the formation of vegetable mould-第23章

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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　
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　hydrochloric；　nitric　and　sulphuric　acids；　diluted　as　in　the　
Pharmacopoeia；　produced　less　effect　than　did　the　acids　in　the　
intestines　of　the　worms。

Mr。　A。　A。　Julien　has　lately　collected　all　the　extant　information　
about　the　acids　generated　in　humus；　which；　according　to　some　
chemists；　amount　to　more　than　a　dozen　different　kinds。　　These　
acids；　as　well　as　their　acid　salts　（i。e。；　in　combination　with　
potash；　soda；　and　ammonia）；　act　energetically　on　carbonate　of　lime　
and　on　the　oxides　of　iron。　　It　is　also　known　that　some　of　these　
acids；　which　were　called　long　ago　by　Thenard　azohumic；　are　enabled　
to　dissolve　colloid　silica　in　proportion　to　the　nitrogen　which　they　
contain。　｛64｝　　In　the　formation　of　these　latter　acids　worms　
probably　afford　some　aid；　for　Dr。　H。　Johnson　informs　me　that　by　
Nessler's　test　he　found　0。018　per　cent。　of　ammonia　in　their　
castings。

It　may　be　here　added　that　I　have　recently　been　informed　by　Dr。　
Gilbert　〃that　several　square　yards　on　his　lawn　were　swept　clean；　
a