﻿J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  97 
  

  

  of 
  East 
  Rock 
  and 
  110 
  below 
  that 
  of 
  Indian 
  Head, 
  and 
  proba- 
  

   bly 
  sandstone 
  intervened 
  for 
  the 
  greater 
  part 
  of 
  this 
  depth 
  ; 
  

   for 
  the 
  two 
  Rocks 
  face 
  one 
  another 
  with 
  steep 
  slopes, 
  as 
  well 
  

   brought 
  out 
  on 
  the 
  map, 
  Plate 
  III. 
  These 
  continue 
  to 
  be 
  

  

  Section 
  of 
  Indian 
  Head. 
  

  

  steep 
  to 
  the 
  very 
  foot 
  of 
  each, 
  where 
  they 
  approach 
  one 
  an- 
  

   other 
  down 
  the 
  eastern 
  slopes. 
  Their 
  bases 
  are 
  here 
  in 
  inde- 
  

   pendent 
  valleys, 
  designated 
  on 
  the 
  map 
  by 
  the 
  letters 
  E 
  and 
  I, 
  

   separated 
  by 
  a 
  low 
  trap 
  ridge, 
  R, 
  so 
  that 
  East 
  Rock 
  and 
  

   Indian 
  Head, 
  although 
  the 
  trap 
  extends 
  over 
  the 
  surface 
  of 
  the 
  

   gap 
  from 
  one 
  to 
  the 
  other, 
  are 
  nowhere 
  united 
  at 
  base. 
  The 
  

   eastward 
  sloping 
  valley, 
  I, 
  lying 
  at 
  the 
  northeast 
  foot, 
  of 
  Indian 
  

   Head 
  is 
  continued 
  in 
  a 
  westward 
  sloping 
  valley 
  I', 
  at 
  its 
  north- 
  

   western 
  foot, 
  and 
  the 
  two 
  together 
  define 
  its 
  outline. 
  The 
  low 
  

   trap 
  ridge 
  R, 
  between 
  E 
  and 
  I, 
  although 
  consisting 
  at 
  surface 
  

   mostly 
  of 
  blocks 
  of 
  trap, 
  has 
  a 
  solid 
  ledge 
  in 
  its 
  lower 
  part. 
  

   It 
  probably 
  crosses 
  the 
  gap 
  westward 
  ; 
  and 
  the 
  Summer 
  House, 
  

   near 
  201 
  on 
  Plate 
  III, 
  may 
  be 
  on 
  its 
  western 
  part. 
  The 
  valley 
  

   E, 
  at 
  the 
  southeast 
  foot 
  of 
  East 
  Rock, 
  is 
  perhaps, 
  a 
  result 
  of 
  

   glacial 
  action 
  ; 
  but 
  why 
  there 
  should 
  be 
  two 
  valleys 
  side-by-side 
  

   if 
  erosion 
  made 
  either, 
  is 
  not 
  explained. 
  

  

  The 
  trap 
  of 
  Indian 
  Head 
  rises 
  from 
  the 
  bottom 
  of 
  the 
  small 
  

   valley 
  just 
  mentioned 
  apparently 
  in 
  two 
  half-separated 
  streams 
  

   instead 
  of 
  one 
  even 
  stream 
  ; 
  but 
  this 
  feature 
  may 
  be 
  a 
  result 
  

   of 
  erosion. 
  The 
  eastern 
  outline 
  of 
  the 
  trap 
  (see 
  Plate 
  III) 
  is 
  in 
  

   a 
  line 
  w 
  T 
  ith 
  the 
  eastern 
  of 
  the 
  East 
  Rock 
  trap, 
  indicating 
  that 
  

   the 
  supply-fissure 
  corresponded 
  in 
  direction 
  with 
  the 
  outer 
  and 
  

   not 
  the 
  inner 
  of 
  the 
  East 
  Rock 
  courses 
  of 
  fissures. 
  The 
  two 
  

   Rocks, 
  although 
  alike 
  in 
  features, 
  are 
  to 
  a 
  large 
  degree 
  inde- 
  

   pendent. 
  Abrasion 
  helped 
  to 
  deepen 
  the 
  gap 
  between 
  them, 
  

   but 
  more 
  by 
  the 
  removal 
  of 
  sandstone 
  than 
  of 
  trap. 
  

  

  Indian 
  Head 
  is 
  peculiar 
  in 
  having 
  a 
  long 
  eastward 
  projection 
  

   from 
  the 
  southern 
  end. 
  It 
  is 
  described 
  on 
  a 
  following 
  page. 
  

  

  The 
  mode 
  of 
  origin 
  of 
  the 
  trap-masses 
  of 
  East 
  Rock 
  and 
  

   Indian 
  Head 
  — 
  by 
  a 
  forced 
  flow 
  of 
  lava, 
  opening 
  through 
  its 
  

   uplifting 
  action, 
  a 
  chamber 
  in 
  the 
  sandstone 
  for 
  its 
  accommo- 
  

   dation 
  — 
  entitles 
  the 
  two 
  to 
  be 
  called 
  laccoliths. 
  Through 
  

   degradation, 
  stripping 
  them 
  of 
  the 
  covering 
  of 
  sandstone, 
  they 
  

   stand 
  side-by-side 
  — 
  a 
  pair 
  of 
  laccoliths. 
  

  

  