rejettent；〃　&c。　｛74｝
  As　the　amount　of　trituration　which　the　particles　of　earth　undergo
  in　the　gizzards　of　worms　possesses　some　interest　（as　we　shall
  hereafter　see）；　I　endeavoured　to　obtain　evidence　on　this　head　by
  carefully　examining　many　of　the　fragments　which　had　passed　through
  their　alimentary　canals。　　With　worms　living　in　a　state　of　nature；
  it　is　of　course　impossible　to　know　how　much　the　fragments　may　have
  been　worn　before　they　were　swallowed。　　It　is；　however；　clear　that
  worms　do　not　habitually　select　already　rounded　particles；　for
  sharply　angular　bits　of　flint　and　of　other　hard　rocks　were　often
  found　in　their　gizzards　or　intestines。　　On　three　occasions　sharp
  spines　from　the　stems　of　rose…bushes　were　thus　found。　　Worms　kept
  in　confinement　repeatedly　swallowed　angular　fragments　of　hard　tile；
  coal；　cinders；　and　even　the　sharpest　fragments　of　glass。
  Gallinaceous　and　struthious　birds　retain　the　same　stones　in　their
  gizzards　for　a　long　time；　which　thus　become　well　rounded；　but　this
  does　not　appear　to　be　the　case　with　worms；　judging　from　the　large
  number　of　the　fragments　of　tiles；　glass　beads；　stones；　&c。；
  commonly　found　in　their　castings　and　intestines。　　So　that　unless
  the　same　fragments　were　to　pass　repeatedly　through　their　gizzards；
  visible　signs　of　attrition　in　the　fragments　could　hardly　be
  expected；　except　perhaps　in　the　case　of　very　soft　stones。
  I　will　now　give　such　evidence　of　attrition　as　I　have　been　able　to
  collect。　　In　the　gizzards　of　some　worms　dug　out　of　a　thin　bed　of
  mould　over　the　chalk；　there　were　many　well…rounded　small　fragments
  of　chalk；　and　two　fragments　of　the　shells　of　a　land…mollusc　（as
  ascertained　by　their　microscopical　structure）；　which　latter　were
  not　only　rounded　but　somewhat　polished。　　The　calcareous　concretions
  formed　in　the　calciferous　glands；　which　are　often　found　in　their
  gizzards；　intestines；　and　occasionally　in　their　castings；　when　of
  large　size；　sometimes　appeared　to　have　been　rounded；　but　with　all
  calcareous　bodies　the　rounded　appearance　may　be　partly　or　wholly
  due　to　their　corrosion　by　carbonic　acid　and　the　humus…acids。　　In
  the　gizzards　of　several　worms　collected　in　my　kitchen　garden　near　a
  hothouse；　eight　little　fragments　of　cinders　were　found；　and　of
  these；　six　appeared　more　or　less　rounded；　as　were　two　bits　of
  brick；　but　some　other　bits　were　not　at　all　rounded。　　A　farm…road
  near　Abinger　Hall　had　been　covered　seven　years　before　with　brick…
  rubbish　to　the　depth　of　about　6　inches；　turf　had　grown　over　this
  rubbish　on　both　sides　of　the　road　for　a　width　of　18　inches；　and　on
  this　turf　there　were　innumerable　castings。　　Some　of　them　were
  coloured　of　a　uniform　red　owing　to　the　presence　of　much　brick…dust；
  and　they　contained　many　particles　of　brick　and　of　hard　mortar　from
  1　to　3　mm。　in　diameter；　most　of　which　were　plainly　rounded；　but　all
  these　particles　may　have　been　rounded　before　they　were　protected　by
  the　turf　and　were　swallowed；　like　those　on　the　bare　parts　of　the
  road　which　were　much　worn。　　A　hole　in　a　pasture…field　had　been
  filled　up　with　brick…rubbish　at　the　same　time；　viz。；　seven　years
  ago；　and　was　now　covered　with　turf；　and　here　the　castings　contained
  very　many　particles　of　brick；　all　more　or　less　rounded；　and　this
  brick…rubbish；　after　being　shot　into　the　hole；　could　not　have
  undergone　any　attrition。　　Again；　old　bricks　very　little　broken；
  together　with　fragments　of　mortar；　were　laid　down　to　form　walks；
  and　were　then　covered　with　from　4　to　6　inches　of　gravel；　six　little
  fragments　of　brick　were　extracted　from　castings　collected　on　these
  walks；　three　of　which　were　plainly　worn。　　There　were　also　very　many
  particles　of　hard　mortar；　about　half　of　which　were　well　rounded；
  and　it　is　not　credible　that　these　could　have　suffered　so　much
  corrosion　from　the　action　of　carbonic　acid　in　the　course　of　only
  seven　years。
  Much　better　evidence　of　the　attrition　of　hard　objects　in　the
  gizzards　of　worms；　is　afforded　by　the　state　of　the　small　fragments
  of　tiles　or　bricks；　and　of　concrete　in　the　castings　thrown　up　where
  ancient　buildings　once　stood。　　As　all　the　mould　covering　a　field
  passes　every　few　years　through　the　bodies　of　worms；　the　same　small
  fragments　will　probably　be　swallowed　and　brought　to　the　surface
  many　times　in　the　course　of　centuries。　　It　should　be　premised　that
  in　the　several　following　cases；　the　finer　matter　was　first　washed
  away　from　the　castings；　and　then　all　the　particles　of　bricks；　tiles
  and　concrete　were　collected　without　any　selection；　and　were
  afterwards　examined。　　Now　in　the　castings　ejected　between　the
  tesserae　on　one　of　the　buried　floors　of　the　Roman　villa　at　Abinger；
  there　were　many　particles　（from　to　2　mm。　in　diameter）　of　tiles　and
  concrete；　which　it　was　impossible　to　look　at　with　the　naked　eye　or
  through　a　strong　lens；　and　doubt　for　a　moment　that　they　had　almost
  all　undergone　much　attrition。　　I　speak　thus　after　having　examined
  small　water…worn　pebbles；　formed　from　Roman　bricks；　which　M。　Henri
  de　Saussure　had　the　kindness　to　send　me；　and　which　he　had　extracted
  from　sand　and　gravel　beds；　deposited　on　the　shores　of　the　Lake　of
  Geneva；　at　a　former　period　when　the　water　stood　at　about　two　metres
  above　its　present　level。　　The　smallest　of　these　water…worn　pebbles
  of　brick　from　Geneva　resembled　closely　many　of　those　extracted　from
  the　gizzards　of　worms；　but　the　larger　ones　were　somewhat　smoother。
  Four　castings　found　on　the　recently　uncovered；　tesselated　floor　of
  the　great　room　in　the　Roman　villa　at　Brading；　contained　many
  particles　of　tile　or　brick；　of　mortar；　and　of　hard　white　cement；
  and　the　majority　of　these　appeared　plainly　worn。　　The　particles　of
  mortar；　however；　seemed　to　have　suffered　more　corrosion　than
  attrition；　for　grains　of　silex　often　projected　from　their　surfaces。
  Castings　from　within　the　nave　of　Beaulieu　Abbey；　which　was
  destroyed　by　Henry　VIII。；　were　collected　from　a　level　expanse　of
  turf；　overlying　the　buried　tesselated　pavement；　through　which　worm…
  burrows　passed；　and　these　castings　contained　innumerable　particles
  of　tiles　and　bricks；　of　concrete　and　cement；　the　majority　of　which
  had　manifestly　undergone　some　or　much　attrition。　　There　were　also
  many　minute　flakes　of　a　micaceous　slate；　the　points　of　which　were
  rounded。　　If　the　above　supposition；　that　in　all　these　cases　the
  same　minute　fragments　have　passed　several　times　through　the
  gizzards　of　worms；　be　rejected；　notwithstanding　its　inherent
  probability；　we　must　then　assume　that　in　all　the　above　cases　the
  many　rounded　fragments　found　in　the　castings　had　all　accidentally
  undergone　much　attrition　before　they　were　swallowed；　and　this　is
  highly　improbable。
  On　the　other　hand　it　must　be　stated　that　fragments　of　ornamental
  tiles；　somewhat　harder　than　common　tiles　or　bricks；　which　had　been
  swallowed　only　once　by　worms　kept　in　confinement；　were　with　the
  doubtful　exception　of　one　or　two　of　the　smallest　grains；　not　at　all
  rounded。　　Nevertheless　some　of　them　appeared　a　little　worn；　though
  not　rounded。　　Notwithstanding　these　cases；　if　we　consider　the
  evidence　above　given；　there　can　be　little　doubt　that　the　fragments；
  which　serve　as　millstones　in　the　gizzards　of　worms；　suffer；　when　of
  a　not　very　hard　texture；　some　amount　of　attrition；　and　that　the
  smaller　particles　in　the　earth；　which　is　habitually　swallowed　in
  such　astonishingly　large　quantities　by　worms；　are　ground　together
  and　are　thus　levigated。　　If　this　be　the　case；　the　〃terra
  tenuissima；〃the　〃pate　excessivement　fine；〃of　which　the　castings
  largely　consist；　is　in　part　due　to　the　mechanical　action　of　the
  gizzard；　｛75｝　and　this　fine　matter；　as　we　shall　see　in　the　next
  chapter；　is　that　which　is　chiefly　washed　away　from　the　innumerable
  castings　on　every　field　during　each　heavy　shower　of　rain。　　If　the
  softer　stones　yield　at　all；　the　harder　ones　will　suffer　some　slight
  amount　of　wear　and　tear。
  The　trituration　of　small　particles　of　stone　in　the　gizzards　of
  worms　is　of　more　importance　under　a　geological　point　of　view　than
  may　at　first　appear　to　be　the　case；　for　Mr。　Sorby　has　clearly　shown
  that　the　ordinary　means　of　disintegration；　namely；　running　water
  and　the　waves　of　the　sea；　act　with　less　and　less　power　on　fragments
  of　rock　the　smaller　they　are。　　〃Hence；〃　as　he　remarks；　〃even　making
  no　allowance　for　the　extra　buoying　up　of　very　minute　particles　by　a
  current　of　water；　depending　on　surface　cohesion；　the　effects　of
  wearing　on　the　form　of　the　grains　must　vary　directly　as　their
  diameter　or　thereabouts。　　If　so；　a　grain　of　1/10　an　inch　in
  diameter　would　be　worn　ten　times　as　much　as　one　of　an　inch　in
  diameter；　and　at　least　a　hundred　times　as　much　as　one　of　1/100　an
  inch　in　diameter。　　Perhaps；　then；　we　may　conclude　that　a　grain　1/10
  of　an　inch　in　diameter　would　be　worn　as　much　or　more　in　drifting　a
  mile　as　a　grain　1/1000　of　an　inch　in　being　drifted　100　miles。　　On
  the　same　principle　a　pebble　one　inch　in　diameter　would　be　worn
  relatively　more　by　being　drifted　only　a　few　hundred　yards。〃　｛76｝
  Nor　should　we　forget；　in　considering　the　power　which　worms　exert　in
  triturating　particles　of　rock；　that　there　is　good　evidence　that　on
  each　acr