﻿100 
  J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  

  

  farther 
  north. 
  Moreover 
  it 
  is 
  raised 
  into 
  rounded 
  ridges, 
  and 
  

   some 
  of 
  these 
  ridges 
  have 
  a 
  high 
  inclined 
  wall 
  on 
  the 
  south 
  

   side. 
  The 
  first 
  of 
  these 
  walls 
  adjoins 
  the 
  Triangle 
  and 
  has 
  a 
  

   height 
  of 
  seventy-five 
  feet, 
  a 
  slope 
  of 
  about 
  45° 
  and 
  an 
  even 
  

   flat 
  surface 
  free 
  from 
  marks 
  of 
  abrasion. 
  Another 
  similar 
  wall 
  

   farther 
  north 
  is 
  thirty 
  feet 
  high. 
  The 
  smaller 
  troughs 
  are 
  

   mostly 
  one 
  to 
  three 
  yards 
  deep. 
  The 
  angle 
  of 
  slope 
  in 
  the 
  

   embossed 
  surface 
  between 
  the 
  300-foot 
  and 
  100-foot 
  contour- 
  

   lines 
  is 
  less 
  than 
  17° 
  ; 
  and 
  in 
  the 
  surface 
  north 
  of 
  it 
  less 
  than 
  

   14°. 
  (5.) 
  The 
  long, 
  hook 
  like 
  point, 
  above 
  referred 
  to, 
  is 
  not 
  a 
  

   simple 
  ridge 
  of 
  trap, 
  like 
  that 
  from 
  an 
  ordinary 
  fissure, 
  but 
  

   consists, 
  as 
  seen 
  along 
  its 
  northern 
  side 
  (Plate 
  YI), 
  of 
  a 
  series 
  

   of 
  ronnded 
  ridges 
  which 
  increase 
  in 
  height 
  to 
  the 
  westward, 
  

   like 
  those 
  of 
  the 
  elevated 
  surface 
  of 
  trap 
  on 
  the 
  other 
  side 
  of 
  

   the 
  Triangle. 
  Moreover, 
  all 
  these 
  wrinkle-like 
  ridges, 
  concave 
  

   troughs 
  and 
  oblique 
  walls, 
  have 
  a 
  general 
  parallelism. 
  (6.) 
  

   The 
  embossed 
  surface 
  north 
  of 
  the 
  Triangle 
  has 
  lost, 
  through 
  

   glacial 
  abrasion, 
  as 
  a 
  consequence 
  of 
  its 
  elevation 
  above 
  the 
  

   general 
  level, 
  all 
  of 
  the 
  sandstone 
  once 
  covering 
  it, 
  even 
  to 
  the 
  

   foot 
  of 
  the 
  mountain, 
  excepting 
  small 
  portions 
  in 
  two 
  of 
  the 
  

   troughs. 
  Farther 
  north 
  the 
  sandstone 
  remains 
  in 
  some 
  places 
  

   nearly 
  to 
  the 
  300-foot 
  contour-line. 
  (7.) 
  The 
  trap 
  of 
  the 
  

   embossed 
  area 
  that 
  was 
  thus 
  uncovered 
  suffered 
  little 
  from 
  the 
  

   abrasion 
  ; 
  for 
  the 
  rock 
  of 
  the 
  surface 
  has 
  the 
  fineness 
  of 
  grain 
  

   and 
  other 
  characteristics 
  of 
  the 
  contact 
  rock. 
  This 
  is 
  true 
  also 
  

   of 
  the 
  trap 
  of 
  the 
  southeast 
  point. 
  Moreover, 
  in 
  many 
  places 
  

   on 
  this 
  point 
  below 
  300 
  feet, 
  the 
  trap 
  contains 
  imbedded 
  frag- 
  

   ments 
  of 
  the 
  sandstone 
  which 
  fell 
  into 
  it 
  while 
  it 
  was 
  still 
  

   liquid. 
  The 
  trap 
  of 
  other 
  parts 
  of 
  West 
  Rock 
  ridge 
  rarely 
  

   shows 
  evidence 
  of 
  abrasion 
  below 
  a 
  level 
  of 
  300 
  feet. 
  On 
  the 
  

   contrary, 
  above 
  this 
  level 
  it 
  has 
  lost 
  by 
  abrasion 
  the 
  fine-grained, 
  

   brittle 
  crust-portion, 
  and 
  presents 
  at 
  surface 
  the 
  coarseness 
  of 
  

   crystalline 
  texture 
  that 
  belongs 
  to 
  the 
  interior 
  of 
  the 
  mass. 
  

  

  (8.) 
  Another 
  very 
  important 
  feature 
  of 
  "West 
  Rock 
  is 
  its 
  

   affording 
  a 
  long 
  east-and-west 
  section 
  through 
  the 
  breadth 
  of 
  

   a 
  great 
  trap 
  range, 
  exhibiting 
  the 
  contact-plane 
  for 
  several 
  

   hundred 
  feet 
  of 
  the 
  outflowing 
  trap 
  and 
  the 
  underlying 
  sand- 
  

   stone, 
  as 
  described 
  and 
  figured 
  beyond. 
  

  

  The 
  map, 
  Plate 
  YI, 
  has 
  the 
  walls, 
  troughs, 
  and 
  ridges 
  of 
  

   the 
  surface 
  shaded, 
  to 
  bring 
  out 
  better 
  these 
  features 
  of 
  the 
  

   original 
  surface 
  of 
  the 
  trap. 
  The 
  southern 
  front 
  of 
  the 
  Pock 
  

   has 
  been 
  made 
  by 
  degradation 
  and 
  hence 
  has 
  no 
  shading. 
  The 
  

   southeastern 
  point 
  owes 
  its 
  straight 
  outline 
  on 
  the 
  south 
  side 
  

   to 
  the 
  quarrymen 
  and 
  the 
  joints 
  in 
  the 
  trap. 
  The 
  map 
  shows 
  

   what 
  remained 
  of 
  the 
  point 
  in 
  1880. 
  There 
  is 
  much 
  less 
  now. 
  

  

  The 
  Supply-fissure. 
  — 
  The 
  inclination 
  and 
  width 
  of 
  the 
  fis- 
  

   sures 
  supplying 
  the 
  liquid 
  trap 
  for 
  the 
  West 
  Rock 
  range 
  are 
  

  

  