the formation of vegetable mould-第25章

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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　acre　of　land；　which　is　sufficiently　damp　and　not　too　sandy；　
gravelly　or　rocky　for　worms　to　inhabit；　a　weight　of　more　than　ten　
tons　of　earth　annually　passes　through　their　bodies　and　is　brought　
to　the　surface。　　The　result　for　a　country　of　the　size　of　Great　
Britain；　within　a　period　not　very　long　in　a　geological　sense；　such　
as　a　million　years；　cannot　be　insignificant；　for　the　ten　tons　of　
earth　has　to　be　multiplied　first　by　the　above　number　of　years；　and　
then　by　the　number　of　acres　fully　stocked　with　worms；　and　in　
England；　together　with　Scotland；　the　land　which　is　cultivated　and　
is　well　fitted　for　these　animals；　has　been　estimated　at　above　32　
million　acres。　　The　product　is　320　million　million　tons　of　earth。



CHAPTER　VITHE　DENUDATION　OF　THE　LANDcontinued。



Denudation　aided　by　recently　ejected　castings　flowing　down　inclined　
grass…covered　surfacesThe　amount　of　earth　which　annually　flows　
downwardsThe　effect　of　tropical　rain　on　worm　castingsThe　finest　
particles　of　earth　washed　completely　away　from　castingsThe　
disintegration　of　dried　castings　into　pellets；　and　their　rolling　
down　inclined　surfacesThe　formation　of　little　ledges　on　hill…
sides；　in　part　due　to　the　accumulation　of　disintegrated　castings
Castings　blown　to　leeward　over　level　landAn　attempt　to　estimate　
the　amount　thus　blownThe　degradation　of　ancient　encampments　and　
tumuliThe　preservation　of　the　crowns　and　furrows　on　land　
anciently　ploughedThe　formation　and　amount　of　mould　over　the　
Chalk　formation。

We　are　now　prepared　to　consider　the　more　direct　part　which　worms　
take　in　the　denudation　of　the　land。　　When　reflecting　on　sub…aerial　
denudation；　it　formerly　appeared　to　me；　as　it　has　to　others；　that　a　
nearly　level　or　very　gently　inclined　surface；　covered　with　turf；　
could　suffer　no　loss　during　even　a　long　lapse　of　time。　　It　may；　
however；　be　urged　that　at　long　intervals；　debacles　of　rain　or　
water…spouts　would　remove　all　the　mould　from　a　very　gentle　slope；　
but　when　examining　the　steep；　turf…covered　slopes　in　Glen　Roy；　I　
was　struck　with　the　fact　how　rarely　any　such　event　could　have　
happened　since　the　Glacial　period；　as　was　plain　from　the　well…
preserved　state　of　the　three　successive　〃roads〃　or　lake…margins。　　
But　the　difficulty　in　believing　that　earth　in　any　appreciable　
quantity　can　be　removed　from　a　gently　inclined　surface；　covered　
with　vegetation　and　matted　with　roots；　is　removed　through　the　
agency　of　worms。　　For　the　many　castings　which　are　thrown　up　during　
rain；　and　those　thrown　up　some　little　time　before　heavy　rain；　flow　
for　a　short　distance　down　an　inclined　surface。　　Moreover　much　of　
the　finest　levigated　earth　is　washed　completely　away　from　the　
castings。　　During　dry　weather　castings　often　disintegrate　into　
small　rounded　pellets；　and　these　from　their　weight　often　roll　down　
any　slope。　　This　is　more　especially　apt　to　occur　when　they　are　
started　by　the　wind；　and　probably　when　started　by　the　touch　of　an　
animal；　however　small。　　We　shall　also　see　that　a　strong　wind　blows　
all　the　castings；　even　on　a　level　field；　to　leeward；　whilst　they　
are　soft；　and　in　like　manner　the　pellets　when　they　are　dry。　　If　the　
wind　blows　in　nearly　the　direction　of　an　inclined　surface；　the　
flowing　down　of　the　castings　is　much　aided。

The　observations　on　which　these　several　statements　are　founded　must　
now　be　given　in　some　detail。　　Castings　when　first　ejected　are　
viscid　and　soft；　during　rain；　at　which　time　worms　apparently　prefer　
to　eject　them；　they　are　still　softer；　so　that　I　have　sometimes　
thought　that　worms　must　swallow　much　water　at　such　times。　　However　
this　may　be；　rain；　even　when　not　very　heavy；　if　long　continued；　
renders　recently…ejected　castings　semi…fluid；　and　on　level　ground　
they　then　spread　out　into　thin；　circular；　flat　discs；　exactly　as　
would　so　much　honey　or　very　soft　mortar；　with　all　traces　of　their　
vermiform　structure　lost。　　This　latter　fact　was　sometimes　made　
evident；　when　a　worm　had　subsequently　bored　through　a　flat　circular　
disc　of　this　kind；　and　heaped　up　a　fresh　vermiform　mass　in　the　
centre。　　These　flat　subsided　discs　have　been　repeatedly　seen　by　me　
after　heavy　rain；　in　many　places　on　land　of　all　kinds。

On　the　flowing　of　wet　castings；　and　the　rolling　of　dry　
disintegrated　castings　down　inclined　surfaces。When　castings　are　
ejected　on　an　inclined　surface　during　or　shortly　before　heavy　rain；　
they　cannot　fail　to　flow　a　little　down　the　slope。　　Thus；　on　some　
steep　slopes　in　Knole　Park；　which　were　covered　with　coarse　grass　
and　had　apparently　existed　in　this　state　from　time　immemorial；　I　
found　（Oct。　22；　1872）　after　several　wet　days　that　almost　all　the　
many　castings　were　considerably　elongated　in　the　line　of　the　slope；　
and　that　they　now　consisted　of　smooth；　only　slightly　conical　
masses。　　Whenever　the　mouths　of　the　burrows　could　be　found　from　
which　the　earth　had　been　ejected；　there　was　more　earth　below　than　
above　them。　　After　some　heavy　storms　of　rain　（Jan。　25；　1872）　two　
rather　steeply　inclined　fields　near　Down；　which　had　formerly　been　
ploughed　and　were　now　rather　sparsely　clothed　with　poo