﻿X864,] 
  MTJRCHISON 
  AND 
  HARKNESS 
  PERMIAN. 
  163 
  

  

  afford 
  further 
  proof 
  that 
  the 
  dolomitizing 
  influence 
  had 
  ceased 
  to 
  ope- 
  

   rate 
  anterior 
  to 
  their 
  deposition. 
  Dr. 
  Blyth 
  finds 
  that 
  these 
  rocks 
  are 
  

   composed 
  of 
  small 
  rounded 
  quartz-granules, 
  with 
  a 
  cementing 
  matrix 
  

   of 
  silicates 
  of 
  lime 
  and 
  iron, 
  very 
  little 
  magnesia 
  being 
  present. 
  

   Their 
  deep-red 
  colour, 
  owing 
  to 
  oxide 
  of 
  iron 
  as 
  in 
  the 
  lower 
  sand- 
  

   stones, 
  is 
  merely 
  incrusting, 
  and 
  can 
  be 
  dissolved 
  off, 
  leaving 
  the 
  

   granules 
  perfectly 
  colourless. 
  

  

  The 
  mode 
  in 
  which 
  the 
  magnesian 
  breccias 
  and 
  limestones 
  occur 
  in 
  

   the 
  north-west 
  of 
  England 
  leads 
  to 
  the 
  conclusion 
  that 
  the 
  dolomiti- 
  

   zing 
  agent 
  must 
  have 
  acted 
  from 
  above. 
  Had 
  this 
  cause 
  exercised 
  its 
  

   influence 
  from 
  below, 
  it 
  would 
  have 
  operated 
  powerfully 
  on 
  the 
  lower 
  

   breccias; 
  and 
  yet 
  these, 
  as 
  seen 
  in 
  the 
  north-west 
  of 
  England, 
  are 
  per- 
  

   fectly 
  unchanged, 
  while 
  the 
  higher 
  breccias 
  always 
  exhibit 
  the 
  mag- 
  

   nesian-limestone 
  character. 
  These 
  latter 
  have 
  also 
  been 
  subjected 
  

   to 
  a 
  force 
  which 
  dissolved 
  a 
  portion 
  of 
  carbonate 
  of 
  lime 
  from 
  the 
  

   fragments, 
  converting 
  them 
  into 
  potato 
  -stones 
  ; 
  the 
  dissolved 
  por- 
  

   tion, 
  combining 
  with 
  carbonate 
  of 
  magnesia, 
  furnished 
  the 
  materials 
  

   out 
  of 
  which 
  the 
  regular 
  stratified 
  dolomites 
  were 
  formed, 
  or, 
  be- 
  

   coming 
  mixed 
  with 
  other 
  mineral 
  matter, 
  constituted 
  its 
  cementing 
  

   matrix 
  ; 
  in 
  which 
  condition 
  we 
  have 
  it, 
  in 
  the 
  plant-beds 
  of 
  Hilton, 
  

   giving 
  rise 
  to 
  dolomitic 
  sandstones. 
  

  

  Concerning 
  the 
  dolomitizing 
  cause, 
  this 
  has 
  been 
  frequently 
  dis- 
  

   cussed 
  both 
  by 
  chemists 
  and 
  geologists. 
  One 
  circumstance, 
  namely, 
  

   the 
  occurrence 
  of 
  gypsum, 
  either 
  in 
  beds 
  associated 
  with 
  magnesian 
  

   limestone 
  or 
  in 
  deposits 
  appertaining 
  to 
  the 
  period 
  when 
  this 
  kind 
  

   of 
  mineral 
  matter 
  was 
  very 
  abundant, 
  seems 
  to 
  connect 
  gypsum 
  

   with 
  dolomites, 
  since 
  at 
  St. 
  Bees 
  Head 
  the 
  former 
  occurs 
  in 
  red 
  

   shales 
  immediately 
  overlying 
  magnesian 
  limestone, 
  and 
  in 
  several 
  

   portions 
  of 
  the 
  Yale 
  of 
  the 
  Eden 
  gypsum 
  and 
  red 
  shales 
  are 
  the 
  ex- 
  

   clusive 
  representatives 
  of 
  the 
  Middle 
  Permian 
  series. 
  This 
  associa- 
  

   tion 
  supports, 
  to 
  some 
  extent, 
  the 
  conclusion 
  that 
  the 
  magnesia 
  of 
  the 
  

   dolomites 
  originally 
  existed 
  as 
  a 
  sulphate 
  of 
  this 
  earth, 
  from 
  which 
  

   was 
  derived 
  the 
  sulphuric 
  acid 
  combined 
  with 
  the 
  lime 
  in 
  gypsum. 
  

  

  § 
  8. 
  Conclusion. 
  

   The 
  Permian 
  rocks 
  of 
  the 
  county 
  of 
  Durham 
  have 
  hitherto 
  been 
  

   regarded 
  as 
  the 
  type 
  of 
  this 
  formation 
  in 
  England. 
  In 
  this 
  county, 
  

   however, 
  excepting 
  the 
  middle 
  member, 
  the 
  strata 
  appertaining 
  

   to 
  this 
  portion 
  of 
  the 
  Palaeozoic 
  period 
  are 
  very 
  imperfectly 
  repre- 
  

   sented. 
  Of 
  this 
  group 
  the 
  base 
  is 
  almost 
  absent, 
  the 
  Penrith 
  sand- 
  

   stones 
  occurring 
  only 
  to 
  a 
  very 
  slight 
  extent 
  as 
  soft 
  sandstones, 
  sup- 
  

   porting 
  the 
  base 
  of 
  the 
  magnesian 
  limestones 
  — 
  the 
  equivalents 
  of 
  the 
  

   Hilton 
  series 
  — 
  and 
  in 
  many 
  localities 
  these 
  sandstones 
  disappear 
  

   altogether. 
  In 
  the 
  county 
  of 
  Durham 
  the 
  portions 
  of 
  the 
  Permian 
  

   strata 
  which 
  occur 
  are 
  transgressive 
  on 
  the 
  coal-measures, 
  and 
  

   are 
  spread 
  out 
  upon 
  the 
  eroded 
  surfaces 
  of 
  the 
  latter 
  rocks. 
  At 
  

   Tynemouth, 
  beneath 
  the 
  " 
  brecciated 
  limestone 
  " 
  forming 
  the 
  upper 
  

   portion 
  of 
  the 
  cliffs, 
  this 
  eroded 
  surface 
  is 
  seen 
  in 
  the 
  light-coloured 
  

   sandstone 
  supporting 
  the 
  Permian 
  breccia-beds. 
  This 
  light- 
  coloured 
  

   sandstone 
  is 
  said 
  to 
  pass 
  downwards 
  into 
  another 
  sandstone 
  with 
  

  

  m2 
  

  

  * 
  

  

  