the formation of vegetable mould-第31章
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have consequently subsided。 The unequal subsidence thus caused；
probably explains the great cracks which may be seen in many
ancient walls； as well as their inclination from the perpendicular。
CHAPTER VTHE ACTION OF WORMS IN THE DENUDATION OF THE LAND。
Evidence of the amount of denudation which the land has undergone
Sub…aerial denudationThe deposition of dustVegetable mould； its
dark colour and fine texture largely due to the action of worms
The disintegration of rocks by the humus…acids Similar acids
apparently generated within the bodies of wormsThe action of
these acids facilitated by the continued movement of the particles
of earthA thick bed of mould checks the disintegration of the
underlying soil and rocks。 Particles of stone worn or triturated
in the gizzards of wormsSwallowed stones serve as mill…stones
The levigated state of the castingsFragments of brick in the
castings over ancient buildings well rounded。 The triturating
power of worms not quite insignificant under a geological point of
view。
No one doubts that our world at one time consisted of crystalline
rocks； and that it is to their disintegration through the action of
air； water； changes of temperature； rivers； waves of the sea；
earthquakes and volcanic outbursts； that we owe our sedimentary
formations。 These after being consolidated and sometimes
recrystallized； have often been again disintegrated。 Denudation
means the removal of such disintegrated matter to a lower level。
Of the many striking results due to the modern progress of geology
there are hardly any more striking than those which relate to
denudation。 It was long ago seen that there must have been an
immense amount of denudation； but until the successive formations
were carefully mapped and measured； no one fully realised how great
was the amount。 One of the first and most remarkable memoirs ever
published on this subject was that by Ramsay； ｛57｝ who in 1846
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
