﻿98 
  J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  

  

  Snake 
  Rock. 
  — 
  In 
  Snake 
  Rock, 
  a 
  broad 
  mass 
  of 
  trap 
  measur- 
  

   ing 
  about 
  900 
  by 
  450 
  feet 
  in 
  its 
  two 
  diameters 
  lies 
  encased 
  in 
  

   sandstone. 
  The 
  greatest 
  height 
  of 
  the 
  trap 
  is 
  but 
  160 
  feet, 
  

   and 
  that 
  of 
  the 
  sandstone 
  west 
  of 
  it 
  over 
  200 
  feet. 
  The 
  trap 
  

   covers 
  the 
  eastern 
  slope 
  of 
  the 
  Rock 
  nearly 
  to 
  its 
  foot, 
  thus 
  

   showing 
  that 
  the 
  supply-fissure 
  was 
  on 
  that 
  side, 
  as 
  in 
  other 
  

   parts 
  of 
  the 
  East 
  Rock 
  series, 
  and 
  also 
  indicating 
  by 
  its 
  steep- 
  

   ness 
  that 
  the 
  fissure 
  was 
  much 
  inclined. 
  At 
  the 
  south 
  end 
  of 
  

   the 
  Rock, 
  in 
  the 
  yard 
  behind 
  the 
  north 
  corner 
  of 
  the 
  Basser- 
  

   mann 
  house, 
  at 
  a 
  junction 
  of 
  the 
  trap 
  and 
  sandstone, 
  the 
  dip 
  

   is 
  about 
  45° 
  ; 
  and 
  this 
  is 
  direct 
  evidence 
  as 
  to 
  the 
  inclination. 
  

  

  The 
  area 
  of 
  trap 
  of 
  Snake 
  Rock 
  has 
  on 
  the 
  north 
  the 
  width 
  

   of 
  that 
  of 
  Indian 
  Head 
  ; 
  and 
  the 
  mass 
  may 
  hence 
  owe 
  its 
  in- 
  

   creased 
  width 
  northward 
  to 
  an 
  outflow. 
  If 
  so, 
  Snake 
  Rock 
  

   contains 
  a 
  half-emerged 
  laccolith, 
  its 
  summit 
  exposed, 
  but 
  the 
  

   western 
  wall 
  of 
  sandstone 
  still 
  standing 
  and 
  overtopping 
  the 
  

   trap. 
  The 
  sandstone 
  shows 
  everywhere 
  the 
  effects 
  of 
  hot 
  

   vapors 
  in 
  all 
  their 
  varied 
  forms, 
  and 
  before 
  encroachments 
  

   were 
  made 
  by 
  a 
  brewery 
  there 
  was 
  a 
  fine 
  display 
  of 
  columnar 
  

   sandstone 
  in 
  the 
  southwestern 
  bluff. 
  

  

  Origin 
  of 
  the 
  breaks 
  in 
  the 
  East 
  Mock 
  series. 
  

  

  The 
  prominent 
  breaks 
  in 
  the 
  East 
  Rock 
  series 
  are 
  that 
  be- 
  

   tween 
  Indian 
  Head 
  and 
  Snake 
  Rock, 
  and 
  that 
  separating 
  the 
  

   small 
  northern 
  area, 
  AA', 
  from 
  the 
  main 
  East 
  Rock 
  mass, 
  BB 
  ; 
  . 
  

  

  The 
  Indian 
  Head 
  and 
  Snake 
  Rock 
  masses, 
  CC 
  and 
  DD', 
  

   approach 
  one 
  another 
  bluntly 
  within 
  a 
  hundred 
  yards, 
  and 
  the 
  

   area 
  of 
  sandstone 
  between 
  has 
  parallel 
  sides, 
  as 
  the 
  map, 
  

   Plate 
  III, 
  shows. 
  In 
  view 
  of 
  the 
  steep 
  pitch 
  of 
  the 
  supply- 
  

   fissure, 
  a 
  catastrophe 
  to 
  the 
  western 
  or 
  overhanging 
  wall 
  is 
  a 
  

   most 
  probable 
  explanation 
  of 
  the 
  break 
  between 
  them. 
  The 
  

   checking 
  of 
  so 
  great 
  a 
  stream 
  for 
  a 
  length 
  of 
  100 
  yards 
  

   might 
  be 
  expected 
  to 
  open 
  escape- 
  ways 
  in 
  some 
  direction. 
  

   The 
  long 
  eastern 
  tail-like 
  projection 
  from 
  Indian 
  Head, 
  C'C", 
  

   is 
  the 
  result 
  of 
  outflow 
  along 
  an 
  east-and-west 
  fissure. 
  The 
  

   pitch 
  of 
  the 
  fissure, 
  as 
  the 
  position 
  of 
  the 
  trap 
  shows, 
  was 
  

   about 
  25° 
  to 
  the 
  northward. 
  Its 
  southern 
  front 
  is 
  steep 
  and 
  

   rocky, 
  the 
  northern, 
  gentle 
  and 
  grass-covered. 
  It 
  may 
  be 
  that 
  

   this 
  supply 
  fissure 
  was 
  the 
  escape-way 
  then 
  made, 
  and 
  the 
  trap 
  

   the 
  part 
  of 
  the 
  stream 
  that 
  would 
  have 
  occupied 
  the 
  interval 
  

   had 
  no 
  such 
  catastrophe 
  occurred. 
  

  

  The 
  relations 
  of 
  the 
  northern 
  trap-mass 
  of 
  the 
  series, 
  A 
  A', 
  

   to 
  BB 
  / 
  are 
  doubtful. 
  Yet 
  it 
  is 
  probable 
  that 
  the 
  trap 
  of 
  A 
  A' 
  

   was 
  ejected 
  from 
  the 
  north 
  end 
  of 
  one 
  of 
  the 
  two 
  East 
  Rock 
  

   fissures, 
  or 
  lines 
  of 
  fissures. 
  The 
  ledge 
  of 
  very 
  hard 
  sandstone 
  

   which 
  extends 
  southward 
  from 
  near 
  the 
  south 
  end 
  of 
  A 
  A', 
  passes 
  

   by 
  the 
  east 
  side 
  of 
  the 
  dike-wall 
  he 
  ; 
  and 
  it 
  probably 
  derived 
  

   its 
  position 
  and 
  its 
  excessive 
  consolidation 
  and 
  lost 
  bedding 
  to 
  

  

  