Читать книгу The Birth-Time of the World and Other Scientific Essays - John Joly - Страница 6

Table of Contents

THE subject of denudation is at once one of the most interesting

and one of the most complicated with which the geologist has to

deal. While its great results are apparent even to the most

casual observer, the factors which have led to these results are

in many cases so indeterminate, and in some cases apparently so

variable in influence, that thoughtful writers have even claimed

precisely opposite effects as originating from, the same cause.

Indeed, it is almost impossible to deal with the subject without

entering upon controversial matters. In the following pages I

shall endeavour to keep to broad issues which are, at the present

day, either conceded by the greater number of authorities on the

subject, or are, from their strictly quantitative character, not

open to controversy.

It is evident, in the first place, that denudation—or the wearing

away of the land surfaces of the earth—is mainly a result of the

circulation of water from the ocean to the land, and back again

to the ocean. An action entirely conditioned by solar heat, and

without which it would completely cease and further change upon

the land come to an end.

To what actions, then, is so great a potency of the

30

circulating water to be traced? Broadly speaking, we may classify

them as mechanical and chemical. The first involves the

separation of rock masses into smaller fragments of all sizes,

down to the finest dust. The second involves the actual solution

in the water of the rock constituents, which may be regarded as

the final act of disintegration. The rivers bear the burden both

of the comminuted and the dissolved materials to the sea. The mud

and sand carried by their currents, or gradually pushed along

their beds, represent the former; the invisible dissolved matter,

only to be demonstrated to the eye by evaporation of the water or

by chemical precipitation, represents the latter.

The results of these actions, integrated over geological time,

are enormous. The entire bulk of the sedimentary rocks, such as

sandstones, slates, shales, conglomerates, limestones, etc., and

the salt content of the ocean, are due to the combined activity

of mechanical and solvent denudation. We shall, later on, make an

estimate of the magnitude of the quantities actually involved.

In the Swiss valleys we see torrents of muddy water hurrying

along, and if we follow them up, we trace them to glaciers high

among the mountains. From beneath the foot of the glacier, we

find, the torrent has birth. The first debris given to the river

is derived from the wearing of the rocky bed along which the

glacier moves. The river of ice bequeaths to the river of

water—of which it is the parent—the spoils which it has won from

the rocks

31

The work of mechanical disintegration is, however, not restricted

to the glacier's bed. It proceeds everywhere over the surface of

the rocks. It is aided by the most diverse actions. For instance,

the freezing and expansion of water in the chinks and cracks in

those alpine heights where between sunrise and sunset the heat of

summer reigns, and between sunset and sunrise the cold of winter.

Again, under these conditions the mere change of surface

temperature from night to day severely stresses the surface

layers of the rocks, and, on the same principles as we explain

the fracture of an unequally heated glass vessel, the rocks

cleave off in slabs which slip down the steeps of the mountain

and collect as screes in the valley. At lower levels the

expansive force of vegetable growth is not unimportant, as all

will admit who have seen the strong roots of the pines

penetrating the crannies of the rocks. Nor does the river which

flows in the bed of the valley act as a carrier only. Listening

carefully we may detect beneath the roar of the alpine torrent

the crunching and knocking of descending boulders. And in the

potholes scooped by its whirling waters we recognise the abrasive

action of the suspended sand upon the river bed.

A view from an Alpine summit reveals a scene of remarkable

desolation (Pl. V, p. 40). Screes lie piled against the steep

slopes. Cliffs stand shattered and ready to fall in ruins. And

here the forces at work readily reveal themselves. An occasional

wreath of white smoke among

32

the far-off peaks, followed by a rumbling reverberation, marks

the fall of an avalanche. Water everywhere trickles through the

shaly _débris_ scattered around. In the full sunshine the rocks are

almost too hot to bear touching. A few hours later the cold is

deadly, and all becomes a frozen silence. In such scenes of

desolation and destruction, detrital sediments are actively being

generated. As we descend into the valley we hear the deep voice

of the torrents which are continually hurrying the disintegrated

rocks to the ocean.

A remarkable demonstration of the activity of mechanical

denudation is shown by the phenomenon of "earth pillars." The

photograph (Pl. IV.) of the earth pillars of the Val d'Hérens

(Switzerland) shows the peculiar appearance these objects

present. They arise under conditions where large stones or

boulders are scattered in a deep deposit of clay, and where much

of the denudation is due to water scour. The large boulders not

only act as shelter against rain, but they bind and consolidate

by their mere weight the clay upon which they rest. Hence the

materials underlying the boulders become more resistant, and as

the surrounding clays are gradually washed away and carried to

the streams, these compacted parts persist, and, finally, stand

like walls or pillars above the general level. After a time the

great boulders fall off and the underlying clay becomes worn by

the rainwash to fantastic spikes and ridges. In the Val d'Hérens

the earth pillars are formed

33

of the deep moraine stuff which thickly overlies the slopes of

the valley. The wall of pillars runs across the axis of the

valley, down the slope of the hill, and crosses the road, so that

it has to be tunnelled to permit the passage of traffic. It is

not improbable that some additional influence—possibly the

presence of lime—has hardened the material forming the pillars,

and tended to their preservation.

Denudation has, however, other methods of work than purely

mechanical; methods more noiseless and gentle, but not less

effective, as the victories of peace ate no less than those of

war.

Over the immense tracts of the continents chemical work proceeds

relentlessly. The rock in general, more especially the primary

igneous rock, is not stable in presence of the atmosphere and of

water. Some of the minerals, such as certain silicates and

carbonates, dissolve relatively fast, others with extreme

slowness. In the process of solution chemical actions are

involved; oxidation in presence of the free oxygen of the

atmosphere; attack by the feeble acid arising from the solution

of carbon dioxide in water; or, again, by the activity of certain

acids—humous acids—which originate in the decomposition of

vegetable remains. These chemical agents may in some instances,

_e.g._ in the case of carbonates such as limestone or

dolomite—bring practically the whole rock into solution. In other

instances—_e.g._ granites, basalts, etc.—they may remove some of

the