  Features. 
  — 
  (1.) 
  While 
  the 
  general 
  course 
  of 
  the 
  West 
  Rock 
  

   Range 
  is 
  north-and-south, 
  the 
  western 
  foot 
  of 
  the 
  blunt 
  south- 
  

   ern 
  extremity 
  bends 
  round 
  to 
  an 
  eastward 
  course, 
  and 
  ends 
  

   with 
  north 
  30° 
  east. 
  The 
  summit 
  of 
  the 
  ridge 
  also 
  curves, 
  in 
  

   its 
  last 
  500 
  yards, 
  around 
  to 
  S. 
  70° 
  E. 
  or 
  nearly 
  to 
  east-by-west. 
  

   Its 
  height 
  in 
  this 
  part 
  is 
  399 
  to 
  405 
  feet 
  above 
  high 
  tide, 
  the 
  

   geodetic 
  station 
  at 
  the 
  extremity 
  being 
  399 
  feet. 
  The 
  eastern 
  

   foot 
  of 
  the 
  ridge 
  has 
  no 
  corresponding 
  bend. 
  

  

  (2.) 
  The 
  trap 
  of 
  the 
  Rock 
  is 
  a 
  continued 
  mass 
  instead 
  of 
  

   being 
  divided 
  into 
  several 
  masses 
  through 
  a 
  multiplication 
  of 
  

   outlets. 
  But 
  it 
  has 
  a 
  large 
  bay 
  of 
  sandstone, 
  of 
  triangular 
  out- 
  

   line, 
  in 
  its 
  southeastern 
  portion, 
  which 
  from 
  its 
  form 
  is 
  called 
  

   the 
  Triangle. 
  (3.) 
  South 
  of 
  the 
  Triangle 
  there 
  is 
  a 
  prolonged 
  

   hook-like 
  point 
  making 
  the 
  southeast 
  termination 
  of 
  the 
  trap. 
  

  

  (4.) 
  North 
  of 
  the 
  Triangle 
  commences 
  the 
  tiap 
  of 
  the 
  west 
  

   slope 
  of 
  the 
  mountain. 
  For 
  a 
  distance 
  of 
  500 
  feet 
  near 
  the 
  

   foot, 
  increasing 
  to 
  800 
  feet 
  above, 
  the 
  surface 
  of 
  the 
  trap 
  is 
  

   here 
  elevated 
  sixty 
  to 
  eighty 
  feet 
  or 
  more 
  above 
  the 
  level 
  

  

  * 
  The 
  dotted 
  line 
  on 
  Plate 
  II 
  is 
  the 
  north 
  limit 
  of 
  the 
  map, 
  Plate 
  VI. 
  Heights 
  

   C 
  to 
  Oa 
  are 
  plane-table 
  results 
  of 
  Prof. 
  H. 
  A. 
  Newton, 
  from 
  Bache's 
  399 
  as 
  base. 
  

  

  