﻿1875.] 
  

  

  Prismatic 
  Structure 
  of 
  Basalt. 
  

  

  183 
  

  

  normal 
  hexagonal 
  form 
  is 
  returned 
  to 
  in 
  such 
  a 
  manner 
  as 
  to 
  require 
  only 
  

   the 
  minimum 
  expenditure 
  of 
  work. 
  

  

  The 
  conditions 
  producing 
  greater 
  or 
  less 
  interspaces 
  between 
  the 
  

   prisms, 
  which 
  may 
  vary 
  from 
  point 
  to 
  point 
  of 
  the 
  same 
  mass, 
  are 
  pointed 
  

   out, 
  as 
  also 
  those 
  which 
  cause 
  the 
  spaces 
  between 
  successive 
  joints 
  in 
  

   adjacent 
  prisms 
  to 
  coincide 
  in 
  successive 
  planes, 
  transverse 
  to 
  their 
  axes 
  

   or 
  the 
  contrary. 
  

  

  The 
  author 
  then 
  proceeds 
  to 
  discuss 
  the 
  various 
  positions 
  in 
  space, 
  and 
  

   relatively 
  to 
  each 
  other, 
  which 
  the 
  axes 
  of 
  the 
  prisms 
  must 
  assume, 
  

   dependent 
  on 
  the 
  general 
  law, 
  as 
  already 
  stated, 
  that 
  the 
  axes 
  of 
  the 
  

   prisms, 
  however 
  produced, 
  are 
  always 
  normal 
  to 
  successive 
  isothermal 
  

   couches 
  or 
  planes 
  at 
  the 
  splitting 
  temperature, 
  taken 
  in 
  succession 
  within 
  

   the 
  mass. 
  

  

  If 
  the 
  mass 
  be 
  tabular, 
  as 
  already 
  assumed, 
  and 
  cooling 
  take 
  place 
  only 
  

   from 
  the 
  top 
  surface, 
  the 
  prisms 
  will 
  be 
  straight 
  and 
  vertical, 
  extending 
  

   from 
  top 
  to 
  bottom 
  nearly 
  of 
  the 
  tabular 
  mass, 
  and 
  being 
  separated 
  from 
  

   the 
  bottom 
  on 
  which 
  it 
  rests 
  by 
  a 
  more 
  or 
  less 
  thick 
  layer 
  of 
  irregular 
  

   angular 
  fragments, 
  or 
  of 
  badly 
  conducting 
  material, 
  tufa, 
  scoriae, 
  &c, 
  the 
  

   convex 
  surfaces 
  of 
  the 
  cross 
  joints 
  all 
  pointing 
  downwards. 
  If 
  the 
  mass 
  cool 
  

   both 
  from 
  the 
  bottom 
  and 
  the 
  top, 
  the 
  prisms, 
  vertical 
  and 
  straight, 
  will 
  

   split 
  upwards 
  and 
  downwards, 
  and 
  meet 
  in 
  an 
  irregular 
  intermediate 
  

   stratum 
  of 
  angular 
  fragments, 
  the 
  convex 
  surfaces 
  of 
  the 
  joints 
  of 
  the 
  

   lower 
  prisms 
  pointing 
  upwards, 
  and 
  the 
  respective 
  lengths 
  of 
  the 
  upper 
  

   and 
  lower 
  ranges 
  depending 
  on 
  their 
  relative 
  rates 
  of 
  cooling. 
  If 
  the 
  

   tabular 
  mass 
  cool 
  also 
  from 
  one 
  or 
  more 
  of 
  its 
  sides, 
  as 
  by 
  an 
  abutting 
  

   wall 
  of 
  rock, 
  prisms 
  will 
  be 
  produced 
  with 
  their 
  axes 
  perpendicular 
  to 
  that 
  

   wall, 
  and 
  will 
  be 
  separated 
  from 
  the 
  vertical 
  ranges 
  of 
  prisms 
  by 
  an 
  

   inclined 
  stratum 
  of 
  angular 
  fragments. 
  Also, 
  if 
  the 
  basalt 
  fill 
  a 
  crevasse 
  

   producing 
  a 
  dyke, 
  the 
  prisms 
  formed 
  by 
  cooling 
  will 
  be 
  generally 
  trans- 
  

   verse 
  to 
  the 
  plane 
  of 
  its 
  walls, 
  and 
  meet 
  somewhere 
  towards 
  the 
  centre 
  in 
  

   a 
  stratum 
  of 
  more 
  or 
  less 
  irregular 
  fragments, 
  due 
  in 
  all 
  cases 
  to 
  the 
  irre- 
  

   gular 
  contractions 
  at 
  the 
  extremities 
  of 
  the 
  prisms 
  breaking 
  up 
  their 
  

   mass 
  there 
  into 
  wholly 
  irregular 
  forms. 
  If 
  the 
  upper 
  and 
  cooling 
  sur- 
  

   face 
  have 
  a 
  curved 
  convex 
  contour, 
  the 
  prisms 
  will 
  be 
  taper 
  and 
  convergent 
  

   from 
  the 
  surface 
  of 
  the 
  mass 
  ; 
  and, 
  on 
  the 
  contrary, 
  if 
  the 
  cooling 
  surface 
  

   have 
  a 
  concave 
  contour, 
  or 
  rest 
  upon 
  a 
  concave 
  bottom, 
  the 
  prisms 
  will 
  

   be 
  divergent 
  from 
  the 
  interior 
  of 
  the 
  mass, 
  the 
  natural 
  law 
  of 
  economy 
  of 
  

   work 
  limiting 
  the 
  length 
  or 
  amount 
  of 
  taper 
  in 
  either 
  case 
  and 
  the 
  

   length 
  of 
  the 
  prisms, 
  and 
  at 
  a 
  certain 
  length 
  of 
  prism 
  a 
  new 
  range 
  of 
  

   larger 
  diameter 
  partially 
  or 
  wholly 
  then 
  commencing. 
  The 
  convergence 
  

   or 
  divergence 
  are 
  simple 
  consequences 
  of 
  the 
  general 
  law, 
  that 
  the 
  split- 
  

   ting 
  takes 
  place 
  always 
  normal 
  to 
  the 
  isothermal 
  couches 
  which 
  are 
  at 
  

   the 
  splitting 
  temperature. 
  

  

  The 
  author 
  then 
  proceeds 
  to 
  develop 
  and 
  illustrate 
  by 
  diagrams 
  some 
  

   of 
  the 
  varied 
  and 
  curious 
  combinations 
  which 
  are 
  observable 
  in 
  nature, 
  

  

  VOL. 
  XXIII. 
  Q 
  

  

  