that　they　stand　up　above　the　general　surface　merely　from　resisting
  air；　rain　and　frost　better　than　the　adjoining　formations。　　It　has
  rarely　been　the　good　fortune　of　a　geologist　to　bring　conviction　to
  the　minds　of　his　fellow…workers　on　a　disputed　point　by　a　single
  memoir；　but　Mr。　Whitaker；　of　the　Geological　Survey　of　England；　was
  so　fortunate　when；　in　1867；　he　published　his　paper　〃On　sub…aerial
  Denudation；　and　on　Cliffs　and　Escarpments　of　the　Chalk。〃　｛58｝
  Before　this　paper　appeared；　Mr。　A。　Tylor　had　adduced　important
  evidence　on　sub…aerial　denudation；　by　showing　that　the　amount　of
  matter　brought　down　by　rivers　must　infallibly　lower　the　level　of
  their　drainage　basins　by　many　feet　in　no　immense　lapse　of　time。
  This　line　of　argument　has　since　been　followed　up　in　the　most
  interesting　manner　by　Archibald　Geikie；　Croll　and　others；　in　a
  series　of　valuable　memoirs。　｛59｝　　For　the　sake　of　those　who　have
  never　attended　to　this　subject；　a　single　instance　may　be　here
  given；　namely；　that　of　the　Mississippi；　which　is　chosen　because　the
  amount　of　sediment　brought　down　by　this　great　river　has　been
  investigated　with　especial　care　by　order　of　the　United　States
  Government。　　The　result　is；　as　Mr。　Croll　shows；　that　the　mean　level
  of　its　enormous　area　of　drainage　must　be　lowered　1/4566　of　a　foot
  annually；　or　1　foot　in　4566　years。　　Consequently；　taking　the　best
  estimate　of　the　mean　height　of　the　North　American　continent；　viz。
  748　feet；　and　looking　to　the　future；　the　whole　of　the　great
  Mississippi　basin　will　be　washed　away；　and　〃brought　down　to　the
  sea…level　in　less　than　4；500；000　years；　if　no　elevation　of　the　land
  takes　place。〃　　Some　rivers　carry　down　much　more　sediment　relatively
  to　their　size；　and　some　much　less　than　the　Mississippi。
  Disintegrated　matter　is　carried　away　by　the　wind　as　well　as　by
  running　water。　　During　volcanic　outbursts　much　rock　is　triturated
  and　is　thus　widely　dispersed；　and　in　all　arid　countries　the　wind
  plays　an　important　part　in　the　removal　of　such　matter。　　Wind…driven
  sand　also　wears　down　the　hardest　rocks。　　I　have　shown　｛60｝　that
  during　four　months　of　the　year　a　large　quantity　of　dust　is　blown
  from　the　north…western　shores　of　Africa；　and　falls　on　the　Atlantic
  over　a　space　of　1600　miles　in　latitude；　and　for　a　distance　of　from
  300　to　600　miles　from　the　coast。　　But　dust　has　been　seen　to　fall　at
  a　distance　of　1030　miles　from　the　shores　of　Africa。　　During　a　stay
  of　three　weeks　at　St。　Jago　in　the　Cape　Verde　Archipelago；　the
  atmosphere　was　almost　always　hazy；　and　extremely　fine　dust　coming
  from　Africa　was　continually　falling。　　In　some　of　this　dust　which
  fell　in　the　open　ocean　at　a　distance　of　between　330　and　380　miles
  from　the　African　coast；　there　were　many　particles　of　stone；　about
  1/1000　of　an　inch　square。　　Nearer　to　the　coast　the　water　has　been
  seen　to　be　so　much　discoloured　by　the　falling　dust；　that　a　sailing
  vessel　left　a　track　behind　her。　　In　countries；　like　the　Cape　Verde
  Archipelago；　where　it　seldom　rains　and　there　are　no　frosts；　the
  solid　rock　nevertheless　disintegrates；　and　in　conformity　with　the
  views　lately　advanced　by　a　distinguished　Belgian　geologist；　De
  Koninck；　such　disintegration　may　be　attributed　in　chief　part　to　the
  action　of　the　carbonic　and　nitric　acids；　together　with　the　nitrates
  and　nitrites　of　ammonia；　dissolved　in　the　dew。
  In　all　humid；　even　moderately　humid；　countries；　worms　aid　in　the
  work　of　denudation　in　several　ways。　　The　vegetable　mould　which
  covers；　as　with　a　mantle；　the　surface　of　the　land；　has　all　passed
  many　times　through　their　bodies。　　Mould　differs　in　appearance　from
  the　subsoil　only　in　its　dark　colour；　and　in　the　absence　of
  fragments　or　particles　of　stone　（when　such　are　present　in　the
  subsoil）；　larger　than　those　which　can　pass　through　the　alimentary
  canal　of　a　worm。　　This　sifting　of　the　soil　is　aided；　as　has　already
  been　remarked；　by　burrowing　animals　of　many　kinds；　especially　by
  ants。　　In　countries　where　the　summer　is　long　and　dry；　the　mould　in
  protected　places　must　be　largely　increased　by　dust　blown　from　other
  and　more　exposed　places。　　For　instance；　the　quantity　of　dust
  sometimes　blown　over　the　plains　of　La　Plata；　where　there　are　no
  solid　rocks；　is　so　great；　that　during　the　〃gran　seco；〃　1827　to
  1830；　the　appearance　of　the　land；　which　is　here　unenclosed；　was　so
  completely　changed　that　the　inhabitants　could　not　recognise　the
  limits　of　their　own　estates；　and　endless　lawsuits　arose。　　Immense
  quantities　of　dust　are　likewise　blown　about　in　Egypt　and　in　the
  south　of　France。　　In　China；　as　Richthofen　maintains；　beds　appearing
  like　fine　sediment；　several　hundred　feet　in　thickness　and　extending
  over　an　enormous　area；　owe　their　origin　to　dust　blown　from　the　high
  lands　of　central　Asia。　｛61｝　　In　humid　countries　like　Great　Britain；
  as　long　as　the　land　remains　in　its　natural　state　clothed　with
  vegetation；　the　mould　in　any　one　place　can　hardly　be　much　increased
  by　dust；　but　in　its　present　condition；　the　fields　near　high　roads；
  where　there　is　much　traffic；　must　receive　a　considerable　amount　of
  dust；　and　when　fields　are　harrowed　during　dry　and　windy　weather；
  clouds　of　dust　may　be　seen　to　be　blown　away。　　But　in　all　these
  cases　the　surface…soil　is　merely　transported　from　one　place　to
  another。　　The　dust　which　falls　so　thickly　within　our　houses
  consists　largely　of　organic　matter；　and　if　spread　over　the　land
  would　in　time　decay　and　disappear　almost　entirely。　　It　appears；
  however；　from　recent　observations　on　the　snow…fields　of　the　Arctic
  regions；　that　some　little　meteoric　dust　of　extra　mundane　origin　is
  continually　falling。
  The　dark　colour　of　ordinary　mould　is　obviously　due　to　the　presence
  of　decaying　organic　matter；　which；　however；　is　present　in　but　small
  quantities。　　The　loss　of　weight　which　mould　suffers　when　heated　to
  redness　seems　to　be　in　large　part　due　to　water　in　combination　being
  dispelled。　　In　one　sample　of　fertile　mould　the　amount　of　organic
  matter　was　ascertained　to　be　only　1。76　per　cent。；　in　some
  artificially　prepared　soil　it　was　as　much　as　5。5　per　cent。；　and　in
  the　famous　black　soil　of　Russia　from　5　to　even　12　per　cent。　｛62｝
  In　leaf…mould　formed　exclusively　by　the　decay　of　leaves　the　amount
  is　much　greater；　and　in　peat　the　carbon　alone　sometimes　amounts　to
  64　per　cent。；　but　with　these　latter　cases　we　are　not　here
  concerned。　　The　carbon　in　the　soil　tends　gradually　to　oxidise　and
  to　disappear；　except　where　water　accumulates　and　the　climate　is
  cool；　｛63｝　so　that　in　the　oldest　pasture…land　there　is　no　great
  excess　of　organic　matter；　notwithstanding　the　continued　decay　of
  the　roots　and　the　underground　stems　of　plants；　and　the　occasional
  addition　of　manure。　　The　disappearance　of　the　organic　matter　from
  mould　is　probably　much　aided　by　its　being　brought　again　and　again
  to　the　surface　in　the　castings　of　worms。
  Worms；　on　the　other　hand；　add　largely　to　the　organic　matter　in　the
  soil　by　the　astonishing　number　of　half…decayed　leaves　which　they
  draw　into　their　burrows　to　a　depth　of　2　or　3　inches。　　They　do　this
  chiefly　for　obtaining　food；　but　partly　for　closing　the　mouths　of
  their　burrows　and　for　lining　the　upper　part。　　The　leaves　which　they
  consume　are　moistened；　torn　into　small　shreds；　partially　digested；
  and　intimately　commingled　with　earth；　and　it　is　this　process　which
  gives　to　vegetable　mould　its　uniform　dark　tint。　　It　is　known　that
  various　kinds　of　acids　are　generated　by　the　decay　of　vegetable
  matter；　and　from　the　contents　of　the　intestines　of　worms　and　from
  their　castings　being　acid；　it　seems　probable　that　the　process　of
  digestion　induces　an　analogous　chemical　change　in　the　swallowed；
  triturated；　and　half…decayed　leaves。　　The　large　quantity　of
  carbonate　of　lime　secreted　by　the　calciferous　glands　apparently
  serves　to　neutralise　the　acids　thus　generated；　for　the　digestive
  fluid　of　worms　will　not　act　unless　it　be　alkaline。　　As　the　contents
  of　the　upper　part　of　their　intestines　are　acid；　the　acidity　can
  hardly　be　due　to　the　presence　of　uric　acid。　　We　may　therefore
  conclude　that　the　acids　in　the　alimentary　canal　of　worms　are　formed
  during　the　digestive　process；　and　that　probably　they　are　nearly　of
  the　same　nature　as　those　in　ordinary　mould　or　humus。　　The　latter
  are　well　known　to　have　the　power　of　de…oxidising　or　dissolving　per…
  oxide　of　iron；　as　may　be　seen　wherever　peat　overlies　red　sand；　or
  where　a　rotten　root　penetrates　such　sand。　　Now　I　kept　some　worms　in
  a　pot　filled　with　very　fine　reddish　sand；　consisting　of　minute
  particles　of　silex　coated　with　the　red　oxide　of　iron；　and　the
  burrows；　which　the　worms　made　through　this　sand；　were　lined　or
  coated　in　the　usual　manner　with　their　castings；　formed　of　the　sand
  mingled　with　their　intestinal　secretions　and　the　refuse　of　the
  digested　leaves；　and　this　sand　had　almost　wholly　lost　its　red
  colour。　　When　small　portions　of　it　were　placed　under　the
  microscope；　most　of　the　grains　were　seen　to　be　transparent　and
  colourless；　owing　to　the　dissolution　of　the　oxide；　whilst　almost
  all　the　grains　taken　from　other　parts　of　the　pot　were　coated　with
  the　oxide。　　Acetic　acid　produced　hardly　any　effect　on　his　sand；　and
  even