﻿92 
  J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  

  

  besides, 
  ledges 
  of 
  trap 
  along 
  the 
  east 
  side 
  appear 
  to 
  indicate 
  

   that 
  the 
  supply 
  of 
  liquid 
  rock 
  was 
  from 
  the 
  eastward, 
  like 
  that 
  

   of 
  East 
  Rock. 
  On 
  this 
  view 
  it 
  is 
  the 
  northern 
  mass 
  of 
  the 
  

   East 
  Rock 
  series. 
  

  

  East 
  Rock 
  proper. 
  — 
  The 
  trap 
  mass 
  BB', 
  or 
  East 
  Rock, 
  

   curves 
  around 
  from 
  K 
  25° 
  E., 
  on 
  the 
  north 
  to 
  N. 
  75° 
  W. 
  at 
  

   the 
  southwest 
  extremity. 
  Adding 
  to 
  it 
  the 
  Indian 
  Head 
  mass, 
  

   it 
  euds 
  in 
  an 
  east-and-west 
  dike, 
  and 
  ds 
  a 
  complete 
  crescent 
  in 
  

   outline. 
  It 
  has 
  a 
  bold 
  columnar 
  front, 
  in 
  which 
  the 
  columns 
  

   incline 
  about 
  22° 
  from 
  a 
  vertical 
  — 
  the 
  position, 
  being, 
  as 
  is 
  

   usual, 
  at 
  right 
  angles 
  to 
  the 
  mean 
  dip 
  of 
  the 
  tilted 
  sandstone. 
  

   A 
  view 
  of 
  the 
  southwest 
  front 
  of 
  the 
  Rock 
  is 
  presented 
  on 
  

   Plate 
  IV. 
  Plate 
  V 
  illustrates 
  the 
  character 
  and 
  inclined 
  

   position 
  of 
  the 
  columns, 
  and 
  shows 
  the 
  contrast 
  in 
  the 
  latter 
  

   respect 
  with 
  Pine 
  Rock. 
  

  

  The 
  upper 
  200 
  feet 
  are 
  of 
  trap. 
  The 
  junction 
  of 
  the 
  col- 
  

   umnar 
  trap 
  with 
  the 
  sandstone 
  is 
  exposed 
  to 
  view 
  at 
  several 
  

   points 
  along 
  the 
  front. 
  One 
  such 
  exposure 
  may 
  be 
  seen 
  when 
  

   crossing 
  the 
  Orange 
  Street 
  bridge. 
  The 
  view 
  in 
  Plate 
  IY, 
  in 
  

   which 
  the 
  bridge 
  appears 
  in 
  the 
  .foreground, 
  has 
  the 
  exposure 
  

   half 
  way 
  up 
  the 
  front 
  to 
  the 
  right. 
  The 
  height 
  of 
  the 
  junction 
  

   plane 
  above 
  mean 
  tide 
  at 
  this 
  place 
  is 
  155 
  feet. 
  Another 
  is 
  

   faintly 
  indicated 
  on 
  the 
  same 
  plate 
  directly 
  below 
  the 
  Refresh- 
  

   ment 
  House 
  ; 
  the 
  height 
  of 
  the 
  junction 
  is 
  there 
  150 
  feet. 
  In 
  

   other 
  exposures 
  of 
  the 
  junction-plane 
  to 
  the 
  north, 
  the 
  height 
  

   is 
  less 
  and 
  becomes 
  only 
  85 
  feet 
  near 
  the 
  Rock 
  Lane 
  bridge 
  ; 
  

   and 
  it 
  is 
  also 
  less 
  to 
  the 
  south 
  being 
  but 
  132^- 
  feet 
  at 
  B', 
  the 
  

   southwest 
  angle 
  of 
  the 
  trap 
  mass. 
  Since 
  the 
  strike 
  of 
  the 
  

   sandstone 
  of 
  the 
  legion 
  is 
  about 
  N. 
  30° 
  W., 
  the 
  sandstone 
  (or 
  

   the 
  junction 
  plane) 
  has 
  its 
  greatest 
  height, 
  155 
  feet, 
  where 
  the 
  

   front 
  has 
  this 
  direction 
  ; 
  and 
  the 
  bedding 
  of 
  the 
  sandstone 
  in 
  

   the 
  section 
  for 
  this 
  reason 
  appears 
  to 
  be 
  horizontal. 
  The 
  

   diminished 
  height 
  to 
  the 
  northward 
  is 
  owing 
  mainly 
  to 
  the 
  

   exposures 
  being 
  at 
  a 
  lower 
  level 
  on 
  the 
  junction-plane 
  because 
  

   of 
  the 
  changed 
  direction 
  of 
  the 
  front, 
  it 
  becoming 
  N. 
  10° 
  E. 
  

   near 
  Rock 
  Lane 
  bridge. 
  Through 
  this 
  interval 
  the 
  trap 
  retains 
  

   its 
  thickness 
  of 
  about 
  200 
  feet. 
  North 
  of 
  Rock 
  Lane 
  bridge 
  

   the 
  underlying 
  sandstone 
  is 
  wholly 
  covered 
  by 
  debris, 
  so 
  that 
  

   the 
  position 
  of 
  the 
  junction-plane 
  is 
  doubtful. 
  

  

  The 
  supply 
  of 
  the 
  trap 
  forming 
  East 
  Rock 
  came 
  up, 
  as 
  the 
  

   slope 
  of 
  its 
  surface 
  shows, 
  from 
  the 
  eastward 
  ; 
  and 
  it 
  continues 
  

   rising 
  westward 
  to 
  the 
  western 
  and 
  southwestern 
  margin 
  of 
  

   the 
  summit. 
  The 
  slope 
  from 
  the 
  summit 
  eastward 
  and 
  north- 
  

   ward 
  is 
  gradual 
  for 
  about 
  300 
  yards, 
  and 
  then 
  it 
  pitches 
  off 
  at 
  

   an 
  angle 
  of 
  45° 
  to 
  50 
  / 
  along 
  the 
  course 
  of 
  one 
  of 
  its 
  dikes. 
  

  

  The 
  position 
  of 
  the 
  dike, 
  and 
  thereby 
  of 
  the 
  supply 
  -fissure, 
  

   is 
  well 
  exhibited 
  at 
  be. 
  A 
  bare 
  wall 
  of 
  trap, 
  50 
  to 
  55 
  feet 
  in 
  

  

  