the formation of vegetable mould-第34节

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again　arrest　the　castings。　　An　irregular　ledge　when　once　formed　
would　also　tend　to　become　more　regular　and　horizontal　by　some　of　
the　castings　rolling　laterally　from　the　higher　to　the　lower　parts；　
which　would　thus　be　raised。　　Any　projection　beneath　a　ledge　would　
not　afterwards　receive　disintegrated　matter　from　above；　and　would　
tend　to　be　obliterated　by　rain　and　other　atmospheric　agencies。　　
There　is　some　analogy　between　the　formation；　as　here　supposed；　of　
these　ledges；　and　that　of　the　ripples　of　wind…drifted　sand　as　
described　by　Lyell。　｛78｝

The　steep；　grass…covered　sides　of　a　mountainous　valley　in　
Westmoreland；　called　Grisedale；　was　marked　in　many　places　with　
innumerable　lines　of　miniature　cliffs；　with　almost　horizontal；　
little　ledges　at　their　bases。　　Their　formation　was　in　no　way　
connected　with　the　action　of　worms；　for　castings　could　not　anywhere　
be　seen　（and　their　absence　is　an　inexplicable　fact）；　although　the　
turf　lay　in　many　places　over　a　considerable　thickness　of　boulder…
clay　and　moraine　rubbish。　　Nor；　as　far　as　I　could　judge；　was　the　
formation　of　these　little　cliffs　at　all　closely　connected　with　the　
trampling　of　cows　or　sheep。　　It　appeared　as　if　the　whole　
superficial；　somewhat　argillaceous　earth；　while　partially　held　
together　by　the　roots　of　the　grasses；　had　slided　a　little　way　down　
the　mountain　sides；　and　in　thus　sliding；　had　yielded　and　cracked　in　
horizontal　lines；　transversely　to　the　slope。

Castings　blown　to　leeward　by　the　wind。We　have　seen　that　moist　
castings　flow；　and　that　disintegrated　castings　roll　down　any　
inclined　surface；　and　we　shall　now　see　that　castings；　recently　
ejected　on　level　grass…covered　surfaces；　are　blown　during　gales　of　
wind　accompanied　by　rain　to　leeward。　　This　has　been　observed　by　me　
many　times　on　many　fields　during　several　successive　years。　　After　
such　gales；　the　castings　present　a　gently　inclined　and　smooth；　or　
sometimes　furrowed；　surface　to　windward；　while　they　are　steeply　
inclined　or　precipitous　to　leeward；　so　that　they　resemble　on　a　
miniature　scale　glacier…ground　hillocks　of　rock。　　They　are　often　
cavernous　on　the　leeward　side；　from　the　upper　part　having　curled　
over　the　lower　part。　　During　one　unusually　heavy　south…west　gale　
with　torrents　of　rain；　many　castings　were　wholly　blown　to　leeward；　
so　that　the　mouths　of　the　burrows　were　left　naked　and　exposed　on　
the　windward　side。　　Recent　castings　naturally　flow　down　an　inclined　
surface；　but　on　a　grassy　field；　which　sloped　between　10　degrees　and　
15　degrees；　several　were　found　after　a　heavy　gale　blown　up　the　
slope。　　This　likewise　occurred　on　another　occasion　on　a　part　of　my　
lawn　where　the　slope　was　somewhat　less。　　On　a　third　occasion；　the　
castings　on　the　steep；　grass…covered　sides　of　a　valley；　down　which　
a　gale　had　blown；　were　directed　obliquely　instead　of　straight　down　
the　slope；　and　this　was　obviously　due　to　the　combined　action　of　the　
wind　and　gravity。　　Four　castings　on　my　lawn；　where　the　downward　
inclination　was　0　　degrees　45　seconds；　1　degree；　3　degrees　and　3　
degrees　30　seconds　（mean　2　degrees　45　seconds）　towards　the　north…
east；　after　a　heavy　south…west　gale　with　rain；　were　divided　across　
the　mouths　of　the　burrows　and　weighed　in　the　manner　formerly　
described。　　The　mean　weight　of　the　earth　below　the　mouths　of　
burrows　and　to　leeward；　was　to　that　above　the　mouths　and　on　the　
windward　side　as　2。75　to　1；　whereas　we　have　seen　that　with　several　
castings　which　had　flowed　down　slopes　having　a　mean　inclination　of　
9　degrees　26　seconds；　and　with　three　castings　where　the　inclination　
was　above　12　degrees；　the　proportional　weight　of　the　earth　below　to　
that　above　the　burrows　was　as　only　2　to　1。　　These　several　cases　
show　how　efficiently　gales　of　wind　accompanied　by　rain　act　in　
displacing　recently　ejected　castings。　　We　may　therefore　conclude　
that　even　a　moderately　strong　wind　will　produce　some　slight　effect　
on　them。

Dry　and　indurated　castings；　after　their　disintegration　into　small　
fragments　or　pellets；　are　sometimes；　probably　often；　blown　by　a　
strong　wind　to　leeward。　　This　was　observed　on　four　occasions；　but　I　
did　not　sufficiently　attend　to　this　point。　　One　old　casting　on　a　
gently　sloping　bank　was　blown　quite　away　by　a　strong　south…west　
wind。　　Dr。　King　believes　that　the　wind　removes　the　greater　part　of　
the　old　crumbling　castings　near　Nice。　　Several　old　castings　on　my　
lawn　were　marked　with　pins　and　protected　from　any　disturbance。　　
They　were　examined　after　an　interval　of　10　weeks；　during　which　time　
the　weather　had　been　alternately　dry　and　rainy。　　Some；　which　were　
of　a　yellowish　colour　had　been　washed　almost　completely　away；　as　
could　be　seen　by　the　colour　of　the　surrounding　ground。　　Others　had　
completely　disappeared；　and　these　no　doubt　had　been　blown　away。　　
Lastly；　others　still　remained　and　would　long　remain；　as　blades　of　
grass　had　grown　through　them。　　On　poor　pasture…land；　which　has　
never　been　rolled　and　has　not　been　much　trampled　on　by　animals；　the　
whole　surface　is　sometimes　dotted　with　little　pimples；　through　and　
on　which　grass　grows；　and　these　pimples　consist　of　old　worm…
castings。

In　all　the　many　observed　cases　of　soft　castings　blown　to　leeward；　
this　had　been　effected　by　strong　winds　accompanied　by　rain。　　As　
such　winds　in　England　generally　blow　from　the　south　and　south…west；　
earth　must　on　the　whole　tend　to　travel　over　our　fields　in　a　north　
and　north…east　direction。　　This　fact　is　interesting；　because　it　
might　be　thought　that　none　could　be　removed　from　a　level；　grass…
covered　surface　by　any　means。　　In　thick　and　level　woods；　protected　
from　the　wind；　castings　will　never　be　removed　as　long　as　the　wood　
lasts；　and　mould　will　here　tend　to　accumulate　to　the　depth　at　which　
worms　can　work。　　I　tried　to　procure　evidence　as　to　how　much　mould　
is　blown；　whilst　in　the　state　of　castings；　by　our　wet　southern　
gales　to　the　north…east；　over　open　and　flat　land；　by　looking　to　the　
level　of　the　surface　on　opposite　sides　of　old　trees　and　hedge…rows；　
but　I　failed　owing　to　the　unequal　growth　of　the　roots　of　trees　and　
to　most　pasture…land　having　been　formerly　ploughed。

On　an　open　plain　near　Stonehenge；　there　exist　shallow　circular　
trenches；　with　a　low　embankment　outside；　surrounding　level　spaces　
50　yards　in　diameter。　　These　rings　appear　very　ancient；　and　are　
believed　to　be　contemporaneous　with　the　Druidical　stones。　　Castings　
ejected　within　these　circular　spaces；　if　blown　to　the　north…east　by　
south…west　winds　would　form　a　layer　of　mould　within　the　trench；　
thicker　on　the　north…eastern　than　on　any　other　side。　　But　the　site　
was　not　favourable　for　the　action　of　worms；　for　the　mould　over　the　
surrounding　Chalk　formation　with　flints；　was　only　3。37　inches　in　
thickness；　from　a　mean　of　six　observations　made　at　a　distance　of　10　
yards　outside　the　embankment。　　The　thickness　of　the　mould　within　
two　of　the　circular　trenches　was　measured　every　5　yards　all　round；　
on　the　inner　sides　near　the　bottom。　　My　son　Horace　protracted　these　
measurements　on　paper；　and　though　the　curved　line　representing　the　
thickness　of　the　mould　was　extremely　irregular；　yet　in　both　
diagrams　it　could　be　seen　to　be　thicker　on　the　north…eastern　side　
than　elsewhere。　　When　a　mean　of　all　the　measurements　in　both　the　
trenches　was　laid　down　and　the　line　smoothed；　it　was　obvious　that　
the　mould　was　thickest　in　the　quarter　of　the　circle　between　north…
west　and　north…east；　and　thinnest　in　the　quarter　between　south…east　
and　south…west；　especially　at　this　latter　point。　　Besides　the　
foregoing　measurements；　six　others　were　taken　near　together　in　one　
of　the　circular　trenches；　on　the　north…east　side；　and　the　mould　
here　had　a　mean　thickness　of　2。29　inches；　while　the　mean　of　six　
other　measurements　on　the　south…west　side　was　only　1。46　inches。　　
These　observations　indicate　that　the　castings　had　been　blown　by　the　
south…west　winds　from　the　circular　enclosed　space　into　the　trench　
on　the　north…east　side；　but　many　more　measurements　in　other　
analogous　cases　would　be　requisite　for　a　trustworthy　result。

The　amount　of　fine　earth　brought　to　the　surface　under　the　form　of　
castings；　and　afterwards　transported　by　the　winds　accompanied　by　
rain；　or　that　which　flows　and　rolls　down　an　inclined　surface；　no　
doubt　is　small　in　the　course　of　a　few　scores　of　years；　for　
otherwise　all　the　inequalities　in　our　pasture　fields　would　be　
smoothed　within　a　much　shorter　period　than　appears　to　be　the　case。　　
But　the　amount　which　is　thus　transported　in　the　course　of　thousands　
of　years　cannot　fail　to　be　considerable　and　deserves　attention。　　E。　
de　Beaumont　looks　at　the　vegetable　mould　which　everywhere　covers　
the　land　as　a　fixed　line；　from　which　the　amount　of　denudation　may　
be　measured。　｛79｝　　He　ignores　the　continued　formation　of　fresh　
mould　by　the　disintegration　of　the　underlying　rocks　and　fragments　
of　rock；　and　it　is　curious　to　find　how　much　more　philosophical　were　
the　views　maintained　long　ago；　by　Playfair；　who；　in　1802；　wrote；　
〃In　the　permanence　of　a　coat　of　vegetable　mould　on　the　surface　of　
the　earth；　we　have　a　demonstrative　proof　of　the　continued　
destruction　of　the　rocks。〃　｛80｝

Ancient　encampments　and　tumuli。E。　de　Beaumont　adduc