depth　undisturbed　sand　was　reached　（28）
  13。　　In　another　part　of　same　field；　trench　9。5　inches　deep　stopped
  by　concrete　（9。5）
  14。　　In　another　part　of　same　field；　trench　9　inches　deep；　stopped
  by　concrete　（9）
  15。　　In　another　part　of　the　same　field；　trench　24　inches　deep；　when
  sand　was　reached　（16）
  16。　　In　another　part　of　same　field；　trench　30　inches　deep；　when
  stones　were　reached；　at　one　end　of　the　trench　mould　12　inches；　at
  the　other　end　14　inches　thick　（13）
  Small　field　between　〃Old　Works〃　and　〃Shop　Leasows；〃　I　believe
  nearly　as　high　as　the　upper　part　of　the　latter　field。
  17。　　Trench　26　inches　deep　（24）
  18。　　Trench　10　inches　deep；　and　then　came　upon　a　causeway　（10）
  19。　　Trench　34　inches　deep　（30）
  20。　Trench　31　inches　deep　（31）
  Field　on　the　western　side　of　the　space　enclosed　within　the　old
  walls。
  21。　　Trench　28　inches　deep；　when　undisturbed　sand　was　reached　（16）
  22。　　Trench　29　inches　deep；　when　undisturbed　sand　was　reached　（15）
  23。　　Trench　14　inches　deep；　and　then　came　upon　a　building　（14）
  Dr。　Johnson　distinguished　as　mould　the　earth　which　differed；　more
  or　less　abruptly；　in　its　dark　colour　and　in　its　texture　from　the
  underlying　sand　or　rubble。　　In　the　specimens　sent　to　me；　the　mould
  resembled　that　which　lies　immediately　beneath　the　turf　in　old
  pasture…land；　excepting　that　it　often　contained　small　stones；　too
  large　to　have　passed　through　the　bodies　of　worms。　　But　the　trenches
  above　described　were　dug　in　fields；　none　of　which　were　in　pasture；
  and　all　had　been　long　cultivated。　　Bearing　in　mind　the　remarks　made
  in　reference　to　Silchester　on　the　effects　of　long…continued
  culture；　combined　with　the　action　of　worms　in　bringing　up　the　finer
  particles　to　the　surface；　the　mould；　as　so　designated　by　Dr。
  Johnson；　seems　fairly　well　to　deserve　its　name。　　Its　thickness；
  where　there　was　no　causeway；　floor　or　walls　beneath；　was　greater
  than　has　been　elsewhere　observed；　namely；　in　many　places　above　2
  ft。；　and　in　one　spot　above　3　ft。　　The　mould　was　thickest　on　and
  close　to　the　nearly　level　summit　of　the　field　called　〃Shop
  Leasows；〃　and　in　a　small　adjoining　field；　which；　as　I　believe；　is
  of　nearly　the　same　height。　　One　side　of　the　former　field　slopes　at
  an　angle　of　rather　above　2　degrees；　and　I　should　have　expected　that
  the　mould；　from　being　washed　down　during　heavy　rain；　would　have
  been　thicker　in　the　lower　than　in　the　upper　part；　but　this　was　not
  the　case　in　two　out　of　the　three　trenches　here　dug。
  In　many　places；　where　streets　ran　beneath　the　surface；　or　where　old
  buildings　stood；　the　mould　was　only　8　inches　in　thickness；　and　Dr。
  Johnson　was　surprised　that　in　ploughing　the　land；　the　ruins　had
  never　been　struck　by　the　plough　as　far　as　he　had　heard。　　He　thinks
  that　when　the　land　was　first　cultivated　the　old　walls　were　perhaps
  intentionally　pulled　down；　and　that　hollow　places　were　filled　up。
  This　may　have　been　the　case；　but　if　after　the　desertion　of　the　city
  the　land　was　left　for　many　centuries　uncultivated；　worms　would　have
  brought　up　enough　fine　earth　to　have　covered　the　ruins　completely；
  that　is　if　they　had　subsided　from　having　been　undermined。　　The
  foundations　of　some　of　the　walls；　for　instance　those　of　the　portion
  still　standing　about　20　feet　above　the　ground；　and　those　of　the
  marketplace；　lie　at　the　extraordinary　depth　of　14　feet；　but　it　is
  highly　improbable　that　the　foundations　were　generally　so　deep。　　The
  mortar　employed　in　the　buildings　must　have　been　excellent；　for　it
  is　still　in　parts　extremely　hard。　　Wherever　walls　of　any　height
  have　been　exposed　to　view；　they　are；　as　Dr。　　Johnson　believes；
  still　perpendicular。　　The　walls　with　such　deep　foundations　cannot
  have　been　undermined　by　worms；　and　therefore　cannot　have　subsided；
  as　appears　to　have　occurred　at　Abinger　and　Silchester。　　Hence　it　is
  very　difficult　to　account　for　their　being　now　completely　covered
  with　earth；　but　how　much　of　this　covering　consists　of　vegetable
  mould　and　how　much　of　rubble　I　do　not　know。　　The　market…place；　with
  the　foundations　at　a　depth　of　14　feet；　was　covered　up；　as　Dr。
  Johnson　believes；　by　between　6　and　24　inches　of　earth。　　The　tops　of
  the　broken…down　walls　of　a　caldarium　or　bath；　9　feet　in　depth；　were
  likewise　covered　up　with　nearly　2　feet　of　earth。　　The　summit　of　an
  arch；　leading　into　an　ash…pit　7　feet　in　depth；　was　covered　up　with
  not　more　than　8　inches　of　earth。　　Whenever　a　building　which　has　not
  subsided　is　covered　with　earth；　we　must　suppose；　either　that　the
  upper　layers　of　stone　have　been　at　some　time　carried　away　by　man；
  or　that　earth　has　since　been　washed　down　during　heavy　rain；　or
  blown　down　during　storms；　from　the　adjoining　land；　and　this　would
  be　especially　apt　to　occur　where　the　land　has　long　been　cultivated。
  In　the　above　cases　the　adjoining　land　is　somewhat　higher　than　the
  three　specified　sites；　as　far　as　I　can　judge　by　maps　and　from
  information　given　me　by　Dr。　　Johnson。　　If；　however；　a　great　pile　of
  broken　stones；　mortar；　plaster；　timber　and　ashes　fell　over　the
  remains　of　any　building；　their　disintegration　in　the　course　of
  time；　and　the　sifting　action　of　worms；　would　ultimately　conceal　the
  whole　beneath　fine　earth。
  Conclusion。　The　cases　given　in　this　chapter　show　that　worms　have
  played　a　considerable　part　in　the　burial　and　concealment　of　several
  Roman　and　other　old　buildings　in　England；　but　no　doubt　the　washing
  down　of　soil　from　the　neighbouring　higher　lands；　and　the　deposition
  of　dust；　have　together　aided　largely　in　the　work　of　concealment。
  Dust　would　be　apt　to　accumulate　wherever　old　broken…down　walls
  projected　a　little　above　the　then　existing　surface　and　thus
  afforded　some　shelter。　　The　floors　of　the　old　rooms；　halls　and
  passages　have　generally　sunk；　partly　from　the　settling　of　the
  ground；　but　chiefly　from　having　been　undermined　by　worms；　and　the
  sinking　has　commonly　been　greater　in　the　middle　than　near　the
  walls。　　The　walls　themselves；　whenever　their　foundations　do　not　lie
  at　a　great　depth；　have　been　penetrated　and　undermined　by　worms；　and
  have　consequently　subsided。　　The　unequal　subsidence　thus　caused；
  probably　explains　the　great　cracks　which　may　be　seen　in　many
  ancient　walls；　as　well　as　their　inclination　from　the　perpendicular。
  CHAPTER　VTHE　ACTION　OF　WORMS　IN　THE　DENUDATION　OF　THE　LAND。
  Evidence　of　the　amount　of　denudation　which　the　land　has　undergone
  Sub…aerial　denudationThe　deposition　of　dustVegetable　mould；　its
  dark　colour　and　fine　texture　largely　due　to　the　action　of　worms
  The　disintegration　of　rocks　by　the　humus…acids　Similar　acids
  apparently　generated　within　the　bodies　of　wormsThe　action　of
  these　acids　facilitated　by　the　continued　movement　of　the　particles
  of　earthA　thick　bed　of　mould　checks　the　disintegration　of　the
  underlying　soil　and　rocks。　　Particles　of　stone　worn　or　triturated
  in　the　gizzards　of　wormsSwallowed　stones　serve　as　mill…stones
  The　levigated　state　of　the　castingsFragments　of　brick　in　the
  castings　over　ancient　buildings　well　rounded。　　The　triturating
  power　of　worms　not　quite　insignificant　under　a　geological　point　of
  view。
  No　one　doubts　that　our　world　at　one　time　consisted　of　crystalline
  rocks；　and　that　it　is　to　their　disintegration　through　the　action　of
  air；　water；　changes　of　temperature；　rivers；　waves　of　the　sea；
  earthquakes　and　volcanic　outbursts；　that　we　owe　our　sedimentary
  formations。　　These　after　being　consolidated　and　sometimes
  recrystallized；　have　often　been　again　disintegrated。　　Denudation
  means　the　removal　of　such　disintegrated　matter　to　a　lower　level。
  Of　the　many　striking　results　due　to　the　modern　progress　of　geology
  there　are　hardly　any　more　striking　than　those　which　relate　to
  denudation。　　It　was　long　ago　seen　that　there　must　have　been　an
  immense　amount　of　denudation；　but　until　the　successive　formations
  were　carefully　mapped　and　measured；　no　one　fully　realised　how　great
  was　the　amount。　　One　of　the　first　and　most　remarkable　memoirs　ever
  published　on　this　subject　was　that　by　Ramsay；　｛57｝　who　in　1846
  showed　that　in　Wales　from　9000　to　11；000　feet　in　thickness　of　solid
  rock　had　been　stripped　off　large　tracks　of　country。　　Perhaps　the
  plainest　evidence　of　great　denudation　is　afforded　by　faults　or
  cracks；　which　extend　for　many　miles　across　certain　districts；　with
  the　strata　on　one　side　raised　even　ten　thousand　feet　above　the
  corresponding　strata　on　the　opposite　side；　and　yet　there　is　not　a
  vestige　of　this　gigantic　displacement　visible　on　the　surface　of　the
  land。　　A　huge　pile　of　rock　has　been　planed　away　on　one　side　and　not
  a　remnant　left。
  Until　the　last　twenty　or　thirty　years；　most　geologists　thought　that
  the　waves　of　the　sea　were　the　chief　agents　in　the　work　of
  denudation；　but　we　may　now　feel　sure　that　air　and　rain；　aided　by
  streams　and　rivers；　are　much　more　powerful　agents；that　is　if　we
  consider　the　whole　area　of　the　land。　　The　long　lines　of　escarpment
  which　stretch　across　several　parts　of　England　were　formerly
  considered　to　be　undoubtedly　ancient　coast…lines；　but　we　now　know
  that　they　stand　up　above　the　general　surface　merely　from　resisting
  air；　rain　and　frost　better　than　the　adjoining　formations。　　It　has
  rar