The Elements of Geology

to the stream ways and are carried by the streams to the sea, where they are rebuilt into rocky layers. When again the rocks are lifted to form land the process will begin anew; again they will cr

rries we shall learn a little of some

STRATA, and the thin leaves into which each stratum may sometimes be split are termed LAMINAE. To a greater or less degree these layers di

ttom. These are found in all rocks and are known as joints. Two sets of joints, running at about right

Most of the grains are of a clear glassy mineral called quartz. These quartz grains are very hard and will scratch the s

monly ferruginous,-hydrated iron oxide, or iron rust, forming the bond, somewhat as in the case of iron nails which have rusted together. The strongest and most

gentle slopes the rock is covered with a soil composed of sand, which evidently is crumbled sandstone, and dark carbonaceous matter derived from the decay of vegetation. Clearly it is by the dissolving of the cement that the rock thus break

rock is sound and is quarried for building; but the altered upper layers are too soft and broken to be used for this purpose. If the limestone is laminated, the laminae here have split apart, although below the

piece of the stone placed in a test tube with hydrochloric acid dissolves with brisk effervescence, leavi

the clay the rock has disintegrated into meal where the cement between the limestone grains has been removed, while beneath this the laminae are split apart where the cement has been dissolved only along the pl

r rock surface is pitted. Joints are widened as they approach the surface, and alo

in as cardboard in places, and close joints may break the rock into small rhombic blocks. On the upper surface we note that the shale has weathered to a clayey soil in which all trac

his group includes all rocks made of sediments, their materials having settled either in water upon the bottoms of rivers, lakes, or seas, or on dry land, as in the case of deposits made by the wind a

ded according to their composi

ose sand and gravel, sandstone, quartzite, and conglomer

ding limestone and a soft white rock fo

se three classes pass by mixture into one another. Thus there are limy and

ized from a molten mass. Examining a piece of granite, the most conspicuous crystals which meet the eye are those of feldspar. They are commonly pink, white, or yellow, and break along smooth cleavage planes which reflect the li

und in a surface sheet of stony clay called the drift. Of the different minerals composing granite, quartz alone remains unaltered. Mica weathers to detached flakes which have lost their elasticity. The feldspar crystals have lost their luster and hardness, and even have dec

undergo. The agencies by which these changes are brought about we will now take up under two divis

ICAL WOR

rom the soil humus acids and carbon dioxide, both of which are constantly being generated there by the decay of organic matter. So both rain and ground water are charged with active chemical agents, by the help of

s therefore "hard." On boiling the water for some time the carbon dioxide gas is expelled, the whole of the lime carbonate can no longer be held in solution, and much of it is thrown down to form a crust or "scale" in the ket

n the limestone quarry, and where the mantle of waste has been removed

, a mineral composed of hydrated sulphate of lime, and so soft that it may be scratched with the finger nail,

of its erection, one may estimate how many years on the average it has taken for weathering to loosen fine grains on the polished surface, so that they may be rubbed off with the finger, to destroy the polish, to round the shar

relief. As the shells are made of crystalline carbonate of lime, we may infer whether

water containing carbon dioxide, forming alkaline carbonates. These carbonates are freely soluble and contribute potash and soda to soils and river waters. By the removal of the soluble ingredients of feldspar there is left the silicate of alumina, united with water or hydrated, in the condition of a fine plastic clay which, when white and pure

ned with rusty films. Oxygen and water have here united with the iron, forming hydrated iron oxide. The effects of

ing iron sulphate (green vitriol), a soluble compound, and insoluble hydrated iron oxide, which as a mineral is known as limonite. Several large masses of iron sulphide were placed some years ago on the la

ECHANICAL D

ract. When the difference in temperature between day and night is considerable, the repeated strains of sudden expansion and contraction at

he interior remains comparatively cool. By day the surface shell expands and tends to break loose from the mass of the stone.

ity to absorb heat, and which therefore contract and expand in different ratios. In heating and cooling these g

he roots of plants is blown away over large areas, leaving the rocks bare to the blazing sun in a cloudless sky. The air is dry, and the heat received by the earth by day is therefore rap

thwestern Texas there are frequently heard crackling noises as the rocks of that arid region throw off scales from a fraction of an inch to four inches in thickness, and loud reports are made as huge bowlders split apart. Desert pebbles weakened by long exposure to heat and

has dried out. Water also seeps along joints and bedding planes and gathers in all seams and crevices. Water expands in freezing, ten cubic inches of water freezing to about eleven cubic inches of ice. As water freezes in the rifts and pores of rocks it expands with the irresistible force ill

rock summits upon which I have stood has been nothing but a piled-up heap of fragments" (Fig. 7). In Iceland, in Spitsbergen, in Kamchat

NIC

self into some minute rift, and as it grows slowly wedges the rock apart. Moreover, the acids of the root corrode the rocks with which they are in contact.

et bare of soil; but where lichens are destroying the rock less r

ation is scant or absent. On the contrary, the decay of rocks under the chemical action of water is favored by a warm, moist climate and abundant vegetation. Frost and heat

il may

ing planes assists in the breaking up and de

s never to lay stones on edge, but a

o pieces in early winter, when stones which hav

states often keep their quarry

tone should not be

in fineness of grain and in ratios of e

ntain from readily draining away are more apt to hold their pores ten elevenths full of water

nges, a dark rock or one of a light color?

LE OF RO

sive substances which it contains, leaving their insoluble residues as residual clays and sands upon the surface. As a result there is everywhere forming a mantle of rock waste which covers the land. It is well to ima

is constantly being removed as on the rate at which it is forming. On the face of cliffs it is absent, for here waste is remo

et that it can be removed with pick and shovel. About Atlanta, Georgia, the rocks are completely rotted for one hundred feet from the surface, whil

has been removed to the least extent. The country rock on which it rests is a limestone with about ten per cent of

geological agencies. Residual waste is unstratified. It contains no substances which have not been derived from the weathering of the parent rock. There is a

ns nodules of flint, and we may infer that it has been by the decay and removal of thick masses of limestone that the residual layer of clay and flints has been

nd bowlders composed of many different minerals and rocks that the country rock cannot furnish. Hence the drift cannot have been formed by the decay of the rock of

same in rocks of widely different nature, being chiefly quartz, silicate of alumina, and iron oxide. By the removal of their soluble parts

om that in which they were formed under sea or under ground. In open air, where they are attacked by various destructive agents, few of the rock- making minerals are stable compou

e? In what respects would you expect that the mantle of waste would differ in warm hum

lants, and is then known as soil. The coarser waste beneath is sometimes spoken of as subsoil. Soil usually contains more or less dark, carbonaceous, decaying organic matter, called humus, and is then often termed the humus layer. Soil forms not only on waste produced in place from the rock benea

he surface, or we may find it a layer a foot or more thick, dark, or even black, above, and growing gradually lighter in color as it passes by insensible

ts. Earthworms and other animals whose home is in the waste drag them into their burrows either for food or to line their nests. Trees overthrown by the wind, roots and all, turn over the soil and subsoil and mingle them together. Bacteria also work in the waste and contribute to its enrich

s, extending sometimes as much as fourteen feet below the surface, and build mounds which may reach as high above it. In

tral Africa explorers have walked for miles through forests every tree of which was plastered with these galleries of mud. Each grain of earth used in their construction is moistened and cemented by slime as i

ontain. They treat it with their digestive acids, grind it in their stony gizzards, and void it in castings on the surface of the ground. It was estimated by Darwin that in many parts of England e

brought to the surface, and the rich organic matter which plants and animals have taken from the atmosphere is plowed under. Thus Na

e life of plants and animals. The organic world is built on the ruins of the inorganic, and

oad way, we may view it as a globe of solid rock,-the lithosphere,-surrounded by two mobile envelopes: the envelope of air,-THE ATMOSPHERE, and the envelope of water,-THE HYDROSPHERE. Under the action of solar energy these envelopes are in constant motion. Water from the hydrosphere is continual

y particle. The causes which set its particles in motion, and the differ

THE MANTLE

o the stream and were carried by it onward toward the sea. This process is going on everywhere. Slowly it may be, and with many interruptions, but surely, the waste of the land moves down

all when wedged off by frost or by roots of trees, and when detached by any other agency. On slopes of waste, water freezes in chinks between stones, and in pores between particles of soil, and wedges them apart. Animals and plants stir the waste, heat expands it, cold contracts it, the strokes of the raindrops drive

egrees and left in the open air was found to creep down the slope at the

the falling drops, and washes them down all slopes to within reach of permanent streams. On surfaces unprotected by vegetation, as on plow

ut as fast as formed. Against the walls of an abbey built on a slope in Wales seven hundred years ago, the creeping waste has gathered on the uphill side to a depth of seven feet. The slow-flowing sheet of waste is ofte

the rate of its removal. Talus forms rapidly in climates where mechanical disintegration is most effective, where rocks are readily broken into blocks because closely jointed and thinly bedded rather than massive, and where they are firm eno

ndstone breaks down into incoherent sand grains, which in dry climates, where unprotected by vegetation, may be blown away as fast as they fall, le

greater with coarse and angular fragments than with fine rounded grains. Sooner or later a talus reaches that equilibrium where the amount removed from its surface ju

n more rapid movement than on slopes more gentle, but mountain sides and hills and plains alike come to be mantled with sheets of waste which everywhere is creeping tow

which is going on at all times everywhere about us, are the star

in avalanches to the valleys below. These rushing torrents of snow sweep their tracks c

ong rains and after winter frosts the cohesion between the waste and the sound rock beneath is loosened by seeping water underground

ion. The upper layers, including perhaps the entire mountain side, have been cut across by the valley trench and are left supported only on the inclined surface of the underlying rocks. Water may percolate underground along this surface and loosen the cohesion between the upper and the underlying strata by converting the upper surface of a shale to soft wet clay, by dissolving layers of a limestone, or by removing the cement of a s

for a mile, and clouds of limestone dust were spread over the surrounding country. The debris formed a dam one thousand feet high, extending for two miles along the valley. A lake gathered behind this barrier, gradually rising until it overtopped it in a

one slipped into the Yangtse River in China, reducing the widt

. If spread evenly over a surface of twenty-eight square miles, the material would cover it to a depth of six hundred and

dashed across Champlain Street, wrecking a number of houses and causing the dea

its traces of landslides, so common there is this method of the

PTURE BY

f the forms into which rock ma

een broken along their joints and bedding planes are no longer angular, as are those of the layers below. The edges and corners of the

he surface, leaving the more resistant in relief. Thus massive limestones become pitted where the weather drills out the weaker portions. In these pits, when once

project as cliffs, while the softer weather back to slopes covered with the talus of the harder

ill left as hills and mountain ridges, as in the valleys and mountains of eastern Pennsylvania. But in such instances the lowering of the surface of the w

some, as limestone and gypsum for example, are soluble. Even hard insoluble rocks are weak under the attack of the weather

ons are left behind where the rock is more resistant or where the attack for any

yers in which these fragments were imbedded they are left to strew the surface in the same way as are the residual flints of southern Missouri. flat-topped, because of the protection of a resistant horizontal capping lay

uin, leaving here and there an isolated tower; the tower crumbles to a lonely pillar, soon to be overthrown. The various and often picturesque shapes of monuments depend on the kind of rock, the attitude of the strata, and the agent by which they are chiefly carved. Thus pillar

from the vertical blows of raindrops, and thus come to stand on pedestals of some height. One may sometim

. Running water and glacier ice have cut these folds and blocks into masses divided by deep valleys; but it is by the weather,

ns to sharp, tusklike peaks and ragged, serrat

d stones is incessant. Mountain climbers who have camped at its base tell how huge rocks from time to time come leaping down its precipices, followed by trains of dislodged smaller fragments and rock dust; and how a

harp mountain peaks in a warm arid region

h a cover of waste and vegetation, favors the pro

same way weathering at last reduces to rounded hills the earth blocks cut by streams or formed in any other way. High mountains may at first be sculptured by the weather to savage peaks (

exposed to the weather by a cubic foot at a corner a; by one situated in the middle of an edge b; by one in the cen

s the outcome of a long chain of causes. Thus, in order that the mountain peak may be carved by the agents of disintegration, the waste must be rapidly removed,-a work done by many agents, including some which we are yet to study; and in order that the waste

emains above sea level. If waste were not removed, it would grow to be so thick as to protect the rock beneath from further weathering, and the processes of destruction which we have studied would be brought to an end.

s of the crust in old soils and in rocks pitted and decayed, telling of old land surfaces long wasted by the weather. Ever since the dry land appeared these agencies have been as now quietly and unceasingly at

past. The mountain peak, the rounded hill, the wide plain which lies where hills and mountains once stood, tell clearly of the great results which slow processes will reach when given long time in which to do their work. We should accustom ourselves also to think of the results which weathering will sooner or later bring to pass. The tombstone and the bowlder of the field, w