that　which　was　collected。　　The　last　collection　was　made　on　October
  27th；　1871；　i。e。；　367　days　after　the　square　had　been　marked　out　and
  the　surface　cleared　of　all　pre…existing　castings。　　The　collected
  castings；　after　being　well　dried；　weighed　7。453　pounds；　and　this
  would　give；　for　an　acre　of　the　same　kind　of　land；　16。1　tons　of
  annually　ejected　dry　earth。
  SUMMARY　OF　THE　FOUR　FOREGOING　CASES。
  （1。）　　Castings　ejected　near　Nice　within　about　a　year；　collected　by
  Dr。　King　on　a　square　foot　of　surface；　calculated　to　yield　per　acre
  14。58　tons。
  （2。）　　Castings　ejected　during　about　40　days　on　a　square　yard；　in　a
  field　of　poor　pasture　at　the　bottom　of　a　large　valley　in　the　Chalk；
  calculated　to　yield　annually　per　acre　18。12　tons。
  （3。）　　Castings　collected　from　a　square　yard　on　an　old　terrace　at
  Leith　Hill　Place；　during　369　days；　calculated　to　yield　annually　per
  acre　7。56　tons。
  （4。）　　Castings　collected　from　a　square　yard　on　Leith　Hill　Common
  during　367　days；　calculated　to　yield　annually　per　acre　16。1　tons。
  The　thickness　of　the　layer　of　mould；　which　castings　ejected　during
  a　year　would　form　if　uniformly　spread　out。As　we　know；　from　the
  two　last　cases　in　the　above　summary；　the　weight　of　the　dried
  castings　ejected　by　worms　during　a　year　on　a　square　yard　of
  surface；　I　wished　to　learn　how　thick　a　layer　of　ordinary　mould　this
  amount　would　form　if　spread　uniformly　over　a　square　yard。　　The　dry
  castings　were　therefore　broken　into　small　particles；　and　whilst
  being　placed　in　a　measure　were　well　shaken　and　pressed　down。　　Those
  collected　on　the　Terrace　amounted　to　124。77　cubic　inches；　and　this
  amount；　if　spread　out　over　a　square　yard；　would　make　a　layer　0。9627
  inch　in　thickness。　　Those　collected　on　the　Common　amounted　to
  197。56　cubic　inches；　and　would　make　a　similar　layer　0。1524　inch　in
  thickness；
  These　thicknesses　must；　however；　be　corrected；　for　the　triturated
  castings；　after　being　well　shaken　down　and　pressed；　did　not　make
  nearly　so　compact　a　mass　as　vegetable　mould；　though　each　separate
  particle　was　very　compact。　　Yet　mould　is　far　from　being　compact；　as
  is　shown　by　the　number　of　air…bubbles　which　rise　up　when　the
  surface　is　flooded　with　water。　　It　is　moreover　penetrated　by　many
  fine　roots。　　To　ascertain　approximately　by　how　much　ordinary
  vegetable　mould　would　be　increased　in　bulk　by　being　broken　up　into
  small　particles　and　then　dried；　a　thin　oblong　block　of　somewhat
  argillaceous　mould　（with　the　turf　pared　off）　was　measured　before
  being　broken　up；　was　well　dried　and　again　measured。　　The　drying
  caused　it　to　shrink　by　1/7　of　its　original　bulk；　judging　from
  exterior　measurements　alone。　　It　was　then　triturated　and　partly
  reduced　to　powder；　in　the　same　manner　as　the　castings　had　been
  treated；　and　its　bulk　now　exceeded　（notwithstanding　shrinkage　from
  drying）　by　1/16　that　of　the　original　block　of　damp　mould。
  Therefore　the　above　calculated　thickness　of　the　layer；　formed　by
  the　castings　from　the　Terrace；　after　being　damped　and　spread　over　a
  square　yard；　would　have　to　be　reduced　by　1/16；　and　this　will　reduce
  the　layer　to　0。09　of　an　inch；　so　that　a　layer　0。9　inch　in　thickness
  would　be　formed　in　the　course　of　ten　years。　　On　the　same　principle
  the　castings　from　the　Common　would　make　in　the　course　of　a　single
  year　a　layer　0。1429　inch；　or　in　the　course　of　10　years　1。429　inch；
  in　thickness。　　We　may　say　in　round　numbers　that　the　thickness　in
  the　former　case　would　amount　to　nearly　1　inch；　and　in　the　second
  case　to　nearly　1。5　inch　in　10　years。
  In　order　to　compare　these　results　with　those　deduced　from　the　rates
  at　which　small　objects　left　on　the　surfaces　of　grass…fields　become
  buried　（as　described　in　the　early　part　of　this　chapter）；　we　will
  give　the　following　summary：…
  SUMMARY　OF　THE　THICKNESS　OF　THE　MOULD　ACCUMULATED　OVER　OBJECTS　LEFT
  STREWED　ON　THE　SURFACE；　IN　THE　COURSE　OF　TEN　YEARS。
  The　accumulation　of　mould　during　14。75　years　on　the　surface　of　a
  dry；　sandy；　grass…field　near　Maer　Hall；　amounted　to　2。2　inches　in
  10　years。
  The　accumulation　during　21。5　years　on　a　swampy　field　near　Maer
  Hall；　amounted　to　nearly　1。9　inch　in　10　years。
  The　accumulation　during　7　years　on　a　very　swampy　field　near　Maer
  Hall　amounted　to　2。1　inches　in　10　years。
  The　accumulation　during　29　years；　on　good；　argillaceous　pasture…
  land　over　the　Chalk　at　Down；　amounted　to　2。2　inches　in　10　years。
  The　accumulation　during　30　years　on　the　side　of　a　valley　over　the
  Chalk　at　Down；　the　soil　being　argillaceous；　very　poor；　and　only
  just　converted　into　pasture　（so　that　it　was　for　some　years
  unfavourable　for　worms）；　amounted　to　0。83　inch　in　10　years。
  In　these　cases　（excepting　the　last）　it　may　be　seen　that　the　amount
  of　earth　brought　to　the　surface　during　10　years　is　somewhat　greater
  than　that　calculated　from　the　castings　which　were　actually　weighed。
  This　excess　may　be　partly　accounted　for　by　the　loss　which　the
  weighed　castings　had　previously　undergone　through　being　washed　by
  rain；　by　the　adhesion　of　particles　to　the　blades　of　the　surrounding
  grass；　and　by　their　crumbling　when　dry。　　Nor　must　we　overlook　other
  agencies　which　in　all　ordinary　cases　add　to　the　amount　of　mould；
  and　which　would　not　be　included　in　the　castings　that　were
  collected；　namely；　the　fine　earth　brought　up　to　the　surface　by
  burrowing　larvae　and　insects；　especially　by　ants。　　The　earth
  brought　up　by　moles　generally　has　a　somewhat　different　appearance
  from　vegetable　mould；　but　after　a　time　would　not　be　distinguishable
  from　it。　　In　dry　countries；　moreover；　the　wind　plays　an　important
  part　in　carrying　dust　from　one　place　to　another；　and　even　in
  England　it　must　add　to　the　mould　on　fields　near　great　roads。　　But
  in　our　country　these　latter　several　agencies　appear　to　be　of　quite
  subordinate　importance　in　comparison　with　the　action　of　worms。
  We　have　no　means　of　judging　how　great　a　weight　of　earth　a　single
  full…sized　worm　ejects　during　a　year。　　Hensen　estimates　that　53；767
  worms　exist　in　an　acre　of　land；　but　this　is　founded　on　the　number
  found　in　gardens；　and　he　believes　that　only　about　half　as　many　live
  in　corn…fields。　　How　many　live　in　old　pasture　land　is　unknown；　but
  if　we　assume　that　half　the　above　number；　or　26；886　worms　live　on
  such　land；　then　taking　from　the　previous　summary　15　tons　as　the
  weight　of　the　castings　annually　thrown　up　on　an　acre　of　land；　each
  worm　must　annually　eject　20　ounces。　　A　full…sized　casting　at　the
  mouth　of　a　single　burrow　often　exceeds；　as　we　have　seen；　an　ounce
  in　weight；　and　it　is　probable　that　worms　eject　more　than　20　full…
  sized　castings　during　a　year。　　If　they　eject　annually　more　than　20
  ounces；　we　may　infer　that　the　worms　which　live　in　an　acre　of
  pasture　land　must　be　less　than　26；886　in　number。
  Worms　live　chiefly　in　the　superficial　mould；　which　is　usually　from
  4　or　5　to　10　and　even　12　inches　in　thickness；　and　it　is　this　mould
  which　passes　over　and　over　again　through　their　bodies　and　is
  brought　to　the　surface。　　But　worms　occasionally　burrow　into　the
  subsoil　to　a　much　greater　depth；　and　on　such　occasions　they　bring
  up　earth　from　this　greater　depth；　and　this　process　has　gone　on　for
  countless　ages。　　Therefore　the　superficial　layer　of　mould　would
  ultimately　attain；　though　at　a　slower　and　slower　rate；　a　thickness
  equal　to　the　depth　to　which　worms　ever　burrow；　were　there　not　other
  opposing　agencies　at　work　which　carry　away　to　a　lower　level　some　of
  the　finest　earth　which　is　continually　being　brought　to　the　surface
  by　worms。　　How　great　a　thickness　vegetable　mould　ever　attains；　I
  have　not　had　good　opportunities　for　observing；　but　in　the　next
  chapter；　when　we　consider　the　burial　of　ancient　buildings；　some
  facts　will　be　given　on　this　head。　　In　the　two　last　chapters　we
  shall　see　that　the　soil　is　actually　increased；　though　only　to　a
  small　degree；　through　the　agency　of　worms；　but　their　chief　work　is
  to　sift　the　finer　from　the　coarser　particles；　to　mingle　the　whole
  with　vegetable　debris；　and　to　saturate　it　with　their　intestinal
  secretions。
  Finally；　no　one　who　considers　the　facts　given　in　this　chapteron
  the　burying　of　small　objects　and　on　the　sinking　of　great　stones
  left　on　the　surfaceon　the　vast　number　of　worms　which　live　within
  a　moderate　extent　of　ground　on　the　weight　of　the　castings　ejected
  from　the　mouth　of　the　same　burrowon　the　weight　of　all　the
  castings　ejected　within　a　known　time　on　a　measured　spacewill
  hereafter；　as　I　believe；　doubt　that　worms　play　an　important　part　in
  nature。
  CHAPTER　IVTHE　PART　WHICH　WORMS　HAVE　PLAYED　IN　THE　BURIAL　OF
  ANCIENT　BUILDINGS。
  The　accumulation　of　rubbish　on　the　sites　of　great　cities
  independent　of　the　action　of　wormsThe　burial　of　a　Roman　villa　at
  AbingerThe　floors　and　walls　penetrated　by　wormsSubsidence　of　a
  modern　pavementThe　buried　pavement　at　Beaulieu　AbbeyRoman
  villas　at　Chedworth　and　BradingThe　remains　of　the　Roman　town　at
  SilchesterThe　nature　of　the　debris　by　which　the　remains　are
  coveredThe　penetration　of　the　tesselated　floors　and　walls　by
  wormsSubsidence　of　the　floorsThickness　of　the　mouldThe　old
  Roman　city　of　WroxeterThickness　of　the　mouldDepth　of　the
  foundations　of　some　of　the　Buil