﻿104 
  J. 
  D. 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  

  

  fracture-planes 
  or 
  the 
  columnar 
  structure. 
  A 
  reduction 
  so 
  

   nearly 
  to 
  horizontality 
  as 
  that 
  shown 
  in 
  the 
  south 
  front 
  of 
  

   West 
  Rock 
  along 
  with 
  parallelism 
  in 
  the 
  profile 
  of 
  the 
  sum- 
  

   mit 
  may 
  not 
  be 
  common. 
  

  

  West 
  Rock 
  teaches 
  that 
  the 
  section 
  of 
  East 
  Rock 
  in 
  fig. 
  11, 
  

   p. 
  95, 
  may 
  be 
  no 
  exaggeration. 
  Yet 
  it 
  is 
  more 
  probable 
  that 
  the 
  

   original 
  condition 
  was 
  intermediate 
  between 
  this 
  position 
  and 
  

   that 
  indicated 
  in 
  this 
  diagram. 
  

  

  Sections 
  similar 
  to 
  that 
  in 
  the 
  south 
  face 
  of 
  West 
  Rock 
  may 
  

   be 
  looked 
  for, 
  with 
  some 
  probability 
  of 
  success, 
  among 
  many 
  

   of 
  the 
  trap-ranges 
  of 
  the 
  Connecticut 
  Yalley 
  wherever 
  they 
  

   terminate 
  in 
  transverse 
  sections. 
  All 
  that 
  is 
  necessary 
  to 
  

   ascertain 
  the 
  truth 
  is 
  to 
  remove 
  the 
  talus 
  of 
  trap 
  debris. 
  

  

  Three 
  miles 
  east 
  of 
  New 
  Haven 
  (in 
  East 
  Haven) 
  a 
  section 
  

   was 
  opened 
  in 
  cutting 
  for 
  a 
  carriage-road 
  through 
  the 
  second 
  

   trap 
  ridge 
  west 
  of 
  Saltonstall 
  Lake 
  ; 
  it 
  is 
  bat 
  a 
  few 
  rods 
  west 
  of 
  

   the 
  railroad 
  station. 
  The 
  facts 
  are 
  in 
  all 
  respects 
  similar 
  to 
  

   those 
  of 
  West 
  Rock, 
  as 
  shown 
  in 
  the 
  annexed 
  figure. 
  The 
  

  

  trap 
  covers 
  a 
  series 
  of 
  ledges 
  of 
  upturned 
  sandstone, 
  and 
  

   shows 
  no 
  traces 
  of 
  displacement 
  subsequent 
  to 
  its 
  cooling. 
  

   The 
  sandstone 
  is 
  intersected 
  by 
  extensive 
  nearly 
  vertical 
  frac- 
  

   tures, 
  whose 
  surfaces, 
  owing 
  to 
  friction, 
  are 
  scratched 
  and 
  

   polished 
  ; 
  and 
  the 
  larger 
  planes 
  extend 
  up 
  through 
  the 
  sand- 
  

   stone 
  without 
  any 
  appearance 
  of 
  corresponding 
  displacement 
  

   in 
  the 
  trap. 
  Moreover 
  these 
  polished 
  slickensided 
  surfaces 
  

   have 
  the 
  white 
  porcellanous 
  coating 
  common 
  in 
  the 
  region 
  ; 
  

   probably 
  made 
  by 
  the 
  grinding 
  of 
  the 
  feldspar 
  of 
  the 
  sandstone 
  

   in 
  the 
  mutual 
  friction 
  of 
  the 
  walls.* 
  

  

  * 
  At 
  all 
  the 
  East 
  Haven 
  quarries, 
  and 
  in 
  the 
  ledges 
  elsewhere 
  exposed 
  to 
  view, 
  

   these 
  evidences 
  of 
  displacement 
  and 
  of 
  much 
  friction 
  attending 
  it 
  abound. 
  Frag- 
  

   ments 
  as 
  large 
  as 
  the 
  hand, 
  slickensided 
  on 
  both 
  surfaces 
  and 
  over 
  planes 
  of 
  

   cross-fracture, 
  are 
  common; 
  and 
  so 
  are 
  walls 
  of 
  various 
  inclinations 
  hundreds 
  of 
  

   square 
  yards 
  in 
  area. 
  The 
  sloping 
  upper 
  surfaces 
  of 
  the 
  sandstone 
  layers 
  laid 
  

   bare 
  in 
  the 
  quarrying 
  are 
  sometimes 
  polished 
  and 
  scratched 
  in 
  the 
  direction 
  of 
  

   the 
  dip 
  for 
  many 
  square 
  rods. 
  There 
  is 
  abundant 
  evidence 
  of 
  a 
  vast 
  amount 
  of 
  

   movement, 
  though 
  movement 
  in 
  a 
  small 
  way, 
  during 
  the 
  progress 
  of 
  the 
  upturn- 
  

   ing 
  in 
  which 
  the 
  sandstone 
  received 
  its 
  universal 
  eastward 
  dip. 
  

  

  The 
  section 
  represented 
  in 
  rig. 
  15 
  has 
  lost 
  much 
  of 
  its 
  original 
  distinctness 
  by 
  

   the 
  sliding 
  down 
  of 
  debris 
  from 
  above. 
  

  

  