﻿J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  89 
  

  

  a 
  narrow 
  stream 
  of 
  trap 
  (fig. 
  9), 
  coming 
  out 
  above 
  a 
  layer 
  of 
  

   the 
  sandstone 
  5 
  to 
  6 
  feet 
  thick, 
  the 
  main 
  part 
  of 
  the 
  dike 
  appear- 
  

   ing 
  below. 
  This 
  envelope 
  of 
  sandstone 
  by 
  trap 
  continues 
  for 
  30 
  

   feet, 
  when 
  the 
  two 
  parts 
  come 
  together 
  again. 
  The 
  depth 
  

   at 
  which 
  the 
  side 
  stream 
  goes 
  off 
  from 
  the 
  main 
  dike 
  is 
  not 
  

   known. 
  The 
  inclination 
  of 
  the 
  dike 
  is 
  mostly 
  25° 
  to 
  28° 
  (fig. 
  7) 
  

   from 
  a 
  vertical, 
  but 
  at 
  45 
  feet 
  from 
  the 
  west 
  end 
  it 
  becomes 
  40° 
  

   (fig. 
  8), 
  and 
  10 
  feet 
  beyond 
  this, 
  30°. 
  

  

  The 
  sandstone 
  of 
  the 
  Mill 
  Rock 
  region 
  is 
  of 
  all 
  degrees 
  of 
  

   coarseness 
  up 
  to 
  cobble-stone 
  conglomerate 
  ; 
  and 
  no 
  distinction 
  

   is 
  observable 
  between 
  that 
  of 
  the 
  west 
  and 
  east 
  ends. 
  

  

  Origin 
  of 
  the 
  Mill 
  Rock 
  features. 
  

  

  The 
  subdivision 
  of 
  the 
  trap 
  into 
  its 
  four 
  masses 
  may 
  be 
  ex- 
  

   plained 
  in 
  the 
  same 
  way 
  as 
  that 
  in 
  the 
  Pine 
  Hock 
  area. 
  A 
  

   downfall 
  of 
  the 
  northern 
  sandstone 
  wall 
  of 
  the 
  fissure, 
  the 
  

   hanging 
  wall, 
  would 
  account 
  for 
  the 
  separation 
  of 
  AA' 
  and 
  

   BB'. 
  Further, 
  the 
  obstruction 
  thus 
  occasioned 
  to 
  the 
  great 
  as- 
  

   cending 
  stream 
  — 
  its 
  width 
  150 
  to 
  200 
  feet 
  — 
  would 
  have 
  forced 
  

   open 
  passages 
  to 
  the 
  surface 
  for 
  the 
  discharge 
  of 
  the 
  liquid 
  

   trap, 
  and 
  thus 
  may 
  have 
  been 
  produced 
  the 
  small 
  dike 
  C, 
  

   situated 
  near 
  the 
  fissure 
  wall, 
  and 
  the 
  remoter 
  mass 
  D. 
  The 
  

   irregularities 
  of 
  the 
  little 
  dike 
  O, 
  and 
  the 
  situation 
  of 
  both 
  C 
  

   and 
  D 
  to 
  the 
  north 
  of 
  the 
  line 
  of 
  the 
  dike, 
  accord 
  with 
  this 
  

   idea 
  of 
  a 
  downfall 
  of 
  a 
  part 
  of 
  the 
  northern 
  wall. 
  The 
  

   liability 
  to 
  such 
  a 
  catastrophe 
  in 
  a 
  wall 
  made 
  of 
  the 
  rude 
  sand- 
  

   stone 
  3000 
  feet 
  or 
  more 
  high, 
  and 
  having 
  a 
  large 
  inclination, 
  

   was 
  augmented 
  in 
  both 
  Pine 
  Rock 
  and 
  Mill 
  Rock 
  by 
  the 
  tilted 
  

   position 
  and 
  faulted 
  state 
  of 
  the 
  sandstone. 
  The 
  beds 
  had 
  

   already 
  received 
  their 
  eastward 
  dip 
  of 
  15° 
  to 
  25°, 
  and 
  

   breaks 
  and 
  faults 
  innumerable 
  that 
  had 
  been 
  made 
  in 
  the 
  

   adjustment 
  to 
  the 
  new 
  tilted 
  position 
  ; 
  it 
  was 
  therefore 
  a 
  

   tottlish 
  structure 
  overhanging 
  a 
  profound 
  abyss. 
  The 
  fact 
  

   here 
  introduced 
  that 
  the 
  eastward 
  pitch 
  of 
  the 
  sandstone 
  was 
  

   given 
  it 
  before 
  the 
  ejection 
  of 
  the 
  trap 
  is 
  sustained 
  by 
  facts 
  

   reported 
  beyond. 
  But 
  an 
  argument 
  for 
  it 
  is 
  afforded 
  here 
  : 
  

   for 
  if 
  this 
  eastward 
  pitch 
  were 
  of 
  subsequent 
  origin, 
  then 
  the 
  

   Whitney 
  Peak 
  end 
  of 
  the 
  system 
  should 
  be 
  the 
  lowest. 
  In- 
  

   stead 
  of 
  this 
  it 
  is 
  greatly 
  the 
  highest 
  ; 
  the 
  ridge 
  slopes 
  west- 
  

   ward. 
  

  

  It 
  is 
  possible 
  that 
  the 
  fissures 
  of 
  AA' 
  and 
  BB' 
  were, 
  from 
  

   the 
  first, 
  independent 
  fissures 
  to 
  a 
  considerable 
  depth 
  ; 
  for 
  they 
  

   are 
  not 
  in 
  precisely 
  the 
  same 
  line. 
  If 
  this 
  were 
  so, 
  the 
  above 
  

   explanation, 
  while 
  in 
  the 
  chief 
  points 
  right, 
  would 
  require 
  

   some 
  modification. 
  

  

  As 
  in 
  Pine 
  Rock, 
  so 
  with 
  Mill 
  Rock 
  but 
  to 
  a 
  less 
  degree, 
  

   the 
  northward 
  pitch 
  of 
  the 
  dike 
  made 
  it 
  easy 
  of 
  degradation 
  

  

  