﻿J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  95 
  

  

  east 
  to 
  west 
  (or 
  more 
  exactly 
  E.S.E. 
  to 
  W.S.W. 
  since 
  this 
  is 
  

   approximately 
  the 
  direction 
  of 
  a 
  transverse 
  diameter) 
  drawn 
  to 
  

   a 
  scale, 
  fig. 
  11, 
  throws 
  some 
  light 
  on 
  these 
  points. 
  

  

  Section 
  of 
  East 
  Rock, 
  showing 
  the 
  correct 
  profile. 
  

  

  This 
  section 
  is 
  essentially 
  right 
  in 
  its 
  profile, 
  but 
  more 
  or 
  

   less 
  doubtful 
  in 
  its 
  interior 
  lines. 
  The 
  height 
  of 
  the 
  upper 
  

   surface 
  of 
  the 
  outflow 
  where 
  it 
  left 
  the 
  dike 
  at 
  d! 
  is 
  265 
  to 
  270 
  

   feet. 
  It 
  was 
  not 
  less 
  than 
  this 
  ; 
  for 
  we 
  have 
  this 
  height 
  for 
  

   the 
  top 
  of 
  the 
  bare, 
  unabraded 
  wall 
  of 
  trap 
  (adding 
  the 
  part 
  of 
  

   it 
  under 
  the 
  Summer 
  House 
  west 
  of 
  the 
  road). 
  The 
  length 
  of 
  

   the 
  overflow 
  to 
  the 
  present 
  western 
  front, 
  is, 
  as 
  already 
  stated, 
  

   about 
  300 
  yards. 
  The 
  height 
  of 
  the 
  western 
  brow 
  of 
  trap 
  in 
  

   the 
  section 
  is 
  355 
  feet 
  ;* 
  and 
  that 
  of 
  the 
  bottom 
  of 
  the 
  trap 
  in 
  

   the 
  western 
  front, 
  155 
  feet. 
  These 
  are 
  facts 
  ; 
  and 
  the 
  diver- 
  

   gence 
  here 
  from 
  figure 
  10 
  is 
  very 
  great. 
  Further, 
  the 
  mean 
  

   angle 
  of 
  the 
  trap 
  surface 
  over 
  the 
  summit 
  is 
  10° 
  instead 
  of 
  22°, 
  

   the 
  mean 
  dip 
  of 
  the 
  sandstone. 
  The 
  latter 
  dip 
  is 
  shown 
  in 
  

   the 
  lines 
  dn 
  ; 
  and 
  if 
  the 
  floor 
  had 
  originally 
  this 
  pitch 
  through- 
  

   out, 
  the 
  thickness 
  of 
  the 
  trap 
  would 
  have 
  been 
  about 
  450 
  feet, 
  

   this 
  being 
  the 
  distance 
  on 
  the 
  scale 
  of 
  the" 
  section 
  between 
  dn 
  

   and 
  d'n\ 
  while 
  actually 
  it 
  is 
  only 
  200 
  to 
  210 
  feet. 
  

  

  The 
  question 
  arises: 
  How 
  was 
  the 
  lower 
  >slope 
  of 
  10° 
  at- 
  

   tained, 
  and 
  how 
  the 
  lessened 
  thickness. 
  Are 
  they 
  a 
  result 
  of 
  

   wear 
  by 
  glacial 
  or 
  other 
  methods 
  ; 
  or 
  was 
  the 
  present 
  slope 
  

   approximately 
  the 
  original 
  slope 
  of 
  the 
  outflow 
  ? 
  A 
  large 
  

   amount 
  of 
  observation 
  over 
  trap 
  ridges 
  leads 
  me 
  to 
  believe 
  

   that 
  the 
  loss 
  over 
  East 
  and 
  West 
  Rocks 
  by 
  abrasion 
  has 
  been 
  

   small, 
  probably 
  not 
  over 
  50 
  feet. 
  The 
  glacier, 
  as 
  it 
  was 
  shoved 
  

   along, 
  might 
  easily 
  have 
  torn 
  off 
  columns 
  from 
  the 
  front, 
  but 
  

   it 
  would 
  have 
  made 
  little 
  impression 
  on 
  the 
  exposed 
  surfaces. 
  

   Moreover 
  glacial 
  abrasion 
  would 
  hardly 
  have 
  left 
  the 
  highest 
  

   points 
  of 
  the 
  summit 
  so 
  near 
  the 
  western 
  edge. 
  

  

  If 
  the 
  outline 
  of 
  the 
  summit 
  approaches 
  that 
  of 
  the 
  original 
  

   outflow, 
  then 
  — 
  d 
  being 
  the 
  lower 
  limit 
  of 
  the 
  trap 
  on 
  the 
  front 
  

   — 
  a 
  line 
  drawn 
  from 
  d 
  nearly 
  parallel 
  to 
  the 
  summit 
  plane, 
  

  

  * 
  This 
  is 
  the 
  height 
  80 
  feet 
  north 
  of 
  the 
  Summit 
  Refreshment 
  House, 
  just 
  west 
  

   of 
  the 
  road, 
  this 
  being 
  the 
  highest 
  point 
  over 
  this 
  northern 
  half 
  of 
  the 
  summit 
  

   area. 
  

  

  Am. 
  Jour. 
  Sci.— 
  Third 
  Series, 
  Yol. 
  XLII, 
  No. 
  248.— 
  August, 
  1891. 
  

  

  7 
  

  

  