destruction　of　the　rocks。〃　｛80｝
  Ancient　encampments　and　tumuli。E。　de　Beaumont　adduces　the　present
  state　of　many　ancient　encampments　and　tumuli　and　of　old　ploughed
  fields；　as　evidence　that　the　surface　of　the　land　undergoes　hardly
  any　degradation。　　But　it　does　not　appear　that　he　ever　examined　the
  thickness　of　the　mould　over　different　parts　of　such　old　remains。
  He　relies　chiefly　on　indirect；　but　apparently　trustworthy；　evidence
  that　the　slopes　of　the　old　embankments　are　the　same　as　they
  originally　were；　and　it　is　obvious　that　he　could　know　nothing　about
  their　original　heights。　　In　Knole　Park　a　mound　had　been　thrown　up
  behind　the　rifle…targets；　which　appeared　to　have　been　formed　of
  earth　originally　supported　by　square　blocks　of　turf。　　The　sides
  sloped；　as　nearly　as　I　could　estimate　them；　at　an　angle　of　45
  degrees　or　50　degrees　with　the　horizon；　and　they　were　covered；
  especially　on　the　northern　side；　with　long　coarse　grass；　beneath
  which　many　worm…castings　were　found。　　These　had　flowed　bodily
  downwards；　and　others　had　rolled　down　as　pellets。　　Hence　it　is
  certain　that　as　long　as　a　mound　of　this　kind　is　tenanted　by　worms；
  its　height　will　be　continually　lowered。　　The　fine　earth　which　flows
  or　rolls　down　the　sides　of　such　a　mound　accumulates　at　its　base　in
  the　form　of　a　talus。　　A　bed；　even　a　very　thin　bed；　of　fine　earth　is
  eminently　favourable　for　worms；　so　that　a　greater　number　of
  castings　would　tend　to　be　ejected　on　a　talus　thus　formed　than
  elsewhere；　and　these　would　be　partially　washed　away　by　every　heavy
  shower　and　be　spread　over　the　adjoining　level　ground。　　The　final
  result　would　be　the　lowering　of　the　whole　mound；　whilst　the
  inclination　of　the　sides　would　not　be　greatly　lessened。　　The　same
  result　would　assuredly　follow　with　ancient　embankments　and　tumuli；
  except　where　they　had　been　formed　of　gravel　or　of　nearly　pure　sand；
  as　such　matter　is　unfavourable　for　worms。　　Many　old　fortifications
  and　tumuli　are　believed　to　be　at　least　2000　years　old；　and　we
  should　bear　in　mind　that　in　many　places　about　one　inch　of　mould　is
  brought　to　the　surface　in　5　years　or　two　inches　in　10　years。
  Therefore　in　so　long　a　period　as　2000　years；　a　large　amount　of
  earth　will　have　been　repeatedly　brought　to　the　surface　on　most　old
  embankments　and　tumuli；　especially　on　the　talus　round　their　bases；
  and　much　of　this　earth　will　have　been　washed　completely　away。　　We
  may　therefore　conclude　that　all　ancient　mounds；　when　not　formed　of
  materials　unfavourable　to　worms；　will　have　been　somewhat　lowered　in
  the　course　of　centuries；　although　their　inclinations　may　not　have
  been　greatly　changed。
  Fields　formerly　ploughed。From　a　very　remote　period　and　in　many
  countries；　land　has　been　ploughed；　so　that　convex　beds；　called
  crowns　or　ridges；　usually　about　8　feet　across　and　separated　by
  furrows；　have　been　thrown　up。　　The　furrows　are　directed　so　as　to
  carry　off　the　surface　water。　　In　my　attempts　to　ascertain　how　long
  a　time　these　crowns　and　furrows　last；　when　ploughed　land　has　been
  converted　into　pasture；　obstacles　of　many　kinds　were　encountered。
  It　is　rarely　known　when　a　field　was　last　ploughed；　and　some　fields
  which　were　thought　to　have　been　in　pasture　from　time　immemorial
  were　afterwards　discovered　to　have　been　ploughed　only　50　or　60
  years　before。　　During　the　early　part　of　the　present　century；　when
  the　price　of　corn　was　very　high；　land　of　all　kinds　seems　to　have
  been　ploughed　in　Britain。　　There　is；　however；　no　reason　to　doubt
  that　in　many　cases　the　old　crowns　and　furrows　have　been　preserved
  from　a　very　ancient　period。　｛81｝　　That　they　should　have　been
  preserved　for　very　unequal　lengths　of　time　would　naturally　follow
  from　the　crowns；　when　first　thrown　up；　having　differed　much　in
  height　in　different　districts；　as　is　now　the　case　with　recently
  ploughed　land。
  In　old　pasture　fields；　the　mould；　wherever　measurements　were　made；
  was　found　to　be　from　0。5　to　2　inches　thicker　in　the　furrows　than　on
  the　crowns；　but　this　would　naturally　follow　from　the　finer　earth
  having　been　washed　from　the　crowns　into　the　furrows　before　the　land
  was　well　clothed　with　turf；　and　it　is　impossible　to　tell　what　part
  worms　may　have　played　in　the　work。　　Nevertheless　from　what　we　have
  seen；　castings　would　certainly　tend　to　flow　and　to　be　washed　during
  heavy　rain　from　the　crowns　into　the　furrows。　　But　as　soon　as　a　bed
  of　fine　earth　had　by　any　means　been　accumulated　in　the　furrows；　it
  would　be　more　favourable　for　worms　than　the　other　parts；　and　a
  greater　number　of　castings　would　be　thrown　up　here　than　elsewhere；
  and　as　the　furrows　on　sloping　land　are　usually　directed　so　as　to
  carry　off　the　surface　water；　some　of　the　finest　earth　would　be
  washed　from　the　castings　which　had　been　here　ejected　and　be　carried
  completely　away。　　The　result　would　be　that　the　furrows　would　be
  filled　up　very　slowly；　while　the　crowns　would　be　lowered　perhaps
  still　more　slowly　by　the　flowing　and　rolling　of　the　castings　down
  their　gentle　inclinations　into　the　furrows。
  Nevertheless　it　might　be　expected　that　old　furrows；　especially
  those　on　a　sloping　surface；　would　in　the　course　of　time　be　filled
  up　and　disappear。　　Some　careful　observers；　however；　who　examined
  fields　for　me　in　Gloucestershire　and　Staffordshire　could　not　detect
  any　difference　in　the　state　of　the　furrows　in　the　upper　and　lower
  parts　of　sloping　fields；　supposed　to　have　been　long　in　pasture；　and
  they　came　to　the　conclusion　that　the　crowns　and　furrows　would　last
  for　an　almost　endless　number　of　centuries。　　On　the　other　hand　the
  process　of　obliteration　seems　to　have　commenced　in　some　places。
  Thus　in　a　grass　field　in　North　Wales；　known　to　have　been　ploughed
  about　65　years　ago；　which　sloped　at　an　angle　of　15　degrees　to　the
  north…east；　the　depth　of　the　furrows　（only　7　feet　apart）　was
  carefully　measured；　and　was　found　to　be　about　4。5　inches　in　the
  upper　part　of　the　slope；　and　only　1　inch　near　the　base；　where　they
  could　be　traced　with　difficulty。　　On　another　field　sloping　at　about
  the　same　angle　to　the　south…west；　the　furrows　were　scarcely
  perceptible　in　the　lower　part；　although　these　same　furrows　when
  followed　on　to　some　adjoining　level　ground　were　from　2。5　to　3。5
  inches　in　depth。　　A　third　and　closely　similar　case　was　observed。
  In　a　fourth　case；　the　mould　in　a　furrow　in　the　upper　part　of　a
  sloping　field　was　2。5　inches；　and　in　the　lower　part　4。5　inches　in
  thickness。
  On　the　Chalk　Downs　at　about　a　mile　distance　from　Stonehenge；　my　son
  William　examined　a　grass…covered；　furrowed　surface；　sloping　at　from
  8　degrees　to　10　degrees；　which　an　old　shepherd　said　had　not　been
  ploughed　within　the　memory　of　man。　　The　depth　of　one　furrow　was
  measured　at　16　points　in　a　length　of　68　paces；　and　was　found　to　be
  deeper　where　the　slope　was　greatest　and　where　less　earth　would
  naturally　tend　to　accumulate；　and　at　the　base　it　almost
  disappeared。　　The　thickness　of　the　mould　in　this　furrow　in　the
  upper　part　was　2。5　inches；　which　increased　to　5　inches；　a　little
  above　the　steepest　part　of　the　slope；　and　at　the　base；　in　the
  middle　of　the　narrow　valley；　at　a　point　which　the　furrow　if
  continued　would　have　struck；　it　amounted　to　7　inches。　　On　the
  opposite　side　of　the　valley；　there　were　very　faint；　almost
  obliterated；　traces　of　furrows。　　Another　analogous　but　not　so
  decided　a　case　was　observed　at　a　few　miles'　distance　from
  Stonehenge。　　On　the　whole　it　appears　that　the　crowns　and　furrows　on
  land　formerly　ploughed；　but　now　covered　with　grass；　tend　slowly　to
  disappear　when　the　surface　is　inclined；　and　this　is　probably　in
  large　part　due　to　the　action　of　worms；　but　that　the　crowns　and
  furrows　last　for　a　very　long　time　when　the　surface　is　nearly　level。
  Formation　and　amount　of　mould　over　the　Chalk　Formation。Worm…
  castings　are　often　ejected　in　extraordinary　numbers　on　steep；
  grass…covered　slopes；　where　the　Chalk　comes　close　to　the　surface；
  as　my　son　William　observed　near　Winchester　and　elsewhere。　　If　such
  castings　are　largely　washed　away　during　heavy　rains；　it　is
  difficult　to　understand　at　first　how　any　mould　can　still　remain　on
  our　Downs；　as　there　does　not　appear　any　evident　means　for　supplying
  the　loss。　　There　is；　moreover；　another　cause　of　loss；　namely；　in
  the　percolation　of　the　finer　particles　of　earth　into　the　fissures
  in　the　chalk　and　into　the　chalk　itself。　　These　considerations　led
  me　to　doubt　for　a　time　whether　I　had　not　exaggerated　the　amount　of
  fine　earth　which　flows　or　rolls　down　grass…covered　slopes　under　the
  form　of　castings；　and　I　sought　for　additional　information。　　In　some
  places；　the　castings　on　Chalk　Downs　consist　largely　of　calcareous
  matter；　and　here　the　supply　is　of　course　unlimited。　　But　in　other
  places；　for　instance　on　a　part　of　Teg　Down　near　Winchester；　the
  castings　were　all　black　and　did　not　effervesce　with　acids。　　The
  mould　over　the　chalk　was　here　only　from　3　to　4　inches　in　thickness。
  So　again　on　the　plain　near　Stonehenge；　the　mould；　apparently　free
  from　calcareous　matter；　averaged　rather　less　than　3。5　inches　in
  thickness。　　Why　worms　should　penetrate　and　bring　up　chalk　in　some
  places　and　not　in　others　I　do　not　know。
  In　many　distric