﻿SILICA 
  IN 
  THE 
  LOWER 
  CHALK 
  OF 
  BERKSHIRE 
  AND 
  WILTSHIRE. 
  417 
  

  

  probably 
  surface-currents, 
  which 
  gradually 
  lost 
  their 
  force, 
  and 
  

   allowed 
  their 
  burden 
  to 
  subside, 
  whereas 
  the 
  spicules 
  would 
  have 
  

   to 
  be 
  picked 
  up 
  from 
  the 
  sea-floor 
  and 
  carried 
  onward. 
  

  

  The 
  action 
  of 
  gentle 
  bottom-currents 
  may 
  perhaps 
  partly 
  account 
  

   for 
  the 
  wide 
  distribution 
  of 
  the 
  spicules 
  ; 
  but 
  Prof. 
  W. 
  J. 
  Sollas 
  has 
  

   furnished 
  us 
  with 
  a 
  sufficient 
  reason 
  for 
  their 
  wide-spread 
  abun- 
  

   dance. 
  He 
  has 
  found 
  that 
  many, 
  if 
  not 
  all, 
  siliceous 
  sponges 
  are 
  

   continually 
  shedding 
  some 
  of 
  their 
  spicules, 
  and 
  that 
  the 
  number 
  

   of 
  spicules 
  shed 
  by 
  such 
  a 
  sponge 
  during 
  its 
  lifetime 
  must 
  be 
  very 
  

   great, 
  and 
  may 
  possibly 
  be 
  larger 
  than 
  the 
  number 
  of 
  spicules 
  

   existing 
  at 
  any 
  one 
  time 
  in 
  the 
  sponge 
  itself. 
  The 
  facts 
  will 
  be 
  

   found 
  in 
  his 
  Report 
  on 
  the 
  siliceous 
  sponges 
  collected 
  by 
  the 
  ' 
  Chal- 
  

   lenger 
  ' 
  Expedition, 
  but 
  we 
  quote 
  the 
  following 
  from 
  another 
  publi- 
  

   cation 
  : 
  — 
  " 
  In 
  one 
  case 
  (Cydonium 
  neptuni) 
  they 
  [the 
  shed 
  spicules] 
  

   accumulate 
  within 
  certain 
  cavities 
  of 
  the 
  interior 
  of 
  the 
  sponge, 
  

   completely 
  filling 
  them 
  up, 
  so 
  that 
  they 
  look 
  as 
  if 
  stuffed 
  with 
  

   cotton-wool 
  : 
  the 
  quantity 
  of 
  spicules 
  so 
  preserved 
  must, 
  I 
  should 
  

   imagine, 
  amount 
  to 
  at 
  least 
  one-fifth 
  of 
  the 
  total 
  quantity 
  present 
  

   in 
  the 
  sponge 
  ; 
  but 
  these 
  are 
  only 
  what, 
  by 
  an 
  accident 
  of 
  structural 
  

   character, 
  are 
  preserved 
  and 
  can 
  be 
  seen. 
  How 
  many 
  others 
  have 
  been 
  

   extruded 
  at 
  the 
  surface, 
  fallen 
  on 
  the 
  sea-floor, 
  and 
  left 
  no 
  trace 
  of 
  

   their 
  previous 
  connexion 
  with 
  the 
  sponge 
  behind, 
  we 
  do 
  not 
  know, 
  

   and 
  have 
  no 
  means 
  of 
  knowing. 
  But 
  it 
  is 
  a 
  very 
  suggestive 
  fact 
  

   that 
  at 
  one 
  of 
  the 
  stations 
  from 
  which 
  the 
  ' 
  Challenger 
  ' 
  obtained 
  

   sponges 
  in 
  great 
  numbers 
  (Station 
  149, 
  off 
  Kerguelen), 
  the 
  mud 
  of 
  

   the 
  sea-floor 
  from 
  which 
  the 
  sponges 
  were 
  dredged 
  is 
  crammed 
  full 
  

   of 
  sponge-spicules 
  ; 
  and 
  though 
  the 
  majority 
  of 
  these 
  may 
  have 
  been 
  

   derived 
  from 
  dead 
  sponges, 
  yet 
  a 
  goodly 
  proportion 
  may 
  quite 
  fairly 
  

   be 
  regarded 
  as 
  having 
  been 
  cast 
  out 
  from 
  the 
  living 
  sponges"*. 
  

  

  These 
  facts 
  enable 
  us 
  to 
  understand 
  the 
  abundance 
  and 
  the 
  wide 
  

   distribution 
  of 
  the 
  spicules 
  which 
  seem 
  to 
  have 
  supplied 
  the 
  material 
  

   for 
  the 
  formation 
  of 
  the 
  disseminated 
  globular 
  silica. 
  

  

  The 
  next 
  point 
  for 
  consideration 
  is 
  whether 
  the 
  formation 
  and 
  

   accumulation 
  of 
  the 
  globular 
  silica 
  went 
  on 
  contemporaneously 
  with 
  

   the 
  deposition 
  of 
  the 
  calcareous 
  material 
  on 
  the 
  sea-floor, 
  or 
  whether 
  

   the 
  conversion 
  of 
  the 
  spicules 
  into 
  such 
  silica 
  took 
  place 
  after 
  the 
  

   consolidation 
  of 
  the 
  rock. 
  Dr. 
  Hinde 
  believes 
  that, 
  in 
  the 
  case 
  of 
  

   the 
  Malmstones 
  f 
  , 
  the 
  deposition 
  of 
  the 
  silica 
  took 
  place 
  after 
  the 
  

   consolidation 
  of 
  the 
  rock 
  ; 
  he 
  supposes 
  the 
  silica 
  to 
  have 
  been 
  

   derived 
  from 
  the 
  spicules, 
  of 
  which 
  some 
  of 
  the 
  empty 
  casts 
  can 
  be 
  

   seen, 
  and 
  that 
  the 
  material 
  of 
  these 
  was 
  dissolved 
  by 
  water 
  percola- 
  

   ting 
  through 
  the 
  rock, 
  and 
  redeposited 
  in 
  the 
  form 
  of 
  globular 
  

   colloid 
  silica. 
  We 
  find, 
  however, 
  much 
  difficulty 
  in 
  accepting 
  this 
  

   view 
  of 
  the 
  formation 
  of 
  Malmstone, 
  and 
  much 
  more 
  difficulty 
  in 
  

   applying 
  it 
  to 
  the 
  case 
  of 
  the 
  siliceous 
  chalk. 
  

  

  When 
  we 
  consider 
  the 
  occurrence 
  of 
  colloid 
  silica 
  in 
  a 
  similar 
  

   globular 
  condition 
  diffused 
  throughout 
  the 
  calcareous 
  material 
  of 
  a 
  

  

  " 
  A 
  Contribution 
  to 
  the 
  History 
  of 
  Flints," 
  Proc. 
  Roy. 
  Dubl. 
  Soc. 
  vol. 
  vi. 
  

   pt. 
  1 
  (1887). 
  

  

  t 
  Phil. 
  Trans. 
  1885, 
  part 
  ii. 
  p. 
  403. 
  

  

  Q.J.G.S. 
  No. 
  179. 
  2i 
  

  

  