﻿CHAENWOOD-FOEEST 
  EOCKS. 
  199 
  

  

  have 
  expected. 
  The 
  Croft 
  Hill 
  rock 
  thus 
  appears 
  to 
  be 
  intermediate 
  

   between 
  the 
  " 
  southern 
  syenites" 
  of 
  the 
  Forest 
  and 
  the 
  Hornblendic 
  

   granite 
  of 
  Mount 
  Sorrel, 
  on 
  its 
  eastern 
  margin. 
  (T. 
  G. 
  B.) 
  

  

  (5) 
  The 
  " 
  Porphyroid 
  " 
  of 
  Sharpley. 
  

   } 
  25-47 
  | 
  

  

  SiO., 
  67-96 
  

  

  P 
  2 
  0. 
  trace. 
  

  

  MnO 
  trace. 
  

  

  CaO 
  1-69 
  

  

  MgO 
  1-10 
  

  

  K„0 
  trace. 
  

  

  Na 
  2 
  5-07 
  

  

  Total 
  101-29 
  

  

  This 
  rock 
  is 
  very 
  fully 
  discussed 
  in 
  our 
  third 
  communication 
  

   (vol. 
  xxxvi. 
  pp. 
  342-345), 
  in 
  which 
  we 
  come 
  to 
  the 
  conclusion 
  that 
  

   it 
  is 
  probably 
  an 
  altered 
  volcanic 
  tuff. 
  The 
  amount 
  of 
  silica 
  is 
  

   distinctly 
  lower 
  than 
  in 
  the 
  rock 
  which 
  occurs 
  in 
  fragments 
  in 
  the 
  

   agglomerates 
  of 
  this 
  neighbourhood, 
  which 
  gave 
  a 
  percentage 
  of 
  

   about 
  77*7. 
  The 
  rather 
  variable 
  quantity 
  of 
  free 
  quartz 
  in 
  both 
  may, 
  

   however, 
  account 
  for 
  the 
  discrepancy. 
  We 
  are 
  surprised 
  at 
  the 
  

   great 
  predominance 
  of 
  Na 
  2 
  over 
  K 
  2 
  ; 
  for 
  we 
  certainly 
  recognized 
  

   orthoclase, 
  as 
  well 
  as 
  a 
  plagioclastic 
  felspar 
  among 
  those 
  present 
  in 
  

   the 
  Sharpley 
  rock. 
  

  

  The 
  results 
  of 
  the 
  above 
  analyses 
  accord 
  (except 
  in 
  this 
  last 
  

   respect) 
  as 
  nearly 
  as 
  could 
  be 
  expected 
  with 
  those 
  deduced 
  from 
  

   microscopic 
  examination, 
  and 
  are 
  therefore 
  of 
  interest 
  as 
  confirm- 
  

   ing 
  the 
  utility 
  of 
  that 
  method 
  of 
  investigation. 
  (T. 
  G. 
  B.) 
  

  

  Q.J.G.S. 
  No. 
  150. 
  

  

  