﻿546 
  

  

  Mr. 
  I. 
  L. 
  Bell 
  on 
  some 
  

  

  [June 
  17, 
  

  

  Silica 
  54 
  

  

  Alumina 
  24 
  

  

  Protoxide 
  of 
  iron 
  14 
  

  

  Magnesia 
  5 
  

  

  Potash 
  3 
  

  

  100 
  

  

  Now 
  there 
  would 
  be 
  nothing 
  extravagant 
  in 
  the 
  supposition 
  that 
  clay- 
  

   slate 
  of 
  this 
  character 
  reduced 
  to 
  a 
  state 
  of 
  fusion 
  by 
  heat 
  might 
  be 
  

   materially 
  altered 
  in 
  passing 
  through 
  the 
  Mountain 
  Limestone 
  and 
  Mill- 
  

   stone-grit 
  before 
  reaching 
  the 
  surface 
  in 
  the 
  county 
  of 
  Durham, 
  and 
  that 
  

   it 
  then 
  might 
  resemble 
  the 
  composition 
  of 
  the 
  whin-dykes 
  of 
  the 
  district. 
  

   That 
  the 
  Hett 
  Dyke 
  and 
  its 
  parallel 
  one 
  derive 
  their 
  origin 
  from 
  one 
  

   common 
  source, 
  and 
  if 
  changed 
  by 
  subsequent 
  contact 
  with 
  other 
  rocks 
  

   have 
  been 
  altered 
  in 
  each 
  case 
  by 
  the 
  same 
  cause, 
  is 
  apparent 
  on 
  refer- 
  

   ring 
  to 
  their 
  respective 
  compositions. 
  

  

  Hett 
  Dyke. 
  Parallel 
  Dyke. 
  

  

  Silica 
  51-35 
  50-60 
  

  

  Alumina 
  17-61 
  17*38 
  

  

  Protoxide 
  of 
  iron 
  12-04 
  ........ 
  12*22 
  

  

  Lime 
  9*65 
  9-20 
  

  

  Magnesia 
  5 
  "68 
  5-66 
  

  

  Potash 
  1-40 
  1-64 
  

  

  Soda 
  -56 
  -50 
  

  

  Carbonic 
  acid 
  1-53 
  2-57 
  

  

  99-82 
  99-77 
  

  

  In 
  some 
  measure 
  the 
  interior 
  portions 
  of 
  the 
  horizontal 
  bed 
  found 
  in 
  the 
  

   pits 
  resemble 
  these 
  two 
  specimens 
  in 
  the 
  nature 
  of 
  their 
  constituent 
  

   parts, 
  and 
  in 
  physical 
  appearance 
  there 
  is 
  little 
  or 
  no 
  difference 
  among 
  

   them. 
  Fragments 
  taken 
  from 
  the 
  bed 
  gave 
  : 
  — 
  

  

  Silica 
  51-90 
  

  

  Alumina 
  15*46 
  

  

  Protoxide 
  of 
  iron 
  12-87 
  

  

  Lime 
  13*80 
  

  

  Magnesia 
  4*02 
  

  

  Potash 
  1*21 
  

  

  Soda 
  -48 
  

  

  Carbonic 
  acid 
  1*02 
  

  

  100-76 
  

  

  As 
  stated, 
  the 
  specimen 
  just 
  described 
  was 
  obtained 
  from 
  the 
  interior 
  

   of 
  the 
  mass 
  of 
  whin; 
  but 
  its 
  exterior, 
  both 
  on 
  the 
  upper 
  and 
  lower 
  

   surfaces, 
  is 
  covered 
  with 
  a 
  coating 
  10 
  inches 
  thick 
  in 
  the 
  one, 
  and 
  

  

  