﻿82 
  J. 
  I). 
  Dana 
  — 
  Features 
  of 
  non-volcanic 
  Igneous 
  Ejections. 
  

  

  (4) 
  In 
  Pine 
  Rock 
  and 
  Mill 
  Rock, 
  the 
  trap 
  is 
  in 
  dikes, 
  there 
  

   being 
  no 
  evidence 
  of 
  any 
  outflow. 
  Yet 
  these 
  dikes 
  have 
  in 
  

   some 
  of 
  the 
  outlets 
  the 
  great 
  breadth 
  of 
  150 
  to 
  300 
  or 
  more 
  

   feet. 
  

  

  (5) 
  The 
  pitch 
  of 
  these 
  dikes 
  is 
  to 
  the 
  northward 
  ; 
  and 
  its 
  

   angle 
  18° 
  to 
  40° 
  — 
  both 
  characters 
  of 
  unusual 
  interest. 
  

  

  (6) 
  Although 
  neither 
  East 
  Rock, 
  Mill 
  Rock 
  nor 
  Pine 
  Rock 
  

   has 
  a 
  length 
  exceeding 
  a 
  mile 
  and 
  a 
  half, 
  each 
  has 
  three 
  or 
  four 
  

   distinct 
  outlets 
  of 
  trap, 
  separated 
  by 
  intervening 
  sandstone 
  ; 
  

   moreover, 
  there 
  is 
  wide 
  diversity 
  between 
  the 
  Rocks 
  in 
  the 
  

   form 
  and 
  arrangement 
  of 
  these 
  areas 
  of 
  extruded 
  trap, 
  as 
  the 
  

   map 
  illustrates. 
  

  

  (7) 
  The 
  trap 
  of 
  the 
  several 
  ridges, 
  according 
  to 
  examinations 
  

   by 
  E. 
  S. 
  Dana, 
  is 
  true 
  doleryte, 
  free, 
  or 
  nearly 
  so, 
  from 
  chlorite 
  

   and 
  other 
  evidences 
  of 
  interior 
  alteration, 
  and 
  not 
  at 
  all 
  

   vesicular. 
  

  

  (8). 
  Columnar 
  fractures 
  give 
  the 
  rock 
  a 
  rudely 
  columnar 
  

   structure, 
  in 
  which 
  the 
  half-defined 
  columns 
  are 
  four 
  to 
  eight 
  feet 
  

   in 
  diameter. 
  In 
  the 
  west 
  fronts 
  of 
  the 
  north 
  and 
  south 
  ridges 
  

   the 
  rude 
  columns 
  have 
  usually 
  an 
  inclination 
  nearly 
  at 
  right 
  

   angles 
  to 
  the 
  mean 
  dip 
  of 
  the 
  associated 
  sandstone 
  — 
  accord- 
  

   ing 
  thus 
  with 
  the 
  usual 
  rule 
  : 
  perpendicular 
  to 
  the 
  cooling 
  sur- 
  

   faces. 
  But 
  among 
  the 
  columnar 
  fractures, 
  whatever 
  the 
  incli- 
  

   nation 
  of 
  the 
  columns, 
  that 
  plane 
  of 
  fracture 
  or 
  joint 
  which 
  is 
  

   transverse 
  to 
  the 
  sides 
  of 
  the 
  dike 
  or 
  trap-mass 
  and 
  nearly 
  ver- 
  

   tical 
  is 
  the 
  most 
  strongly 
  developed, 
  and 
  consequently 
  the 
  

   trap 
  often 
  cleaves 
  into 
  nearly 
  vertical 
  plates 
  or 
  laminae 
  of 
  

   great 
  extent, 
  much 
  like 
  a 
  laminated 
  rock. 
  There 
  usually 
  is 
  

   also 
  a 
  second 
  easy 
  cleavage-direction, 
  nearly 
  at 
  right 
  angles 
  to 
  

   the 
  former 
  so 
  that 
  rectangular 
  columns 
  sometimes 
  come 
  out 
  

   with 
  great 
  prominence. 
  

  

  (9). 
  The 
  outflows 
  of 
  trap 
  have 
  a 
  floor 
  either 
  of 
  an 
  inclined 
  

   layer 
  of 
  the 
  sandstone 
  or 
  of 
  edges 
  of 
  the 
  upturned 
  layers. 
  

  

  The 
  principal 
  conclusions. 
  — 
  (1). 
  The 
  igneous 
  eruptions 
  of 
  

   the 
  New 
  Haven 
  region 
  took 
  place 
  after 
  the 
  sandstone 
  had 
  been 
  

   upturned 
  ; 
  that 
  is, 
  after 
  the 
  evolution 
  of 
  the 
  Connecticut-valley 
  

   mountain-range 
  in 
  this 
  part 
  of 
  the 
  valley 
  had 
  made 
  great 
  

   progress. 
  

  

  (2). 
  None 
  of 
  them 
  were 
  volcanic 
  eruptions, 
  for 
  there 
  was 
  no 
  

   center 
  of 
  action, 
  no 
  pericentric 
  discharge 
  of 
  volcanic 
  materials. 
  

  

  (3). 
  In 
  the 
  outflows 
  from 
  the 
  fissures 
  (those 
  of 
  East 
  and 
  

   West 
  Rock) 
  the 
  liquid 
  trap 
  did 
  not 
  escape 
  into 
  the 
  open 
  air 
  

   and 
  spread 
  over 
  the 
  surface, 
  but 
  entered 
  between 
  layers 
  of 
  the 
  

   sandstone. 
  

  

  (4). 
  Moreover 
  the 
  flow 
  was 
  not 
  by 
  gravity 
  into 
  spaces 
  that 
  

   had 
  been 
  previously 
  made, 
  but 
  a 
  forced 
  flow 
  that 
  opened 
  

  

  