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  Mr. 
  R. 
  Mallet 
  on 
  the 
  Origin 
  of 
  the 
  [Jan. 
  21, 
  

  

  January 
  21, 
  1875. 
  

  

  The 
  Right 
  Hon. 
  LYON 
  PLAYFAIR, 
  C.B., 
  LL.D., 
  Vice- 
  

   President, 
  in 
  the 
  Chair. 
  

  

  The 
  Presents 
  received 
  were 
  laid 
  on 
  the 
  table, 
  and 
  thanks 
  ordered 
  for 
  

   them. 
  

  

  The 
  following 
  Papers 
  were 
  read 
  : 
  — 
  

  

  I. 
  " 
  On 
  the 
  Origin 
  and 
  Mechanism 
  of 
  Production 
  of 
  the 
  Pris- 
  

   matic 
  (or 
  Columnar) 
  Structure 
  of 
  Basalt." 
  By 
  Robert 
  

   Mallet, 
  F.R.S. 
  Received 
  December 
  12, 
  1874. 
  

  

  (Abstract.) 
  

  

  The 
  author 
  having 
  briefly 
  traced 
  the 
  history 
  of 
  geological 
  opinion 
  on 
  

   this 
  subject, 
  from 
  before 
  the 
  period 
  at 
  which 
  the 
  controversy 
  at 
  which 
  

   the 
  igneous 
  or 
  aqueous 
  origin 
  of 
  basalt 
  might 
  be 
  viewed 
  as 
  settled, 
  and 
  

   having 
  stated 
  the 
  views 
  of 
  some 
  of 
  the 
  more 
  promiuent 
  British 
  authors 
  

   on 
  this 
  subject 
  of 
  recent 
  date, 
  points 
  out 
  that, 
  up 
  to 
  the 
  present, 
  no 
  clear 
  

   and 
  definite 
  theoretic 
  views 
  have 
  been 
  enunciated 
  to 
  account 
  for 
  prismatic 
  

   structure 
  in 
  basalt, 
  and 
  that 
  it 
  is 
  impossible 
  to 
  gather, 
  with 
  any 
  distinct- 
  

   ness, 
  from 
  systematic 
  writers, 
  whether 
  prismatic 
  structure 
  be 
  due 
  to 
  con- 
  

   traction 
  by 
  cooling 
  alone, 
  or 
  whether 
  the 
  structure 
  is 
  due 
  to 
  preexisting 
  

   concretionary 
  or 
  crystalline 
  arrangement 
  of 
  the 
  integral 
  particles 
  of 
  the 
  

   mass, 
  or 
  to 
  this 
  coacting 
  with 
  enormous 
  external 
  pressures, 
  the 
  origin 
  of 
  

   which 
  is 
  left 
  perfectly 
  vague, 
  or 
  to 
  some 
  play 
  of 
  successive 
  and 
  joint 
  

   actions 
  of 
  all 
  these 
  various 
  forces. 
  

  

  Prof. 
  James 
  Thomson, 
  in 
  a 
  paper 
  read 
  some 
  years 
  ago, 
  and 
  since 
  

   repeated 
  at 
  the 
  late 
  Belfast 
  Meeting 
  of 
  the 
  British 
  Association, 
  has 
  pro- 
  

   posed 
  views, 
  in 
  some 
  respects 
  new, 
  tracing 
  prismatic 
  structure 
  to 
  con- 
  

   traction 
  by 
  cooling 
  only, 
  and 
  has 
  expressed 
  entire 
  doubt 
  as 
  to 
  the 
  part 
  

   supposed 
  to 
  be 
  played 
  by 
  concretionary 
  spheroids 
  pressed 
  together. 
  

   Prof. 
  Thomson's 
  views, 
  however, 
  are 
  still 
  far 
  from 
  complete, 
  and 
  the 
  mode 
  

   assigned 
  by 
  him 
  to 
  the 
  production 
  of 
  cup-shaped 
  cross 
  joints 
  in 
  the 
  prisms 
  

   fails 
  to 
  account 
  for 
  the 
  phenomena. 
  

  

  The 
  aim 
  of 
  the 
  author 
  is 
  to 
  point 
  out 
  in 
  this 
  paper 
  that 
  all 
  the 
  salient 
  

   phenomena 
  of 
  the 
  prismatic 
  and 
  jointed 
  structure 
  of 
  basalt, 
  as 
  observable 
  

   in 
  nature, 
  can 
  be 
  accounted 
  for 
  upon 
  the 
  admitted 
  laws 
  of 
  cooling, 
  and 
  

   contraction 
  thereby, 
  of 
  melted 
  rock 
  possessing 
  the 
  known 
  properties 
  of 
  

   basalt, 
  the 
  essential 
  conditions 
  being 
  a 
  very 
  general 
  homogeneity 
  in 
  the 
  

   mass 
  cooling, 
  and 
  that 
  the 
  cooling 
  shall 
  take 
  place 
  slowly, 
  principally 
  

   from 
  one 
  or 
  more 
  of 
  its 
  surfaces. 
  

  

  Thus, 
  taking 
  the 
  simple 
  case 
  of 
  a 
  tabular 
  mass 
  of 
  molten 
  basalt, 
  whose 
  

   top 
  surface 
  is 
  level, 
  the 
  depth 
  being 
  great 
  and 
  the 
  other 
  two 
  dimensions 
  

  

  