triturating　particles　of　rock；　that　there　is　good　evidence　that　on
  each　acre　of　land；　which　is　sufficiently　damp　and　not　too　sandy；
  gravelly　or　rocky　for　worms　to　inhabit；　a　weight　of　more　than　ten
  tons　of　earth　annually　passes　through　their　bodies　and　is　brought
  to　the　surface。　　The　result　for　a　country　of　the　size　of　Great
  Britain；　within　a　period　not　very　long　in　a　geological　sense；　such
  as　a　million　years；　cannot　be　insignificant；　for　the　ten　tons　of
  earth　has　to　be　multiplied　first　by　the　above　number　of　years；　and
  then　by　the　number　of　acres　fully　stocked　with　worms；　and　in
  England；　together　with　Scotland；　the　land　which　is　cultivated　and
  is　well　fitted　for　these　animals；　has　been　estimated　at　above　32
  million　acres。　　The　product　is　320　million　million　tons　of　earth。
  CHAPTER　VITHE　DENUDATION　OF　THE　LANDcontinued。
  Denudation　aided　by　recently　ejected　castings　flowing　down　inclined
  grass…covered　surfacesThe　amount　of　earth　which　annually　flows
  downwardsThe　effect　of　tropical　rain　on　worm　castingsThe　finest
  particles　of　earth　washed　completely　away　from　castingsThe
  disintegration　of　dried　castings　into　pellets；　and　their　rolling
  down　inclined　surfacesThe　formation　of　little　ledges　on　hill…
  sides；　in　part　due　to　the　accumulation　of　disintegrated　castings
  Castings　blown　to　leeward　over　level　landAn　attempt　to　estimate
  the　amount　thus　blownThe　degradation　of　ancient　encampments　and
  tumuliThe　preservation　of　the　crowns　and　furrows　on　land
  anciently　ploughedThe　formation　and　amount　of　mould　over　the
  Chalk　formation。
  We　are　now　prepared　to　consider　the　more　direct　part　which　worms
  take　in　the　denudation　of　the　land。　　When　reflecting　on　sub…aerial
  denudation；　it　formerly　appeared　to　me；　as　it　has　to　others；　that　a
  nearly　level　or　very　gently　inclined　surface；　covered　with　turf；
  could　suffer　no　loss　during　even　a　long　lapse　of　time。　　It　may；
  however；　be　urged　that　at　long　intervals；　debacles　of　rain　or
  water…spouts　would　remove　all　the　mould　from　a　very　gentle　slope；
  but　when　examining　the　steep；　turf…covered　slopes　in　Glen　Roy；　I
  was　struck　with　the　fact　how　rarely　any　such　event　could　have
  happened　since　the　Glacial　period；　as　was　plain　from　the　well…
  preserved　state　of　the　three　successive　〃roads〃　or　lake…margins。
  But　the　difficulty　in　believing　that　earth　in　any　appreciable
  quantity　can　be　removed　from　a　gently　inclined　surface；　covered
  with　vegetation　and　matted　with　roots；　is　removed　through　the
  agency　of　worms。　　For　the　many　castings　which　are　thrown　up　during
  rain；　and　those　thrown　up　some　little　time　before　heavy　rain；　flow
  for　a　short　distance　down　an　inclined　surface。　　Moreover　much　of
  the　finest　levigated　earth　is　washed　completely　away　from　the
  castings。　　During　dry　weather　castings　often　disintegrate　into
  small　rounded　pellets；　and　these　from　their　weight　often　roll　down
  any　slope。　　This　is　more　especially　apt　to　occur　when　they　are
  started　by　the　wind；　and　probably　when　started　by　the　touch　of　an
  animal；　however　small。　　We　shall　also　see　that　a　strong　wind　blows
  all　the　castings；　even　on　a　level　field；　to　leeward；　whilst　they
  are　soft；　and　in　like　manner　the　pellets　when　they　are　dry。　　If　the
  wind　blows　in　nearly　the　direction　of　an　inclined　surface；　the
  flowing　down　of　the　castings　is　much　aided。
  The　observations　on　which　these　several　statements　are　founded　must
  now　be　given　in　some　detail。　　Castings　when　first　ejected　are
  viscid　and　soft；　during　rain；　at　which　time　worms　apparently　prefer
  to　eject　them；　they　are　still　softer；　so　that　I　have　sometimes
  thought　that　worms　must　swallow　much　water　at　such　times。　　However
  this　may　be；　rain；　even　when　not　very　heavy；　if　long　continued；
  renders　recently…ejected　castings　semi…fluid；　and　on　level　ground
  they　then　spread　out　into　thin；　circular；　flat　discs；　exactly　as
  would　so　much　honey　or　very　soft　mortar；　with　all　traces　of　their
  vermiform　structure　lost。　　This　latter　fact　was　sometimes　made
  evident；　when　a　worm　had　subsequently　bored　through　a　flat　circular
  disc　of　this　kind；　and　heaped　up　a　fresh　vermiform　mass　in　the
  centre。　　These　flat　subsided　discs　have　been　repeatedly　seen　by　me
  after　heavy　rain；　in　many　places　on　land　of　all　kinds。
  On　the　flowing　of　wet　castings；　and　the　rolling　of　dry
  disintegrated　castings　down　inclined　surfaces。When　castings　are
  ejected　on　an　inclined　surface　during　or　shortly　before　heavy　rain；
  they　cannot　fail　to　flow　a　little　down　the　slope。　　Thus；　on　some
  steep　slopes　in　Knole　Park；　which　were　covered　with　coarse　grass
  and　had　apparently　existed　in　this　state　from　time　immemorial；　I
  found　（Oct。　22；　1872）　after　several　wet　days　that　almost　all　the
  many　castings　were　considerably　elongated　in　the　line　of　the　slope；
  and　that　they　now　consisted　of　smooth；　only　slightly　conical
  masses。　　Whenever　the　mouths　of　the　burrows　could　be　found　from
  which　the　earth　had　been　ejected；　there　was　more　earth　below　than
  above　them。　　After　some　heavy　storms　of　rain　（Jan。　25；　1872）　two
  rather　steeply　inclined　fields　near　Down；　which　had　formerly　been
  ploughed　and　were　now　rather　sparsely　clothed　with　poor　grass；　were
  visited；　and　many　castings　extended　down　the　slopes　for　a　length　of
  5　inches；　which　was　twice　or　thrice　the　usual　diameter　of　the
  castings　thrown　up　on　the　level　parts　of　these　same　fields。　　On
  some　fine　grassy　slopes　in　Holwood　Park；　inclined　at　angles　between
  8　degrees　and　11　degrees　30　seconds　with　the　horizon；　where　the
  surface　apparently　had　never　been　disturbed　by　the　hand　of　man；
  castings　abounded　in　extraordinary　numbers：　　and　a　space　16　inches
  in　length　transversely　to　the　slope　and　6　inches　in　the　line　of　the
  slope；　was　completely　coated；　between　the　blades　of　grass；　with　a
  uniform　sheet　of　confluent　and　subsided　castings。　　Here　also　in
  many　places　the　castings　had　flowed　down　the　slope；　and　now　formed
  smooth　narrow　patches　of　earth；　6；　7；　and　7。5　inches　in　length。
  Some　of　these　consisted　of　two　castings；　one　above　the　other；　which
  had　become　so　completely　confluent　that　they　could　hardly　be
  distinguished。　　On　my　lawn；　clothed　with　very　fine　grass；　most　of
  the　castings　are　black；　but　some　are　yellowish　from　earth　having
  been　brought　up　from　a　greater　depth　than　usual；　and　the　flowing…
  down　of　these　yellow　castings　after　heavy　rain；　could　be　clearly
  seen　where　the　slope　was　5　degrees；　and　where　it　was　less　than　1
  degree　some　evidence　of　their　flowing　down　could　still　be　detected。
  On　another　occasion；　after　rain　which　was　never　heavy；　but　which
  lasted　for　18　hours；　all　the　castings　on　this　same　gently　inclined
  lawn　had　lost　their　vermiform　structure；　and　they　had　flowed；　so
  that　fully　two…thirds　of　the　ejected　earth　lay　below　the　mouths　of
  the　burrows。
  These　observations　led　me　to　make　others　with　more　care。　　Eight
  castings　were　found　on　my　lawn；　where　the　grass…blades　are　fine　and
  close　together；　and　three　others　on　a　field　with　coarse　grass。　　The
  inclination　of　the　surface　at　the　eleven　places　where　these
  castings　were　collected　varied　between　4　degrees　30　seconds　and　17
  degrees　30　seconds；　the　mean　of　the　eleven　inclinations　being　9
  degrees　26　seconds。　　The　length　of　the　castings　in　the　direction　of
  the　slope　was　first　measured　with　as　much　accuracy　as　their
  irregularities　would　permit。　　It　was　found　possible　to　make　these
  measurements　within　about　of　an　inch；　but　one　of　the　castings　was
  too　irregular　to　admit　of　measurement。　　The　average　length　in　the
  direction　of　the　slope　of　the　remaining　ten　castings　was　2。03
  inches。　　The　castings　were　then　divided　with　a　knife　into　two　parts
  along　a　horizontal　line　passing　through　the　mouth　of　the　burrow；
  which　was　discovered　by　slicing　off　the　turf；　and　all　the　ejected
  earth　was　separately　collected；　namely；　the　part　above　the　hole　and
  the　part　below。　　Afterwards　these　two　parts　were　weighed。　　In　every
  case　there　was　much　more　earth　below　than　above；　the　mean　weight　of
  that　above　being　103　grains；　and　of　that　below　205　grains；　so　that
  the　latter　was　very　nearly　double　the　former。　　As　on　level　ground
  castings　are　commonly　thrown　up　almost　equally　round　the　mouths　of
  the　burrows；　this　difference　in　weight　indicates　the　amount　of
  ejected　earth　which　had　flowed　down　the　slope。　　But　very　many　more
  observations　would　be　requisite　to　arrive　at　any　general　result；
  for　the　nature　of　the　vegetation　and　other　accidental
  circumstances；　such　as　the　heaviness　of　the　rain；　the　direction　and
  force　of　the　wind；　&c。；　appear　to　be　more　important　in　determining
  the　quantity　of　the　earth　which　flows　down　a　slope　than　its　angle。
  Thus　with　four　castings　on　my　lawn　（included　in　the　above　eleven）
  where　the　mean　slope　was　7　degrees　19　seconds；　the　difference　in
  the　amount　of　earth　above　and　below　the　burrows　was　greater　than
  with　three　other　castings　on　the　same　lawn　where　the　mean　slope　was
  12　degrees　5　seconds。
  We　may；　however；　take　the　above　eleven　cases；　which　are　accurate　as
  far　as　they　go；　and　calculate　the　weight　of　the　ejected　earth　which
  annually　flows　down　a　slope　having　a　mean　inclination　of　9　degrees
  26　seconds。　　This　was　done　by　my