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the foundations at a depth of 14 feet； was covered up； as Dr。
Johnson believes； by between 6 and 24 inches of earth。 The tops of
the broken…down walls of a caldarium or bath； 9 feet in depth； were
likewise covered up with nearly 2 feet of earth。 The summit of an
arch； leading into an ash…pit 7 feet in depth； was covered up with
not more than 8 inches of earth。 Whenever a building which has not
subsided is covered with earth； we must suppose； either that the
upper layers of stone have been at some time carried away by man；
or that earth has since been washed down during heavy rain； or
blown down during storms； from the adjoining land； and this would
be especially apt to occur where the land has long been cultivated。
In the above cases the adjoining land is somewhat higher than the
three specified sites； as far as I can judge by maps and from
information given me by Dr。 Johnson。 If； however； a great pile of
broken stones； mortar； plaster； timber and ashes fell over the
remains of any building； their disintegration in the course of
time； and the sifting action of worms； would ultimately conceal the
whole beneath fine earth。
Conclusion。 The cases given in this chapter show that worms have
played a considerable part in the burial and concealment of several
Roman and other old buildings in England； but no doubt the washing
down of soil from the neighbouring higher lands； and the deposition
of dust； have together aided largely in the work of concealment。
Dust would be apt to accumulate wherever old broken…down walls
projected a little above the then existing surface and thus
afforded some shelter。 The floors of the old rooms； halls and
passages have generally sunk； partly from the settling of the
ground； but chiefly from having been undermined by worms； and the
sinking has commonly been greater in the middle than near the
walls。 The walls themselves； whenever their foundations do not lie
at a great depth； have been penetrated and undermined by worms； and
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 dus