thickness。　　Why　worms　should　penetrate　and　bring　up　chalk　in　some
  places　and　not　in　others　I　do　not　know。
  In　many　districts　where　the　land　is　nearly　level；　a　bed　several
  feet　in　thickness　of　red　clay　full　of　unworn　flints　overlies　the
  Upper　Chalk。　　This　overlying　matter；　the　surface　of　which　has　been
  converted　into　mould；　consists　of　the　undissolved　residue　from　the
  chalk。　　It　may　be　well　here　to　recall　the　case　of　the　fragments　of
  chalk　buried　beneath　worm…castings　on　one　of　my　fields；　the　angles
  of　which　were　so　completely　rounded　in　the　course　of　29　years　that
  the　fragments　now　resembled　water…worn　pebbles。　　This　must　have
  been　effected　by　the　carbonic　acid　in　the　rain　and　in　the　ground；
  by　the　humus…acids；　and　by　the　corroding　power　of　living　roots。
  Why　a　thick　mass　of　residue　has　not　been　left　on　the　Chalk；
  wherever　the　land　is　nearly　level；　may　perhaps　be　accounted　for　by
  the　percolation　of　the　fine　particles　into　the　fissures；　which　are
  often　present　in　the　chalk　and　are　either　open　or　are　filled　up
  with　impure　chalk；　or　into　the　solid　chalk　itself。　　That　such
  percolation　occurs　can　hardly　be　doubted。　　My　son　collected　some
  powdered　and　fragmentary　chalk　beneath　the　turf　near　Winchester；
  the　former　was　found　by　Colonel　Parsons；　R。　E。；　to　contain　10　per
  cent。；　and　the　fragments　8　per　cent。　of　earthy　matter。　　On　the
  flanks　of　the　escarpment　near　Abinger　in　Surrey；　some　chalk　close
  beneath　a　layer　of　flints；　2　inches　in　thickness　and　covered　by　8
  inches　of　mould；　yielded　a　residue　of　3。7　per　cent。　of　earthy
  matter。　　On　the　other　hand　the　Upper　Chalk　properly　contains；　as　I
  was　informed　by　the　late　David　Forbes　who　had　made　many　analyses；
  only　from　1　to　2　per　cent。　of　earthy　matter；　and　two　samples　from
  pits　near　my　house　contained　1。3　and　0。6　per　cent。　　I　mention　these
  latter　cases　because；　from　the　thickness　of　the　overlying　bed　of
  red　clay　with　flints；　I　had　imagined　that　the　underlying　chalk
  might　here　be　less　pure　than　elsewhere。　　The　cause　of　the　residue
  accumulating　more　in　some　places　than　in　others；　may　be　attributed
  to　a　layer　of　argillaceous　matter　having　been　left　at　an　early
  period　on　the　chalk；　and　this　would　check　the　subsequent
  percolation　of　earthy　matter　into　it。
  From　the　facts　now　given　we　may　conclude　that　castings　ejected　on
  our　Chalk　Downs　suffer　some　loss　by　the　percolation　of　their　finer
  matter　into　the　chalk。　　But　such　impure　superficial　chalk；　when
  dissolved；　would　leave　a　larger　supply　of　earthy　matter　to　be　added
  to　the　mould　than　in　the　case　of　pure　chalk。　　Besides　the　loss
  caused　by　percolation；　some　fine　earth　is　certainly　washed　down　the
  sloping　grass…covered　surfaces　of　our　Downs。　　The　washing…down
  process；　however；　will　be　checked　in　the　course　of　time；　for
  although　I　do　not　know　how　thin　a　layer　of　mould　suffices　to
  support　worms；　yet　a　limit　must　at　last　be　reached；　and　then　their
  castings　would　cease　to　be　ejected　or　would　become　scanty。
  The　following　cases　show　that　a　considerable　quantity　of　fine　earth
  is　washed　down。　　The　thickness　of　the　mould　was　measured　at　points
  12　yards　apart　across　a　small　valley　in　the　Chalk　near　Winchester。
  The　sides　sloped　gently　at　first；　then　became　inclined　at　about　20
  degrees；　then　more　gently　to　near　the　bottom；　which　transversely
  was　almost　level　and　about　50　yards　across。　　In　the　bottom；　the
  mean　thickness　of　the　mould　from　five　measurements　was　8。3　inches；
  whilst　on　the　sides　of　the　valley；　where　the　inclination　varied
  between　14　degrees　and　20　degrees；　its　mean　thickness　was　rather
  less　than　3。5　inches。　　As　the　turf…covered　bottom　of　the　valley
  sloped　at　an　angle　of　only　between　2　degrees　and　3　degrees；　it　is
  probable　that　most　of　the　8。3…inch　layer　of　mould　had　been　washed
  down　from　the　flanks　of　the　valley；　and　not　from　the　upper　part。
  But　as　a　shepherd　said　that　he　had　seen　water　flowing　in　this
  valley　after　the　sudden　thawing　of　snow；　it　is　possible　that　some
  earth　may　have　been　brought　down　from　the　upper　part；　or；　on　the
  other　hand；　that　some　may　have　been　carried　further　down　the
  valley。　　Closely　similar　results；　with　respect　to　the　thickness　of
  the　mould；　were　obtained　in　a　neighbouring　valley。
  St。　Catherine's　Hill；　near　Winchester；　is　327　feet　in　height；　and
  consists　of　a　steep　cone　of　chalk　about　0。25　of　a　mile　in　diameter。
  The　upper　part　was　converted　by　the　Romans；　or；　as　some　think；　by
  the　ancient　Britons；　into　an　encampment；　by　the　excavation　of　a
  deep　and　broad　ditch　all　round　it。　　Most　of　the　chalk　removed
  during　the　work　was　thrown　upwards；　by　which　a　projecting　bank　was
  formed；　and　this　effectually　prevents　worm…castings　（which　are
  numerous　in　parts）；　stones；　and　other　objects　from　being　washed　or
  rolled　into　the　ditch。　　The　mould　on　the　upper　and　fortified　part
  of　the　hill　was　found　to　be　in　most　places　only　from　2。5　to　3。5
  inches　in　thickness；　whereas　it　had　accumulated　at　the　foot　of　the
  embankment　above　the　ditch　to　a　thickness　in　most　places　of　from　8
  to　9。5　inches。　　On　the　embankment　itself　the　mould　was　only　1　to
  1。5　inch　in　thickness；　and　within　the　ditch　at　the　bottom　it　varied
  from　2。5　to　3。5；　but　was　in　one　spot　6　inches　in　thickness。　　On　the
  north…west　side　of　the　hill；　either　no　embankment　had　ever　been
  thrown　up　above　the　ditch；　or　it　had　subsequently　been　removed；　so
  that　here　there　was　nothing　to　prevent　worm…castings；　earth　and
  stones　being　washed　into　the　ditch；　at　the　bottom　of　which　the
  mould　formed　a　layer　from　11　to　22　inches　in　thickness。　　It　should
  however　be　stated　that　here　and　on　other　parts　of　the　slope；　the
  bed　of　mould　often　contained　fragments　of　chalk　and　flint　which　had
  obviously　rolled　down　at　different　times　from　above。　　The
  interstices　in　the　underlying　fragmentary　chalk　were　also　filled　up
  with　mould。
  My　son　examined　the　surface　of　this　hill　to　its　base　in　a　south…
  west　direction。　　Beneath　the　great　ditch；　where　the　slope　was　about
  24　degrees；　the　mould　was　very　thin；　namely；　from　1。5　to　2。5
  inches；　whilst　near　the　base；　where　the　slope　was　only　3　degrees　to
  4　degrees；　it　increased　to　between　8　and　9　inches　in　thickness。　　We
  may　therefore　conclude　that　on　this　artificially　modified　hill；　as
  well　as　in　the　natural　valleys　of　the　neighbouring　Chalk　Downs；
  some　fine　earth；　probably　derived　in　large　part　from　worm…castings；
  is　washed　down；　and　accumulates　in　the　lower　parts；　notwithstanding
  the　percolation　of　an　unknown　quantity　into　the　underlying　chalk；　a
  supply　of　fresh　earthy　matter　being　afforded　by　the　dissolution　of
  the　chalk　through　atmospheric　and　other　agencies。
  CHAPTER　VIICONCLUSION。
  Summary　of　the　part　which　worms　have　played　in　the　history　of　the
  worldTheir　aid　in　the　disintegration　of　rocksIn　the　denudation
  of　the　landIn　the　preservation　of　ancient　remainsIn　the
  preparation　of　the　soil　for　the　growth　of　plantsMental　powers　of
  wormsConclusion。
  Worms　have　played　a　more　important　part　in　the　history　of　the　world
  than　most　persons　would　at　first　suppose。　　In　almost　all　humid
  countries　they　are　extraordinarily　numerous；　and　for　their　size
  possess　great　muscular　power。　　In　many　parts　of　England　a　weight　of
  more　than　ten　tons　（10；516　kilogrammes）　of　dry　earth　annually
  passes　through　their　bodies　and　is　brought　to　the　surface　on　each
  acre　of　land；　so　that　the　whole　superficial　bed　of　vegetable　mould
  passes　through　their　bodies　in　the　course　of　every　few　years。　　From
  the　collapsing　of　the　old　burrows　the　mould　is　in　constant　though
  slow　movement；　and　the　particles　composing　it　are　thus　rubbed
  together。　　By　these　means　fresh　surfaces　are　continually　exposed　to
  the　action　of　the　carbonic　acid　in　the　soil；　and　of　the　humus…acids
  which　appear　to　be　still　more　efficient　in　the　decomposition　of
  rocks。　　The　generation　of　the　humus…acids　is　probably　hastened
  during　the　digestion　of　the　many　half…decayed　leaves　which　worms
  consume。　　Thus　the　particles　of　earth；　forming　the　superficial
  mould；　are　subjected　to　conditions　eminently　favourable　for　their
  decomposition　and　disintegration。　　Moreover；　the　particles　of　the
  softer　rocks　suffer　some　amount　of　mechanical　trituration　in　the
  muscular　gizzards　of　worms；　in　which　small　stones　serve　as　mill…
  stones。
  The　finely　levigated　castings；　when　brought　to　the　surface　in　a
  moist　condition；　flow　during　rainy　weather　down　any　moderate　slope；
  and　the　smaller　particles　are　washed　far　down　even　a　gently
  inclined　surface。　　Castings　when　dry　often　crumble　into　small
  pellets　and　these　are　apt　to　roll　down　any　sloping　surface。　　Where
  the　land　is　quite　level　and　is　covered　with　herbage；　and　where　the
  climate　is　humid　so　that　much　dust　cannot　be　blown　away；　it　appears
  at　first　sight　impossible　that　there　should　be　any　appreciable
  amount　of　sub…aerial　denudation；　but　worm…castings　are　blown；
  especially　whilst　moist　and　viscid；　in　one　uniform　direction　by　the
  prevalent　winds　which　are　accompanied　by　rain。　　By　these　several
  means　the　superficial　mould　is　prevented　from　accumulating　to　a
  great　thickness；　and　a　thick　bed　of　mould　checks　in　many　ways　the
  disintegration　of　the　underlying　rocks　an