the formation of vegetable mould-第30节

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rejettent；〃　&c。　｛74｝

As　the　amount　of　trituration　which　the　particles　of　earth　undergo　
in　the　gizzards　of　worms　possesses　some　interest　（as　we　shall　
hereafter　see）；　I　endeavoured　to　obtain　evidence　on　this　head　by　
carefully　examining　many　of　the　fragments　which　had　passed　through　
their　alimentary　canals。　　With　worms　living　in　a　state　of　nature；　
it　is　of　course　impossible　to　know　how　much　the　fragments　may　have　
been　worn　before　they　were　swallowed。　　It　is；　however；　clear　that　
worms　do　not　habitually　select　already　rounded　particles；　for　
sharply　angular　bits　of　flint　and　of　other　hard　rocks　were　often　
found　in　their　gizzards　or　intestines。　　On　three　occasions　sharp　
spines　from　the　stems　of　rose…bushes　were　thus　found。　　Worms　kept　
in　confinement　repeatedly　swallowed　angular　fragments　of　hard　tile；　
coal；　cinders；　and　even　the　sharpest　fragments　of　glass。　　
Gallinaceous　and　struthious　birds　retain　the　same　stones　in　their　
gizzards　for　a　long　time；　which　thus　become　well　rounded；　but　this　
does　not　appear　to　be　the　case　with　worms；　judging　from　the　large　
number　of　the　fragments　of　tiles；　glass　beads；　stones；　&c。；　
commonly　found　in　their　castings　and　intestines。　　So　that　unless　
the　same　fragments　were　to　pass　repeatedly　through　their　gizzards；　
visible　signs　of　attrition　in　the　fragments　could　hardly　be　
expected；　except　perhaps　in　the　case　of　very　soft　stones。

I　will　now　give　such　evidence　of　attrition　as　I　have　been　able　to　
collect。　　In　the　gizzards　of　some　worms　dug　out　of　a　thin　bed　of　
mould　over　the　chalk；　there　were　many　well…rounded　small　fragments　
of　chalk；　and　two　fragments　of　the　shells　of　a　land…mollusc　（as　
ascertained　by　their　microscopical　structure）；　which　latter　were　
not　only　rounded　but　somewhat　polished。　　The　calcareous　concretions　
formed　in　the　calciferous　glands；　which　are　often　found　in　their　
gizzards；　intestines；　and　occasionally　in　their　castings；　when　of　
large　size；　sometimes　appeared　to　have　been　rounded；　but　with　all　
calcareous　bodies　the　rounded　appearance　may　be　partly　or　wholly　
due　to　their　corrosion　by　carbonic　acid　and　the　humus…acids。　　In　
the　gizzards　of　several　worms　collected　in　my　kitchen　garden　near　a　
hothouse；　eight　little　fragments　of　cinders　were　found；　and　of　
these；　six　appeared　more　or　less　rounded；　as　were　two　bits　of　
brick；　but　some　other　bits　were　not　at　all　rounded。　　A　farm…road　
near　Abinger　Hall　had　been　covered　seven　years　before　with　brick…
rubbish　to　the　depth　of　about　6　inches；　turf　had　grown　over　this　
rubbish　on　both　sides　of　the　road　for　a　width　of　18　inches；　and　on　
this　turf　there　were　innumerable　castings。　　Some　of　them　were　
coloured　of　a　uniform　red　owing　to　the　presence　of　much　brick…dust；　
and　they　contained　many　particles　of　brick　and　of　hard　mortar　from　
1　to　3　mm。　in　diameter；　most　of　which　were　plainly　rounded；　but　all　
these　particles　may　have　been　rounded　before　they　were　protected　by　
the　turf　and　were　swallowed；　like　those　on　the　bare　parts　of　the　
road　which　were　much　worn。　　A　hole　in　a　pasture…field　had　been　
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　acr