﻿NEGRAIS 
  ROCKS. 
  113 
  

  

  which 
  Mr. 
  W. 
  Blanford 
  described 
  as 
  prevailing 
  among 
  portions 
  of 
  the 
  

   Nummulitic 
  strata 
  in 
  the 
  Bassein 
  district, 
  a 
  feature 
  which 
  makes 
  

   it 
  difficult 
  to 
  separate 
  that 
  group 
  from 
  the 
  rocks 
  I 
  am 
  now 
  describ- 
  

   ing. 
  The 
  alteration 
  to 
  which 
  the 
  beds 
  of 
  this 
  region 
  have 
  been 
  

   subjected 
  would 
  seem, 
  judging 
  by 
  its 
  effect, 
  to 
  be 
  of 
  two 
  sorts, 
  perhaps 
  

   rather 
  marking 
  different 
  periods 
  and 
  degrees 
  of 
  intensity, 
  than 
  different 
  

   sources. 
  There 
  is 
  that 
  wide-spread 
  sub-metamorphism 
  which, 
  though 
  

   somewhat 
  capriciously 
  exerted, 
  yet 
  extends 
  over 
  wide-areas, 
  and 
  affects 
  a 
  

   great 
  thickness 
  of 
  beds, 
  and 
  is 
  manifested 
  by 
  an 
  induration 
  and 
  slight 
  

   change 
  in 
  them, 
  such 
  as 
  we 
  may 
  conceive 
  to 
  be 
  the 
  first 
  stage 
  in 
  the 
  

   process 
  of 
  ordinary 
  metamorphism. 
  This 
  process 
  is 
  a 
  wide-spread 
  one, 
  and 
  

   the 
  shales 
  and 
  sandstones 
  subjected 
  to 
  it 
  nowhere 
  lose 
  their 
  character, 
  

   but 
  are 
  simply 
  indurated 
  and 
  altered 
  to 
  a 
  variable 
  extent 
  by 
  its 
  action. 
  

   There 
  is 
  in 
  addition 
  a 
  more 
  restricted 
  and 
  energetic 
  sort 
  of 
  metamorphism 
  

   which 
  is 
  seen 
  along 
  the 
  coast, 
  in 
  the 
  intense 
  and 
  often 
  abrupt 
  alteration 
  

   of 
  beds 
  of 
  sandstone 
  into 
  cherty 
  masses 
  of 
  a 
  harsh 
  and 
  intraetible 
  

   character. 
  This 
  sort 
  of 
  alteration 
  seems 
  mainly 
  dependant 
  on 
  the 
  

   abundant 
  presence 
  of 
  silica, 
  which 
  has 
  permeated 
  the 
  rock, 
  and 
  to 
  a 
  

   great 
  extent 
  replaced 
  its 
  original 
  constituents 
  ; 
  and 
  this 
  I 
  believe 
  is 
  most 
  

   completely 
  effected 
  where 
  the 
  rock 
  originally 
  contained 
  a 
  considerable 
  

   amount 
  of 
  lime, 
  many 
  of 
  the 
  most 
  silicious 
  masses 
  seen 
  along 
  the 
  coast 
  

   giving 
  the 
  impression 
  that 
  the 
  rock 
  was 
  originally 
  highly 
  calcareous, 
  but 
  

   wherein 
  now 
  silica 
  has 
  almost 
  entirely 
  replaced 
  the 
  lime. 
  I 
  may 
  here 
  

   remark 
  that 
  Dr. 
  Oldham 
  has 
  shown 
  me 
  some 
  fossils 
  forwarded 
  by 
  

   Captain 
  Fryer 
  from 
  some 
  part 
  of 
  the 
  Sandoway 
  or 
  adjoining 
  districts, 
  the 
  

   most 
  remarkable 
  of 
  which 
  were 
  silica-casts 
  of 
  that 
  curious 
  shell, 
  

   Magilus 
  anliqws, 
  now 
  found 
  living 
  on 
  the 
  coast, 
  which 
  prove 
  that 
  the 
  

   effusion 
  of 
  silica 
  by 
  means 
  of 
  springs 
  is 
  still 
  going 
  on 
  in 
  this 
  neighbour- 
  

   hood 
  ; 
  and 
  I 
  have 
  little 
  doubt 
  that 
  such 
  silicious 
  springs 
  played 
  an 
  

   important 
  part 
  in 
  that 
  particular 
  sort 
  of 
  metamorphism 
  I 
  am 
  here 
  

  

  alluding 
  to. 
  

  

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  301 
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