﻿406 
  MESSES. 
  A. 
  J. 
  JTJKES-BKOWNE 
  A.WD 
  W. 
  HILL 
  ON 
  COLLOID 
  

  

  Chalk 
  is 
  about 
  60 
  feet 
  thick, 
  making 
  a 
  total 
  of 
  about 
  220 
  feet 
  for 
  

   the 
  Lower 
  Chalk 
  as 
  a 
  whole. 
  

  

  This 
  thickness 
  appears 
  to 
  be 
  maintained 
  all 
  along 
  the 
  Berkshire 
  

   outcrop 
  and 
  to 
  increase 
  slightly 
  in 
  Wiltshire. 
  Whether 
  there 
  is 
  any- 
  

   thing 
  more 
  than 
  an 
  accidental 
  connexion 
  between 
  the 
  incoming 
  of 
  

   these 
  siliceous 
  beds 
  and 
  the 
  thickening 
  of 
  the 
  Lower 
  Chalk 
  is 
  a 
  

   question 
  that 
  must 
  remain 
  for 
  future 
  inquiry 
  ; 
  we 
  content 
  ourselves 
  

   here 
  with 
  pointing 
  out 
  the 
  coincidence. 
  

  

  Microscopical 
  Characters. 
  

  

  Before 
  entering 
  upon 
  a 
  detailed 
  description 
  of 
  the 
  minute 
  structure 
  

   of 
  the 
  rock-beds 
  found 
  in 
  the 
  sections 
  above 
  mentioned, 
  we 
  may 
  

   premise 
  that 
  they 
  closely 
  resemble 
  some 
  of 
  the 
  sponge-beds 
  which 
  

   have 
  been 
  described 
  by 
  Dr. 
  G. 
  J. 
  Hinde, 
  F.R.S. 
  *, 
  more 
  especially 
  

   some 
  varieties 
  of 
  the 
  malmstones 
  which 
  form 
  part 
  of 
  the 
  so-called 
  

   Upper 
  Greensand 
  of 
  Surrey, 
  Hampshire, 
  and 
  Berkshire. 
  

  

  Dr. 
  Hinde's 
  Memoir 
  contains 
  so 
  much 
  information 
  and 
  has 
  been 
  

   so 
  useful 
  to 
  us 
  in 
  studying 
  these 
  sponge-beds 
  of 
  the 
  Lower 
  Chalk 
  

   that 
  it 
  will 
  be 
  desirable 
  to 
  give 
  a 
  brief 
  resume 
  of 
  his 
  observations 
  

   for 
  comparison 
  with 
  our 
  own 
  results. 
  The 
  Malmstones 
  of 
  Surrey 
  

   and 
  Hampshire 
  are 
  described 
  as 
  consisting 
  of 
  a 
  siliceous 
  or 
  siliceo-cal- 
  

   careous 
  matrix 
  which 
  encloses 
  fragments 
  of 
  sponge-spicules, 
  grains 
  

   and 
  rods 
  of 
  glauconite, 
  and 
  minute 
  scales 
  of 
  silvery 
  mica. 
  The 
  

   proportion 
  of 
  silica 
  varies 
  greatly 
  : 
  sometimes 
  the 
  rock 
  is 
  wholly 
  

   siliceous, 
  as 
  at 
  Farnham, 
  when 
  it 
  is 
  very 
  light 
  and 
  porous 
  : 
  sometimes 
  

   it 
  is 
  largely 
  calcareous 
  and 
  becomes 
  proportionately 
  compact 
  and 
  

   heavy, 
  as 
  at 
  Selborne 
  ; 
  occasionally, 
  as 
  at 
  Godstone, 
  the 
  rock 
  contains 
  

   nodules 
  of 
  bluish 
  chert 
  arranged 
  in 
  lines 
  like 
  flints 
  in 
  the 
  Chalk, 
  but 
  

   not 
  separating 
  easily 
  from 
  the 
  rock-mass. 
  

  

  In 
  the 
  more 
  siliceous 
  malmstones 
  fragments 
  of 
  siliceous 
  sponge- 
  

   spicules 
  are 
  abundant, 
  but 
  less 
  so 
  than 
  the 
  minute 
  hollow 
  cavities 
  

   or 
  casts 
  of 
  spicules 
  which 
  interpenetrate 
  the 
  rock, 
  and 
  to 
  the 
  exis- 
  

   tence 
  of 
  which 
  its 
  porosity 
  is 
  due. 
  In 
  the 
  more 
  calcareous 
  beds 
  

   many 
  of 
  the 
  spicules 
  are 
  replaced 
  by 
  calcite. 
  

  

  The 
  siliceous 
  spicules 
  consist 
  generally 
  of 
  amorphous, 
  not 
  crys- 
  

   talline 
  or 
  chalcedonic, 
  silica. 
  This 
  amorphous 
  or 
  colloid 
  silica, 
  

   when 
  viewed 
  under 
  the 
  microscope 
  by 
  reflected 
  light, 
  has 
  a 
  milky- 
  

   white 
  or 
  opal-like 
  appearance. 
  Mounted 
  in 
  Canada 
  balsam 
  and 
  seen 
  

   by 
  transmitted 
  light 
  some 
  spicules 
  exhibit 
  a 
  distinctly 
  granular 
  tex- 
  

   ture, 
  while 
  others 
  are 
  only 
  traversed 
  in 
  all 
  directions 
  by 
  minute 
  lines 
  

   which 
  can 
  be 
  resolved 
  into 
  incomplete 
  elliptical 
  rings. 
  The 
  spicular 
  

   walls 
  are 
  neutral 
  to 
  polarized 
  light 
  and 
  consist, 
  therefore, 
  of 
  colloid 
  

   silica, 
  though 
  this 
  is 
  not 
  in 
  the 
  hyaline 
  condition 
  of 
  modern 
  sponge- 
  

   spicules. 
  The 
  spicular 
  canals 
  are 
  always 
  filled 
  either 
  with 
  chalce- 
  

   donic 
  silica, 
  or 
  glauconite 
  or 
  some 
  other 
  silicate, 
  and 
  these 
  canal- 
  

   casts 
  often 
  remain 
  in 
  the 
  space 
  from 
  which 
  the 
  spicules 
  themselves 
  

   have 
  been 
  removed. 
  

  

  Besides 
  the 
  sponge-spicules 
  there 
  is 
  an 
  abundance 
  of 
  silica 
  dis- 
  

  

  * 
  Phil. 
  Trans. 
  Roy. 
  Soc. 
  1885, 
  pt. 
  ii. 
  p. 
  403. 
  

  

  