The Elements of Geology

and in the slow movement of waste down all slopes to the stream ways. We now take up the work of water as it descends beneath the

ingled as vapor with the air, it is carried by the winds over sea and land, and condensed it returns to the earth as rain or snow. That part of the rainfall which descen

poration either directly from the soil or through vegetation; the second RUNS OFF over the surface to

S ROCKS, such as granite, clay, and shale, have pores so minute that the water which they take in is held fast within them by capillary attraction, and none drains through. PERVIOUS ROCKS, on the other hand, such as many sandstones, have

tone may have a pore space of 1 in 4. Sand is so porous that it may absorb

ter is constantly seeping downward under gravity, it is evaporated in the waste and its moisture is carried upward by capillarity and the roots of plants to the surface to be evaporated

n those of the hills and valleys. In dry climates permanent ground water may be found only at depths of hundreds of feet. Ground water is held at its height by the fact that its circulation

surface permanent streams are formed, the water either oozing forth along ill-defined areas or issuing at definite points called springs, where it is

hermometer during successive months, we shall find that its temperature remains much the same the year round. In summer it is markedly cooler than the stream; in winter it is warmer and remains

s minerals in solution, but these are usually in quantities so small that they are not perceptible to the taste or feel. But the water of springs is often well charged with soluble minerals; in its slow, long journey underground it has searched out the soluble parts of the rocks through which it seeps and has dissolved as much of them as it could. When spring water is b

o. Such waters are "hard"; when used in washing, the minerals which they contain combine with the fatty acids of soap to form insoluble curdy compounds. When spri

hat of surface streams. So slow is the movement of ground water through the rocks that ev

rate varies according to many conditions, but always is extremely slow. Even through loo

re the descending movement of the water is stopped. The DEEP ZONES OF FLOW occupy any pervious rocks which may be found below the impervious layer which lies nearest to the surface. The upper zone is a vast sheet of water saturating the soil

driven wells, within the city limits, sunk in the sand a few yards from the edge of the stream. Are these wells fed by water from the

eason? Have you ever known wells to go dry? It may be possible to get data from different wells

f the water contained in the higher parts of the stratum, and may reach the surface as a fissure spring. A boring which taps such an aquifer is known as an artesian well, a name derived from a province in France where wells of this kind have been long in use. The rise of the water in artesian wells, and in fissure springs also, depends on the following conditions illustrated in Figure 29. The aquifer di

p is gentle? Which of the two aquifers, their thickness being equal, will have the larger outcrop and

minerals upon which plants depend for their food. It takes away the soluble cements of rocks; it widens fissures and joints and opens winding passages along the bedding planes; it may even remove whole beds of soluble rocks, s

tucky, where an upland is built throughout of thick horizontal beds of limestone. The absence of layers of insoluble or impervious rock in its structure allows a free circulation of ground water within it by

erage height of forty feet. One of the great halls is three hundred feet in width and is overhung by a solid arch of limestone one hundred feet above the floor. Galleries at different levels are connected by well-like pits, some of which

wered by weathering, the roof at last breaks down and the cave becomes an open r

hat the water of rains rapidly drains away through them. Ground water stands low and

finds its way underground by means of sink holes. These are pits, commonly funnel shaped, formed by the enlargement of crevice or joint by perc

ied by lakelets. Great springs mark the point of issue of underground streams, while some rise from beneath the sea. Silver Spring, one of the largest, discharges from a b

feet deep and more, pockmark the surface of the land. The drainage is chiefly subterranean. Surface streams are rare and a portion of their courses is often under ground. Fragmentary valleys come suddenly to an end at walls of rock where the rivers which occupy the valleys plunge into dark tunnels to reappear some miles away. Ground water stands so far below the surface that it cannot be reached by wells, and the inhabitants depend on rain water stored for household uses. The

s whose underground drainage is so perfect that the development of surface streams has been retarded or prevented escape to a large extent the leveling action of su

of limestone above is deposited by evaporation in the air in icicle-like pendants called STALACTITES. As the drops splash on the floor there are built up in the same way thicker masses called STALAGMITES, which may gr

actites formed by the drip of si

ch depths it deposits these minerals in the pores of rocks, cementing their grains together, and in crevices and fissures, forming mineral veins. Thus below the zone of solution where the work of wa

ementation are brought within the zone of solution. Thus there are exposed to view limestones, whose cracks were filled with calcite (cr

constant circulation underground. The process may be illustrated by the deposit of salt crystals in a cup of evaporating brine, but in the latter

center. Nor are they formed in already existing cavities as are geodes. In soft clays concretions may, as they grow, press the clay aside. In many other rocks concretions are made by the process of REPLACEMENT. Molecule by molecule the rock is removed and the mineral of the concretion substituted in its pl

ly worked when "green" than after

s the minerals held in solution. White limy incrustations of this nature cover considerable tracts in northern Mexico. Evaporating beneath the surface, ground water may

activity, such as the Yellowstone National Park, we may believe that the heat of thermal springs is derived from uncooled lavas, perhaps not far below the surface. But when hot springs occur at a distance of hundreds of miles from any volcano, as in the case of the hot springs of Bath, England, it is probable that their waters have risen from the heated rocks of the

re. It is absorbed by highly heated rocks deep below the surface. From time to time some

xposure to the air it is commonly compelled to lay down much of its invisible load in chemical deposits about the spring. T

e from the limestone rocks through which they pass. On reaching the surface the pressure is relieved, the gas escapes, and the lime carbonate is thrown down in

ow building, chiefly by means of algae which cover the bottoms, rims, and sides of the basins and deposit lime carbonate upon them in succes

the springs which formed these ancient deposits discharged near what was then the bottom of the valley, and that a

s, twigs, and other objects over which their waters flow. Such are popularly known as petr

en deposit silica in a white spongy formation known as SILICEOUS SINTER, both by evaporation and by the action of algae which secrete silica

oil, but at a touch it cracks and breaks, as oil would not do. It is a film of hydrated iron oxide, or LIMONITE, and the spring is an iron, or chalybeate, spring. C

ts a few feet below the surface, where their downward progress is arrested by some impervious clay

most red and yellow rocks are decomposed, leaving the rocks of a gray or bluish color, and the soluble iron compounds whi

ring; but out of contact with the air and in the presence of carbon dioxide supplied by

he crust becomes greater than the rocks can bear, and all cavities and pores in them must be completely closed by the enormous pressure which they sustain. Below a depth of even three or four miles it is believed that ground water cannot circulate. Estimating the average pore spaces of