﻿214 
  PROFESSOR 
  FORBES 
  ON 
  THE 
  GEOLOGY 
  

  

  we 
  observe 
  on 
  the 
  same 
  sandstone 
  by 
  another 
  patch 
  of 
  trap 
  tufa 
  on 
  the 
  south 
  

   bank 
  of 
  the 
  Tweed, 
  opposite 
  Dryburgh 
  Abbey, 
  to 
  which 
  I 
  was 
  directed 
  by 
  Mr 
  

   Milne's 
  map, 
  and 
  which 
  perfectly 
  resembles 
  the 
  Melrose 
  tufa 
  ; 
  but 
  it 
  is 
  evidently 
  

   separated 
  from 
  it 
  by 
  the 
  entire 
  mass 
  of 
  the 
  Eildon 
  Hills 
  and 
  the 
  adjoining 
  grey- 
  

   wacke 
  rocks.* 
  

  

  The 
  Melrose 
  tufa 
  is 
  completely 
  lost 
  to 
  the 
  north, 
  in 
  consequence 
  of 
  the 
  ancient 
  

   excavations 
  occasioned 
  by 
  the 
  river 
  Tweed 
  as 
  already 
  mentioned 
  ; 
  it 
  sinks 
  under 
  

   flats 
  and 
  mounds 
  of 
  debris. 
  But 
  it 
  may 
  be 
  traced 
  to 
  the 
  eastward 
  in 
  the 
  bed 
  of 
  the 
  

   Huntly 
  Burn, 
  close 
  to 
  the 
  house 
  of 
  that 
  name, 
  and 
  to 
  the 
  villa 
  of 
  Chiefswood. 
  It 
  

   also 
  extends 
  to 
  the 
  Rhymer's 
  Glen, 
  forming 
  evidently 
  a 
  tongue, 
  which 
  runs 
  up 
  be- 
  

   tween 
  the 
  narrow 
  belt 
  of 
  grey 
  wacke 
  which 
  continues 
  to 
  fringe 
  the 
  trap 
  of 
  Bowden 
  

   Muir 
  and 
  the 
  well-marked 
  grey 
  wacke 
  ridge 
  parallel 
  to 
  it 
  on 
  the 
  north, 
  which 
  

   stretches 
  to 
  Faldonside. 
  The 
  section 
  in 
  the 
  Rhymer's 
  Glen 
  is 
  not 
  without 
  inte- 
  

   rest. 
  Characteristic 
  trap 
  tufa 
  (No. 
  26) 
  first 
  appears 
  from 
  under 
  the 
  detritus 
  of 
  

   the 
  valley 
  in 
  the 
  bed 
  of 
  the 
  small 
  stream. 
  This 
  unquestionably 
  belongs 
  to 
  the 
  

   same 
  mass 
  as 
  the 
  Melrose 
  tufa. 
  It 
  may 
  be 
  traced 
  up 
  the 
  stream 
  of 
  the 
  Rhymer's 
  

   Glen, 
  until 
  it 
  passes 
  into 
  a 
  yellowish 
  felspar 
  rock 
  in 
  a 
  gradual 
  manner, 
  which 
  is 
  

   probably 
  in 
  contact 
  with 
  the 
  grey 
  wacke 
  strata 
  which 
  succeed 
  in 
  almost 
  vertical 
  

   strata. 
  Some 
  of 
  these 
  strata 
  are 
  exceedingly 
  hard, 
  and 
  form 
  the 
  barrier 
  at 
  the 
  

   first 
  waterfall. 
  It 
  is 
  here 
  in 
  contact 
  with 
  a 
  singular 
  bed 
  of 
  a 
  coaly 
  appearance, 
  

   which 
  I 
  believe 
  has 
  been 
  mistaken 
  for 
  an 
  indication 
  of 
  the 
  coal 
  formation, 
  

   which, 
  however, 
  it 
  cannot 
  be, 
  as 
  it 
  is 
  interstratified 
  with 
  the 
  hardened 
  strata 
  of 
  

   greywacke 
  just 
  mentioned, 
  which, 
  it 
  may 
  be 
  added, 
  include 
  traces 
  of 
  common 
  

   galena 
  (No. 
  24), 
  and 
  are 
  traversed 
  by 
  calcspar 
  veins 
  (No. 
  25). 
  The 
  dark 
  bed 
  is 
  a 
  

   shale 
  (No. 
  23) 
  resembling 
  alum 
  shale, 
  mechanically 
  diffusible 
  in 
  water, 
  and 
  in- 
  

   cluding 
  soft 
  whitish 
  fragments 
  resembling 
  steatite. 
  At 
  the 
  highest 
  waterfall 
  in 
  

   the 
  Rhymer's 
  Glen, 
  the 
  greywacke 
  strata 
  (which 
  here 
  run 
  in 
  a 
  direction 
  of 
  east 
  

   by 
  north) 
  are 
  interrupted 
  by 
  a 
  dyke 
  of 
  felspathic 
  trap, 
  sometimes 
  of 
  a 
  purplish, 
  

   sometimes 
  of 
  a 
  yellow 
  colour, 
  and 
  which 
  I 
  have 
  no 
  doubt 
  is 
  the 
  same 
  vein 
  as 
  may 
  

   be 
  discovered 
  in 
  the 
  greywacke 
  on 
  the 
  east 
  side 
  of 
  Cauldshiels 
  Loch 
  (No. 
  20), 
  not 
  

   far 
  from 
  its 
  contact 
  with 
  the 
  main 
  mass 
  of 
  the 
  porphyry 
  of 
  Bowden 
  Muir, 
  of 
  

   which 
  this 
  vein 
  may 
  be 
  an 
  offset. 
  The 
  waterfall 
  above 
  mentioned 
  is 
  unquestion- 
  

  

  * 
  The 
  deposit 
  in 
  question 
  occurs 
  at 
  the 
  house 
  of 
  Holmes, 
  exactly 
  opposite 
  to 
  Dryburgh 
  Abbey. 
  

   Tbe 
  course 
  of 
  the 
  Tweed 
  is 
  here 
  north-north-west 
  to 
  south-south-east. 
  The 
  strata 
  on 
  both 
  sides 
  of 
  

   the 
  tufa 
  mass 
  are 
  red 
  and 
  white 
  sandstone, 
  stratified 
  nearly 
  horizontally 
  with 
  some 
  slate-clay. 
  At 
  

   the 
  north 
  junction 
  the 
  strata 
  cannot 
  be 
  distinctly 
  traced 
  to 
  within 
  50 
  or 
  60 
  yards 
  of 
  the 
  trap; 
  but, 
  

   when 
  the 
  river 
  is 
  low, 
  a 
  better 
  view 
  might 
  be 
  had. 
  The 
  tufa 
  rock, 
  however, 
  is 
  modified 
  and 
  com- 
  

   pacted, 
  including 
  large 
  and 
  small 
  nodules 
  of 
  rounded 
  quartz, 
  and, 
  in 
  one 
  place, 
  includes 
  soft 
  angular 
  

   fragments 
  (perhaps 
  of 
  slate-clay), 
  which 
  give 
  it 
  almost 
  a 
  porphyritic 
  appearance 
  (specimen 
  No. 
  30). 
  

   The 
  characteristic 
  tufa 
  rock 
  (No. 
  31) 
  may 
  be 
  traced 
  100 
  yards 
  or 
  so 
  up 
  a 
  little 
  side 
  ravine, 
  but 
  is 
  

   then 
  completely 
  lost 
  under 
  diluvium. 
  The 
  southern 
  part 
  of 
  the 
  tufaceous 
  mass 
  becomes 
  very 
  com- 
  

   pact, 
  and 
  assumes 
  the 
  character 
  of 
  a 
  very 
  tough 
  felspar 
  porphyry 
  (No. 
  32). 
  The 
  sandstone 
  strata 
  in 
  

   contact 
  with 
  it 
  are 
  hardened 
  and 
  bleached 
  in 
  a 
  remarkable 
  manner. 
  

  

  