﻿1875.] 
  

  

  supposed 
  Changes 
  in 
  Basaltic 
  Veins, 
  

  

  549 
  

  

  position 
  of 
  the 
  outer 
  coverings 
  of 
  the 
  basalt, 
  already 
  described. 
  These 
  

   in 
  all 
  probability 
  would, 
  from 
  their 
  position, 
  be 
  more 
  freely 
  exposed 
  

   to 
  the 
  action 
  of 
  carbonic 
  acid 
  than 
  the 
  interior 
  of 
  the 
  mass 
  of 
  which 
  

   they 
  formed 
  the 
  coating. 
  That 
  these 
  portions 
  are 
  in 
  reality 
  identical 
  

   with 
  the 
  bed 
  itself, 
  merely 
  altered 
  by 
  the 
  rate 
  of 
  cooling, 
  or 
  by 
  a 
  

   change 
  in 
  composition 
  due 
  to 
  absorbing 
  carbonic 
  acid, 
  or 
  by 
  these 
  causes 
  

   combined, 
  is 
  easily 
  seen 
  when 
  the 
  proportions 
  of 
  their 
  fixed 
  elements 
  are 
  

   compared 
  with 
  those 
  as 
  they 
  exist 
  in 
  the 
  whin-rock 
  of 
  the 
  neighbourhood. 
  

  

  - 
  T7 
  . 
  -, 
  , 
  , 
  Covering 
  of 
  Korth 
  dyke 
  Interior 
  of 
  

  

  ±ixea 
  elements. 
  horizontal 
  bed. 
  parallel 
  to 
  Hett 
  Dyke. 
  horizontal 
  bed. 
  

  

  Silica 
  51-56 
  52-05 
  52*05 
  

  

  Al 
  umin 
  a 
  20*72 
  17-88 
  15-50 
  

  

  Protoxide 
  of 
  iron 
  15-47 
  12-58 
  12-90 
  

  

  Lime 
  7*38 
  9-47 
  13-83 
  

  

  Magnesia 
  3-33 
  5-82 
  4-02 
  

  

  Potash 
  and 
  soda 
  1-54 
  2-20 
  ; 
  . 
  . 
  1-70 
  

  

  100-00 
  100-00 
  100-00 
  

  

  The 
  chief 
  change, 
  therefore, 
  in 
  composition 
  which 
  the 
  outer 
  portions 
  of 
  

   the 
  bed 
  of 
  whin 
  have 
  experienced 
  is 
  a 
  much 
  greater 
  absorption 
  of 
  carbonic 
  

   acid, 
  amounting 
  to 
  nearly 
  nine 
  tenths 
  of 
  the 
  total 
  quantity 
  required 
  for 
  

   converting 
  the 
  iron, 
  lime, 
  magnesia, 
  and 
  alkalies 
  into 
  carbonates. 
  

  

  It 
  is 
  perhaps 
  a 
  difficult 
  task 
  to 
  speak 
  with 
  any 
  degree 
  of 
  confidence 
  

   on 
  the 
  precise 
  nature 
  of 
  the 
  causes 
  which 
  have 
  given 
  rise 
  to 
  certain 
  

   differences 
  in 
  the 
  proportions 
  of 
  some 
  of 
  the 
  earths 
  as 
  exhibited 
  in 
  the 
  

   above 
  figures. 
  We 
  may, 
  however, 
  readily 
  imagine 
  that 
  a 
  liquefied 
  rock, 
  

   during 
  its 
  passage 
  through 
  a 
  series 
  of 
  stratified 
  beds 
  of 
  various 
  kinds 
  and 
  

   of 
  different 
  thickness, 
  will 
  continue 
  for 
  a 
  longer 
  or 
  a 
  shorter 
  time 
  in 
  

   contact 
  with 
  any 
  given 
  substance, 
  according 
  to 
  the 
  size 
  of 
  the 
  latter, 
  or 
  

   according 
  to 
  the 
  rate 
  at 
  which 
  the 
  stream 
  of 
  fluid 
  matter 
  for 
  the 
  time 
  

   being 
  is 
  travelling. 
  It 
  is 
  also 
  equally 
  easy 
  to 
  suppose 
  that 
  the 
  exterior 
  may 
  

   solidify 
  by 
  contact 
  with 
  cooler 
  surfaces 
  when 
  the 
  basalt 
  has 
  assumed 
  the 
  

   composition 
  resembling 
  that 
  of 
  the 
  crust 
  or 
  coverings 
  already 
  described. 
  

   The 
  liquid 
  torrent 
  continues 
  to 
  flow 
  through 
  beds 
  in 
  which 
  lime 
  prepon- 
  

   derates, 
  such 
  as 
  the 
  mountain 
  limestone 
  : 
  an 
  additional 
  quantity 
  of 
  

   this 
  earth 
  is 
  dissolved, 
  and 
  the 
  basalt, 
  thus 
  altered 
  in 
  composition, 
  is 
  ex- 
  

   pelled 
  through 
  a 
  kind 
  of 
  gigantic 
  tube 
  formed 
  by 
  the 
  cooling 
  of 
  the 
  first 
  

   portions 
  of 
  the 
  ejected 
  mass. 
  

  

  As 
  a 
  source 
  of 
  silica 
  and 
  alumina 
  I 
  would 
  point 
  to 
  the 
  composition 
  of 
  

   the 
  rocks 
  immediately 
  below 
  the 
  thin 
  seam 
  of 
  coal 
  underlying 
  the 
  hori- 
  

   zontal 
  bed 
  of 
  basalt 
  under 
  consideration. 
  

  

  