﻿162 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Feb. 
  3, 
  

  

  cementing 
  substance 
  generally 
  consists 
  of 
  the 
  same 
  materials 
  as 
  

   those 
  which 
  unite 
  the 
  quartz-granules 
  of 
  the 
  rocks 
  in 
  which 
  they 
  

   occur 
  ; 
  but 
  when 
  they 
  appear 
  among 
  the 
  magnesian 
  breccias, 
  they 
  

   have, 
  however, 
  a 
  different 
  cementing 
  matrix. 
  A 
  portion 
  of 
  one 
  of 
  

   these 
  from 
  the 
  higher 
  part 
  of 
  the 
  Penrith 
  sandstones 
  of 
  Hilton-beck, 
  

   as 
  determined 
  by 
  Dr. 
  Blyth, 
  afforded 
  the 
  following 
  results 
  : 
  — 
  

  

  CaO,C0 
  2 
  16-00 
  

  

  MgO,C0 
  2 
  1-75 
  

  

  Silica 
  76-20 
  

  

  Carbonate 
  of 
  iron 
  and 
  moisture 
  6-05 
  

  

  100-00 
  

   Rejecting 
  the 
  sand 
  and 
  iron 
  from 
  this 
  specimen, 
  and 
  calculating 
  

   the 
  percentage 
  of 
  lime 
  and 
  magnesia, 
  gives 
  the 
  following 
  result 
  : 
  — 
  

  

  CaO,C0 
  2 
  91-36 
  

  

  MgO, 
  C0 
  2 
  8-64 
  

  

  100-00 
  

   In 
  this 
  case 
  the 
  cementing 
  matrix, 
  although 
  of 
  dolomite, 
  has 
  had 
  

   the 
  dolomitizing 
  process 
  much 
  less 
  advanced 
  than 
  is 
  the 
  case 
  with 
  

   the 
  fragments 
  making 
  up 
  the 
  magnesian 
  breccias. 
  

  

  In 
  the 
  Hilton 
  group 
  a 
  specimen 
  of 
  the 
  impure 
  limestones 
  asso- 
  

   ciated 
  with 
  the 
  plant-beds 
  yielded 
  Dr. 
  Blyth 
  the 
  following 
  consti- 
  

   tuents 
  : 
  — 
  

  

  CaO,C0 
  2 
  24-4 
  

  

  MgO,C0 
  2 
  1-7 
  

  

  Si0 
  3 
  70-0 
  

  

  Oxide 
  of 
  iron 
  3-9 
  

  

  100-0 
  

   The 
  dolomitized 
  limestone, 
  the 
  cementing 
  matrix 
  of 
  the 
  sands 
  in 
  

   this 
  stratum, 
  had 
  the 
  following 
  composition 
  : 
  — 
  

  

  CaO, 
  C0 
  2 
  93-1 
  

  

  MgO, 
  C0 
  2 
  6-9 
  

  

  100-0 
  

  

  In 
  this 
  specimen 
  the 
  cementing 
  matrix 
  has 
  a 
  great 
  affinity 
  to 
  that 
  

   of 
  the 
  siliceous 
  vein 
  just 
  described. 
  

  

  Immediately 
  beneath 
  the 
  red 
  shales 
  of 
  the 
  Hilton 
  section 
  a 
  thin 
  

   mass 
  of 
  grey 
  limestone 
  occurs, 
  containing 
  drusy 
  cavities, 
  and 
  much 
  

   resembling 
  in 
  aspect 
  the 
  compact 
  limestone 
  of 
  the 
  county 
  of 
  Durham. 
  

   This 
  limestone 
  approaches 
  very 
  nearly 
  to 
  a 
  true 
  dolomite. 
  

  

  No 
  calcareous 
  rocks 
  are 
  seen 
  in 
  the 
  Hilton 
  section 
  above 
  this 
  

   limestone, 
  but 
  in 
  the 
  Kirkby 
  Stephen 
  section 
  the 
  " 
  hard 
  brockram 
  " 
  

   has 
  a 
  higher 
  position, 
  appearing 
  above 
  the 
  red 
  shales. 
  The 
  un- 
  

   altered 
  condition 
  of 
  the 
  limestone-fragments 
  in 
  this 
  " 
  hard 
  brock- 
  

   ram" 
  indicates 
  the 
  termination 
  of 
  the 
  action 
  of 
  that 
  force 
  which 
  

   gave 
  rise 
  to 
  dolomites, 
  anterior 
  to 
  the 
  period 
  of 
  deposition 
  of 
  these 
  

   " 
  hard 
  brockrams," 
  and 
  shows 
  that 
  the 
  action 
  of 
  the 
  dolomitizing 
  

   agent 
  was 
  confined 
  to 
  the 
  middle 
  portion 
  of 
  the 
  Permian 
  epoch. 
  

  

  The 
  Upper 
  Permian 
  rocks 
  — 
  the 
  St. 
  Bees 
  and 
  Corby 
  sandstones 
  — 
  

  

  