Изображения страниц
PDF
EPUB

land from which it has been removed is lightened and enabled to rise. The wearing-down processes continue

. on the newly raised land and fresh layers are transferred to the lower levels. These presently sink again and the areas lightened by what is called denudation again rise.

Movements in the earth's crust produce mountains. Sometimes weaker areas of the crust collapse, leaving blocks of stronger rock standing above the level of the surrounding country. These blocks are attacked by the agents of denudation, heat, frost, wind, rain, and the gases in the air, and then the surface crumbles and becomes jagged. Rain lies in its hollows, trickles down its sides, and washes away the loose fragments of the rocks. In time deep valleys are worn in the mass along which streams or rivers rush, for ever widening their beds, and the parts left standing between them form mountains.

Other mountains are formed by the crumpling of the earth's crust into parallel ridges and valleys. Others, again, are uplifted in dome-shaped swellings by the inflow underneath them of rock still plastic with the heat of the interior.

Volcanoes are vast heaps of once molten rock and ash piled up round the vents in the earth's crust through which their materials were driven.

The uplift of the land continually exposes fresh surfaces to the forces of denudation. Very often these surfaces consist of primary rock, rich in materials necessary for the development of life, but hitherto inaccessible. It is the work of denudation to set these free by breaking down the rock so that it may be transported by wind and water and refashioned as a secondary rock in which these valuable constituents have become available.

The chief agents of denudation are the oxygen and carbon dioxide in the air, rain, heat, frost, and wind.

Oxygen combines with some of the constituents of rock. These expand with the heat of combination and thus loosen others, and the rock begins to decay and crumble. The destructive power of oxygen is easily

[graphic][merged small]

seen in the rusting of iron, which loses its surface in flakes as a result of exposure to the atmosphere.

Carbon dioxide is dissolved from the air by rain ; the rain penetrates the rocks and the carbon dioxide in it combines with some of the constituents and forms carbonates. This process usually results in the slow decay of the rock; moreover, the carbonates, as, for example, calcium carbonate, of which limestone is formed, are easily dissolved by water containing carbon dioxide, and a limestone country is characterized by deep gorges and caverns which streams and rivers have excavated. The Cheddar caves and the caverns of the Peak district have been made in this way.

a

Rain soaks into the pores of the rock and washes the loose fragments from the surface. Wind beats upon it, laden sometimes with grains of sand, grit, and dust which it has carried from elsewhere, and so helps the grinding-down process.

Heat causes rock to expand. As it cools again it contracts, and the alternate expansion and contraction weaken its structure. Frost, again, causes the expan

. sion of water in the pores and crevices, and as a result the rock splits and is laid more open to the destructive forces of air and water.

Wind and water are the chief agents in laying down beds of secondary rock with the materials of primary rock thus broken up. Wind carries away the lighter particles and deposits them in a sheltered spot; mountain torrents bear along boulders loosened from the mountain-side, as well as pebbles, sand, and mud. As the speed of the current lessens the débris is deposited according to its size: first the boulders, then the pebbles, then beds of sand, and lastly, when the current is slowest, sheets of mud. Thus a river destroys in its upper course and builds up in its lower.

Sandstones, clays, limestones, and coals are the chief kinds of secondary rock.

Sandstones are composed of grains of sand which have become cemented together. They provide valuable building stone.

Clays are composed of the finest water-borne particles. They are deposited as mud and converted by pressure into rock. Besides furnishing material for building bricks, they decay into rich soil, and as they are impervious to water, beds of clay act as reservoirs, from which water is discharged in springs or from which it can be obtained by sinking wells.

Limestone may be formed by the deposit of the calcium carbonate often carried in solution by springs and rivers. Usually, however, its formation is a more complex process. It is extracted from the water by organisms which use it to build up their shells and

[graphic][ocr errors][ocr errors]

STACKS': HARD PARTS OF THE CLIFFS LEFT STANDING AFTER

THE SOFTER PARTS HAVE BEEN WASHED AWAY

From Secrets of the Hills,Craig (Harrap)

skeletons. When they die these parts lie on the sea floor and the accumulation may be cemented into limestone. The limestones make fertile soil and also provide building stone and cement.

Coals are due to the action of plants. These extract carbon dioxide from the air and use the carbon to build their tissues. When masses of decaying vegetation collect in swampy beds they form peat, and if the peat is buried under fresh rock deposits and subjected to heavy pressure it is slowly converted into coal; and so we get

fuel from material existing in the air, but to be obtained from it only by the agency of plants.

Beds of salt are found when lagoons or arms of the sea have dried up, and the saltness of the sea is due to

[graphic][merged small]

rivers, which have been washing salts out of the soil and carrying them in solution to the sea for many millions of years.

The areas built of secondary rock will, it is clear, be the areas most useful to man, but they need continual refertilization, as the constituents required by plants for their food are extracted by them or carried

« ПредыдущаяПродолжить »