﻿102 
  J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  

  

  a 
  tilted 
  layer 
  of 
  the 
  sandstone, 
  the 
  dip 
  of 
  which 
  eastward 
  is 
  

   25°. 
  We 
  are 
  left 
  to 
  conjecture 
  as 
  regards 
  the 
  eastward 
  and 
  

   downward 
  continuation 
  of 
  this 
  layer 
  to 
  the 
  supply-fissure 
  

   (which 
  the 
  further 
  removal 
  of 
  debris 
  might 
  perhaps 
  uncover).. 
  

   But 
  we 
  know 
  that 
  the 
  trap 
  continues 
  up 
  this 
  sloping 
  layer 
  for 
  

   seventy-five 
  yards 
  from 
  the 
  commer 
  cement 
  of 
  the 
  outcrop. 
  

   It 
  conforms 
  to 
  the 
  theoretical 
  view 
  of 
  an 
  outflow 
  as 
  presented 
  

   in 
  fig. 
  10, 
  on 
  page 
  94. 
  

  

  But 
  on 
  reaching 
  the 
  end 
  of 
  the 
  seventy-five 
  yards, 
  there 
  is 
  a 
  

   change. 
  The 
  trap 
  beyond 
  rests 
  on 
  the 
  edges 
  of 
  the 
  layers 
  in 
  a 
  

   series 
  of 
  ledges 
  of 
  the 
  sandstone. 
  Moreover 
  there 
  is 
  but 
  little 
  

   rise 
  westward 
  along 
  the 
  floor; 
  for 
  a 
  line 
  drawn 
  along 
  the 
  top 
  

   of 
  the 
  ledges 
  would 
  be 
  almost 
  horizontal, 
  and 
  have 
  therefore 
  

   near 
  parallelism 
  to 
  the 
  surface 
  of 
  the 
  trap 
  at 
  the 
  summit 
  west 
  

   of 
  the 
  geodetic 
  station. 
  

  

  The 
  following 
  figure 
  represents 
  the 
  eastern 
  extremity 
  of 
  the 
  

   sandstone 
  for 
  a 
  height 
  of 
  fourteen 
  feet, 
  together 
  with 
  the 
  

  

  base 
  of 
  the 
  overlying 
  trap. 
  The 
  rock 
  is 
  partly 
  a 
  hard-baked 
  

   granitic 
  sandstone, 
  and 
  partly 
  the 
  feeble 
  shaly 
  chip-making 
  

   purplish-red 
  sand-rock. 
  The 
  trap 
  columns 
  above 
  the 
  sandstone 
  

   have 
  in 
  the 
  lower 
  part 
  an 
  inclination 
  of 
  20°, 
  approaching 
  thus 
  

   verticality 
  to 
  the 
  surface 
  of 
  the 
  sandstone; 
  but, 
  higher 
  up 
  

   the 
  bluff 
  front, 
  there 
  is 
  a 
  gradual 
  change 
  to 
  5°, 
  which 
  is 
  the 
  

   prevailing 
  inclination.* 
  The 
  upper 
  layer 
  of 
  the 
  sandstone 
  where 
  

   uncovered 
  shows 
  a 
  surface 
  without 
  breaks 
  or 
  much 
  unevenness. 
  

   A 
  section 
  of 
  the 
  sandstone, 
  with 
  the 
  trap 
  above, 
  for 
  the 
  

   next 
  seventy-five 
  yards 
  is 
  represented 
  in 
  the 
  following 
  figure. 
  

   The 
  fact 
  that 
  the 
  trap 
  when 
  melted 
  flowed 
  over 
  the 
  upturned 
  

   edges 
  is 
  manifest. 
  The 
  chip-making 
  rock 
  constitutes 
  much 
  of 
  

   the 
  mass, 
  and 
  at 
  its 
  contact 
  with 
  the 
  trap 
  it 
  is 
  scarcely 
  changed 
  in 
  

   color 
  or 
  texture. 
  The 
  trap 
  is 
  far 
  more 
  finely 
  columnar 
  than 
  

   that 
  to 
  the 
  east 
  over 
  the 
  single 
  sandstone 
  layer, 
  and 
  probably 
  

   because 
  moisture 
  reached 
  the 
  trap 
  freely 
  from 
  between 
  the 
  

   upturned 
  layers. 
  Other 
  sections 
  farther 
  west 
  are 
  of 
  similar 
  

  

  * 
  The 
  angles 
  of 
  inclination 
  here 
  recorded 
  are 
  those 
  presented 
  to 
  an 
  observer 
  

   in 
  the 
  front 
  view 
  of 
  the 
  rock 
  here 
  described. 
  

  

  