CHAPTER II.. The Atmosphere As a Geological Agent.

THE ATMOSPHERE AS A GEOLOGICAL AGENT.

While it is convenient to regard the lithosphere as the earth proper, and the atmosphere as its envelope, the latter is as truly a part of the planet as the former, and its activities and its history are as truly subjects of geological study as the formation of the rocks. This view is in no way vitiated by the fact that the special study of the atmosphere is set apart under the name Meteorology, for in the same way the special study of rocks is set apart under the name Petrology, that of ancient life under the name Paleontology, and that of other phases of the subject under other names. The atmosphere is one of the three great formations of the earth, and as a geological factor takes its place beside the hydrosphere and the lithosphere. It has played a part in the history of the earth comparable to that of the water, though its mass is less and its record more elusive. Unsubstantial as the atmosphere seems when contrasted with the liquid and solid portions of the earth, its extreme mobility and its chemical activity compensate for its lightness and tenuity, and give it a function of the first order of importance.

The atmosphere plays a direct part as (1) a mechanical and (2) a chemical agent, and at the same time serves an indirect function in furnishing favorable conditions under which (3) solar radiation produces temperature effects, and (4) evaporation gives origin to precipitation and stream effects, and furnishes the necessary conditions for land plants and animals, and the important influences that spring from them.

This chapter is devoted to the work of the atmosphere in these and some less notable phases. The consideration of the origin and history of the atmosphere will receive attention later.

A. THE ATMOSPHERE AS A DIRECT AGENCY.

I. Mechanical Work.

The mechanical work of the atmosphere is accomplished chiefly through its movement. A feeble breeze is competent to move particles of dust, and winds of moderate velocity to shift sand. Exceptionally strong winds sometimes move small pebbles, but winds of sufficient force to move larger pieces of rock are rare. It follows, therefore, that the impact of the wind has little direct effect except on surfaces covered with dust and dry sand.

The transportation of material by the wind is limited by the size of the particles to which it has access. Dust particles expose more surface to the wind relative to their mass than sand grains. Winds which are unable to carry sand may still carry dust, and winds which are able to shift sand no more than a trivial distance may blow dust great distances.

The common conception of wind as a horizontal movement of some part of the atmosphere is not altogether accurate. Every obstacle against which wind blows causes deflections of its currents, and some of these deflections are upward. Furthermore, there are exceptional winds, in which the vertical element predominates. Particles of dust are often involved in these upward currents, and by them carried to great heights, and in the upper air are transported great distances.

=Transportation and deposition of dust.=—The universality of the transportation of dust by the wind is well known. No house, no room, and scarcely a drawer can be so tightly closed but that dust enters it, and the movements of dust in the open must be much more considerable. The visible dustiness of the atmosphere in dry regions during wind-storms is adequate and familiar proof of the efficiency of the wind as a transporter of dust.

Under special circumstances, opportunity is afforded for rough determinations of the distance and height to which wind-blown dust is transported. Snow taken from snow-fields in high mountain regions is found to contain a small amount of earthy matter. Its particles are often found to be in part volcanic, even when the place whence the snow was taken is scores or even hundreds of miles from the nearest volcano. There is probably no snow-field so high, or so far from volcanoes, but that volcanic dust reaches it. If this be true of all snow-fields, it is probably true of all land surfaces. In the great Krakatoa eruptions of 1883 large quantities of volcanic ash (pulverized lava) were projected to great heights into the atmosphere. The coarser particles soon settled; but, caught by the currents of the upper atmosphere, many of the finer particles were transported incredible distances. Through all their long journey, the particles of dust were gradually settling from the atmosphere, but not until the dust had traveled repeatedly round the earth did its amount become so small as to cease to make its influence felt in the historic red sunsets which it occasioned. Some of this dust completed the circuit of the earth in 15 days.

In various parts of Kansas and Nebraska there are very considerable beds of volcanic dust, locally as much as 30 feet thick, which must have been transported from volcanic vents by the wind, though there are no known centers of volcanic action, past or present, within some hundreds of miles of some of the localities where the dust occurs. These beds of volcanic dust, so far from its source, may serve as an illustration of the importance of atmospheric movements as a geological force.

Volcanic dust is shot into the atmosphere rather than picked up by it. Dust picked up by the wind is perhaps transported not less widely than volcanic dust, but, after settling, its point of origin is less readily determined. It would perhaps be an exaggeration to say that every square mile of land surface contains particles of dust brought to it by the wind from every other square mile, but such a statement would probably involve much less exaggeration than might at first be supposed.

Examples of extensive deposits of dust other than volcanic are also known. In China there is an extensive earthy formation, the loess, sometimes reaching 1,000 feet in thickness, which von Richtofen believes to have been deposited by the wind. This conclusion has, however, not passed unchallenged. The loess of some other regions has been referred to the same origin, and some of it is quite certainly eolian.

The transportation of dust is important wherever strong winds blow over dry surfaces, free or nearly free of vegetation, and composed of earthy matter. Its effects may be seen in such regions as the sage-brush plains of western North America. The roots of the sage-brush hold the soil immediately about them, but between the clumps of brush, where there is little other vegetation, the wind has often blown away the soil to such an extent that each clump of brush stands up several inches, or even a foot or two, above its surroundings (Fig. 5). Such mounds are often partly due to the lodgment of dust about the bushes.

Where the earthy matter is moist, the cohesion of the particles is great, and the wind cannot pick them up. Furthermore, if the surface is generally moist, it is likely to be covered with vegetation which protects it against the wind. But even where vegetation is prevalent the wind finds many a vulnerable point. Thus on the edges of plains or plateaus facing abrupt valleys, the wind attacks the soil from the side, and in such situations all earthy matter may be stripped from the underlying rock for considerable distances from the edge of the cliff (Fig. 6). This may be seen at numerous points on the lava plateaus of Washington.

The presence of dust in the upper atmosphere during a rain-storm is sometimes the occasion of phenomena which are often misinterpreted. If there be abundant dust in the atmosphere through which rain-drops or snowflakes fall, much of it is gathered up by them, and the water is thereby rendered turbid and the snow discolored. Here is to be found the explanation of “mud-rain,” “blood-rain” (red dust), etc.

Since dust is carried to a considerable extent in the upper atmosphere, its movements and its deposition are little affected by obstacles on the surface of the land. A building or a hedge can only affect the lodgment of that part of the atmospheric dust which comes in contact with it or is swept into its lee. Since most obstacles on the surface of the solid part of the earth reach up but slight distances into the atmosphere, the dust of the greater part of the air settles without especial reference to them, and is spread more or less uniformly over the surface on which it falls.

Much of the dust transported by the wind is carried out over seas or lakes and falls into them. By this means, sedimentation is doubtless going on at the bottom of the whole ocean, and at the bottoms of all lakes. While means of determining the amount of dust blown into the sea are not at hand, it is safe to say that, were such determinations possible, the result, if stated in terms of weight, would be surprising.

=Transportation and deposition of sand.=—In its transportation by the wind, sand is not commonly lifted far above the surface of the land, and its movement is therefore more generally interfered with by surface obstacles than is the movement of dust. A shrub, a tree, a fence, a building, or even a stone may occasion the lodgment of sand in considerable quantity, though it has little effect on the lodgment of dust. The effect of obstacles is illustrated by Fig. 7 (see also Fig. 5). If the obstacle which occasions the lodgment of sand presents a surface which the wind cannot penetrate, such as a wall, sand is dropped abundantly on its windward side as well as on the leeward; but if it be penetrable, like an open fence, the lodgment takes place chiefly on its leeward side. In cultivated regions cases are known where, in a few weeks of dry weather, sand has been drifted into lanes in the lee of hedges to the depth of two or three feet, making them nearly impassable to vehicles.

=Formation of dunes.=—In contrast with dust deposited from the atmosphere, wind-blown sand is commonly aggregated into mounds and ridges in the process of lodgment. These mounds and ridges are dunes. Once a dune is started, it occasions the further lodgment of sand, and is a cause of its own growth. Dunes sometimes reach heights of 200 or 300 feet, but they are much more commonly no more than 10 or 20 feet in height. On plane surfaces, there is a limit in height above which they do not rise, though the limit is different under different conditions. The velocity of wind at the bottom of the air is not so great as that higher up, and as a dune is built up, a level is presently reached where the stronger upper winds sweep away as much sand as is brought to the top. The very even crests of many dune ridges are probably to be accounted for in this way. Wind-blown or eolian sand, not piled up in heaps or ridges, is somewhat widespread, but does not constitute dunes.

=Shapes of dunes.=—Dunes may assume the form of ridges or of hillocks. The ridges may be transverse to the direction of the prevailing wind or parallel with it. Where dunes assume the form of hillocks rather than ridges, a group of them may be elongate in a direction parallel to the dominant wind, or at right angles thereto. The shape assumed by a dune or a group of dunes depends on the abundance of the sand, the strength and direction of the wind, and the shape of the obstacle which occasions the lodgment.

The incipient stages of dune formation are readily seen in many dry, sandy regions. The dune is likely to start in the lee of some obstacle, and to be elongate in the direction of the wind, especially if the wind be strong relative to the supply of sand. This shape is permanently preserved if the proper relations between the supply of sand and strength and direction of wind are preserved. In the dune region of the Indian desert the prevailing winds are alternately the southwest and northeast monsoons, the former being the stronger. The supply of sand comes from the southwest. Near the southwest coast the dune ridges are parallel to the direction of the wind (Fig. 8); in the interior, where the winds are less strong, the dunes are transverse to it (Fig. 9); while between the districts where these two types prevail intermediate forms occur. The transverse dune ridges (Fig. 9) are said to be the result of the lateral growth and erosion of longitudinal dunes. In regions of changeable winds the shape of the dunes is subject to great variation. Dunes are sometimes crescentic, the convexity facing the wind (Fig. 10).

Along coasts, dune ridges are often transverse to the wind, and groups of dune hillocks are frequently elongate in the same direction. Here the source of supply of the sand is itself an elongate belt, often transverse to the dominant wind, and the resulting dunes often have great length transverse to the wind. Where the wind has strong mastery over the sand, the longitudinal tendency is seen, even along coasts.

The shapes of dunes in section, like the shapes in ground-plan, depend on the relative strength and constancy of the winds and the supply of sand. With constant winds and abundant drifting sand, dunes are steep on the lee side (bc, Fig. 11), where the angle of slope is the angle of rest for the sand. It rarely exceeds 23° or 24°. Under the same conditions the windward slope is relatively gentle (ab, Fig. 11). If the winds be variable so that the windward slope of one period becomes the leeward slope of another, and vice versa, this form is not preserved. Thus, by reversal of the wind, the section abc, Fig. 11, may be changed to adc. If the winds and the supply of sand be equal, on the average, from opposite directions, the slopes should, on the average, be equal, though perhaps unequal after any particular storm. The steep slopes of new-made dunes are lost after the sand has ceased to be blown. At some points where the winds erode (scour) more than they deposit, new profiles are developed (Figs. 12 and 13). The erosion profiles may be very irregular if the dunes are partially covered with vegetation. The effect of vegetation in restraining wind erosion is shown in Fig. 14, where plants have preserved a remnant of a dune.

=The topographic map.=—Since dunes as well as other topographic features are conveniently represented on contour maps, and since such maps will be used frequently in the following pages, a general explanation of them is here introduced.

“The features represented on the topographic map are of three distinct kinds: (1) inequalities of surface, called relief, as plains, plateaus, valleys, hills, and mountains; (2) distribution of water, called drainage, as streams, lakes, and swamps; (3) the works of man, called culture, as roads, railroads, boundaries, villages, and cities.

=Relief.=—All elevations are measured from mean sea-level. The heights of many points are accurately determined, and those which are most important are given on the map in figures. It is desirable, however, to give the elevation of all parts of the area mapped, to delineate the horizontal outline, or contour, of all slopes, and to indicate their grade or degree of steepness. This is done by lines connecting points of equal elevation above mean sea-level, the lines being drawn at regular vertical intervals. These lines are called contours, and the uniform vertical space between each two contours is called the contour interval. On the maps of the United States Geological Survey the contours and elevations are printed in brown (see Plate II).

The manner in which contours express elevation, form, and grade is shown in the following sketch and corresponding contour map, Fig. 15.

The sketch represents a river valley between two hills. In the foreground is the sea, with a bay which is partly closed by a hooked sand bar. On each side of the valley is a terrace. From the terrace on the right a hill rises gradually, while from that on the left the ground ascends steeply in a precipice. Contrasted with this precipice is the gentle descent of the slope at the left. In the map each of these features is indicated, directly beneath its position in the sketch, by contours. The following explanation may make clearer the manner in which contours delineate elevation, form, and grade:

1. A contour indicates approximately a certain height above sea-level. In this illustration the contour interval is 50 feet; therefore the contours are drawn at 50, 100, 150, 200 feet, and so on, above sea-level. Along the contour at 250 feet lie all points of the surface 250 feet above sea; and similarly with any other contour. In the space between any two contours are found all elevations above the lower and below the higher contour. Thus the contour at 150 feet falls just below the edge of the terrace, while that at 200 feet lies above the terrace; therefore all points on the terrace are shown to be more than 150 but less than 200 feet above sea. The summit of the higher hill is stated to be 670 feet above sea; accordingly the contour at 650 feet surrounds it. In this illustration nearly all the contours are numbered. Where this is not possible, certain contours—say every fifth one—are accentuated and numbered; the heights of others may then be ascertained by counting up or down from a numbered contour.

2. Contours define the forms of slopes. Since contours are continuous horizontal lines conforming to the surface of the ground, they wind smoothly about smooth surfaces, recede into all reëntrant angles of ravines, and project in passing about prominences. The relations of contour curves and angles to forms of the landscape can be traced in the map and sketch.

3. Contours show the approximate grade of any slope. The vertical space between two contours is the same, whether they lie along a cliff or on a gentle slope; but to rise a given height on a gentle slope one must go farther than on a steep slope, and therefore contours are far apart on gentle slopes and near together on steep ones.

For a flat or gently undulating country a small contour interval is used; for a steep or mountainous country a large interval is necessary. The smallest interval used on the atlas sheets of the Geological Survey is 5 feet. This is used for regions like the Mississippi delta and the Dismal Swamp. In mapping great mountain masses, like those in Colorado, the interval may be 250 feet. For intermediate relief contour intervals of 10, 20, 25, 50, and 100 feet are used.

=Drainage.=—Watercourses are indicated by blue lines. If the streams flow the year round the line is drawn unbroken, but if the channel is dry a part of the year the line is broken or dotted. Where a stream sinks and reappears at the surface, the supposed underground course is shown by a broken blue line. Lakes, marshes, and other bodies of water are also shown in blue, by appropriate conventional signs.

=Culture.=—The works of man, such as road, railroads, and towns, together with boundaries of townships, counties and states, and artificial details, are printed in black.”

=Topography of dune areas.=—From what has been said, it is clear that the topography of dune regions may vary widely, but it is always distinctive. Where the dunes take the form of ridges (Fig. 1, Pl. II), the ridges are often of essentially uniform height and width for considerable distances. If there are parallel ridges, they are often separated by trough-like depressions. Where dunes assume the form of hillocks (Figs. 2 and 3, Pl. II), rather than ridges, the topography is even more distinctive. In some regions, depressions (basins) are associated with the dune hillocks. Occasionally they are hardly less notable than the dunes themselves. A somewhat similar association of hillocks and basins is locally developed by other means, but dunes are made up of sand and usually of sand only, while the composition of similarly shaped hillocks and depressions shaped by other agencies is notably different.

In Fig. 1, Plate II (Five Mile Beach, 8 miles northeast of Cape May, N. J.), the contour interval is 10 feet. There is here but one contour line (the 10-foot contour), though this appears in several places. Since this line connects places 10 feet above sea-level, all places between it and the sea (or marsh) are less than 10 feet above the water, while all places within the lines have an elevation of more than 10 feet. None of them reaches an elevation of 20 feet, since a 20-foot contour does not appear. It will be seen that some of the elevations in Fig. 1 are elongate, while others have the form of mounds.

Fig. 2 (Pl. II) shows dune topography along the Arkansas River in Kansas (near Larned); Fig. 4, dune topography in Nebraska (Lat. 42°, Long. 103°), not in immediate association with a valley or shore; and Fig. 3 shows irregular ridge-like dunes at the head of Lake Michigan. In Fig. 2 the contour interval is 20 feet. All the small hillocks southeast of the river are dunes. Some of them are represented by one contour and some by two. The altitude of the region is considerable, the heavy contour representing an elevation of 2100 feet; but the dunes themselves are rarely more than 20 feet above their surroundings. In Fig. 4, where the contour interval is also 20 feet, there are, besides the numerous hillocks, several depressions (basins). These are represented by hachures inside the contour lines. In some cases there are intermittent lakes (blue) in the depressions. The heavy contour at Spring Lakes in this figure is the contour of 4300 feet. There are two depression contours (4280 and 4260) below it. The bottom of the depression is therefore lower than 4260, but not so low as 4240. In Fig. 3 the contour interval is 10 feet, and the dune ridges north of Miller are more than 50 feet high. The dune ridges here have helped to determine the position of this branch of the Calumet River, and have blocked its former outlet. The present drainage is to the westward.

=Migration of dunes.=—By the continual transfer of sand from its windward to its leeward side, a dune may be moved from one place to another, though continuing to be made up, in large part, of the same sand. In their migration dunes sometimes invade fertile lands, causing so great loss that means are devised for stopping them. The simplest method (employed in France and Holland) is to help vegetation to get a foothold in the sand. The effect of the vegetation is to pin the sand down. As a dune ridge along a coast travels inland, another may be formed behind it. Successions of dune ridges are thus sometimes formed.

A remarkable instance of the migration of a sand dune is recorded on the Kurische Nehrung on the north coast of Germany. The Nehrung consists of a long narrow neck of land composed of sand, lying off the main coast. At the beginning of this century there was a notable dune ridge on one side. Since that time it has migrated a considerable distance, and in its migration it has been brought into the relationships illustrated in the accompanying diagrams (Fig. 16). In 1800 the dune ridge was on one side of a church, which was then in use. In 1839 the ridge had been so far shifted to the leeward as to completely bury the church, and in 1869, its migration had progressed so far as to again discover the building.

When dunes migrate into a timbered region they bury and kill the trees (Fig. 17). In one instance on the coast of Prussia a tall pine forest, covering hundreds of acres, was destroyed during the brief period between 1804 and 1827. At some points in New Jersey orchards have been so far buried within the lifetime of their owners that only the tops of the highest trees are exposed. Trees and other objects once buried may be again discovered by farther migration of the sand (Figs. 18 and 19).

Eolian sand, not aggregated into distinct dunes, is often destructive. Even valleys and cities are sometimes buried by it. Drifting sands had so completely buried Nineveh two centuries after its destruction that its site was unknown.

=Distribution of dunes.=—Dunes are likely to be developed wherever dry sand is exposed to the wind. Their favorite situations are the dry and sandy shores of lakes and seas, sandy valleys, and arid sandy plains.

Along coasts, dunes are likely to be extensively developed only where the prevailing winds are on shore. Thus about Lake Michigan, where the prevailing winds are from the west, dunes are abundant and large on the east shore, and but few and small on the west. In shallow water, shore currents and storm waves often build up a reef of sand a little above the normal level of the water. When the waves subside, the sand dries and the wind heaps it up into dunes. This sequence of events is in progress at many points on the Atlantic Coast. Sandy Hook, New Jersey, and the “beaches” farther south started as barrier ridges. When the waves had built them above normal water-level, the wind re-worked the sand, piling it up into mounds and hillocks (Fig. 1, Pl. II). Such dune belts a little off shore are sometimes turned to good account. They are usually separated from the mainland by a shallow lagoon. Where land is valuable, the lagoon is sometimes filled in, making new land, thus anticipating the result which nature would achieve more slowly. This has been done at some points on the western coast of Europe.

Dunes are likely to occur along stream valleys (Fig. 2, Pl. II), if their bottoms or slopes are of sand, and not covered by vegetation. Dunes along valleys are usually on the side toward which the prevailing winds blow. Thus they are more common on the east side of the Mississippi than on the west. Dunes may be formed in the valley bottoms, but the sand is often blown up out of the valley and lodged on the bluffs above.

Apart from these special classes of situations, any sandy region the surface of which is dry is likely to have its surface material shifted by the wind and piled up into dune ridges or hillocks (Fig. 4, Pl. II). Dunes probably reach their greatest development in the Sahara, where some of them take the form of hillocks, and some the form of ridges. Travelers in that region report that dune ridges are sometimes encountered the faces of which are so high and steep as to be difficult of ascent, and that parties have been obliged to travel miles along their bases before finding a break where crossing was practicable.

=Wind-ripples.=—The surface of the dry sand over which the wind has blown for a few hours is likely to be marked with ripples (Fig. 20) similar to those made on a sandy bottom beneath shallow water, under the influence of waves. Like ripple marks made by the water, wind-ripples have one side (the lee) steeper than the other. While the ripples are, as a rule, but a fraction of an inch high, they throw much light on the origin of the great dune ridges. If the ripples be watched closely during the progress of a wind-storm, they are found gradually to shift their position. Sand is blown up the gentler windward slope to the crest of the ridge and falls down on the other side. The moment it falls below the crest of the ridge to leeward, it is protected against the wind, and is likely to lodge. Wear on the windward side is about equal to deposition on the leeward, and the result is the orderly progression of the ripples in the direction in which the wind is blowing, just as in the case of dune ridges.

=Abrasion by the wind.=—While the effect of the wind on sandy and dusty surfaces may be considerable, its effect on solid rock is relatively slight and accomplished, not by its own impact, but by that of the material it carries. The effect of blown sand on rock surfaces over and against which it is driven is perhaps best understood by recalling the effects of artificial sand-blasts, by means of which glass is etched. In a region where sand is blowing, exposed surfaces of rock suffer from a multitude of blows struck by the sand grains in transit. The result is that such rock surfaces are worn, and worn in a way peculiar to the agency accomplishing the work. If the rock be made up of laminæ which are of unequal hardness, the blown sand digs out the softer ones, leaving the harder projecting as ridges between them. Adjacent masses of harder and softer rock of whatever thickness are similarly affected. The sculpturing thus effected on projecting masses of rock is often picturesque and striking (Figs. 21 and 22), and is most common in arid regions. Details of wind-carving are shown in Fig. 23.

Fig. 1. NEW JERSEY.

U. S. Geol. Surv.

Fig. 2. KANSAS.

U. S. Geol. Surv.

Fig. 3. INDIANA.

U. S. Geol. Surv.

Fig. 4. NEBRASKA.

U. S. Geol. Surv.]

Sand drifted over loose stones lying on the surface often develops flat or flattish faces or facets on them. These facets are likely to be three in number, and the exposed portion of the stone is likely to develop a sort of pyramidal shape, the three flattish surfaces being mutually limited by tolerably well-defined lines (Fig. 24). Thus arise the three-faceted stones (Dreikanter of the Germans) commonly seen where sands have been long in movement.

Not only does the drifting sand wear the surface over which it passes and against which it strikes, but the grains themselves are worn in the process. They are liable to be broken as they strike rock surfaces, and they are likely to strike one another in the atmosphere. In both cases they are subject to wear, and so to reduction to a finer and finer state.

The erosion accomplished by the wind is therefore of various sorts. The impact of the wind itself picks up the fine materials which are already loosened, thus wearing down the surface from which they are removed; the materials picked up wear the rock surfaces against which they are blown, and the transported materials themselves suffer reduction in transit.

=Effects of wind on plants.=—Another effect of wind work is seen in the uprooting of trees (Fig. 25). The uprooting disturbs the surface in such a way as to make loose earth more readily accessible to wind and water. The uprooting of trees on steep slopes often causes the descent of considerable quantities of loose rock and soil. Again, organisms of various sorts (certain types of seeds, germs, etc.), as well as dust and sand, are extensively transported by the wind. While this is important biologically its geological effects are remote.

=Indirect effects of the wind.=—Other dynamic processes are called into being or stimulated by the atmosphere. Winds generate both waves and currents, and both are effective agents in geological work. The results of their activities are discussed elsewhere.

II. The Chemical Work of the Atmosphere.

The chemical work of the atmosphere (including solution and precipitation from solution) is principally accomplished in connection with water, a dry atmosphere having relatively little direct chemical effect on rock or soils.

=Precipitation from solution.=—The water in the soil is constantly evaporating. Such substances as it contains in solution are deposited where the water evaporates, and where evaporation is long continued without re-solution of the substances deposited, the surface becomes coated with an efflorescence of mineral matter. Conspicuous examples are found in the alkali plains of certain areas in the western part of the United States. Since the alkaline efflorescence is the result of evaporation it is connected with the atmosphere, but the material of the efflorescence was brought to its present position by water. The principle involved is illustrated by the white efflorescence which frequently appears on brick walls during the dry days which follow a drenching rain. The water penetrates the brick and mortar and dissolves something of their substance, and when it is evaporated from the surface the material in solution is left behind.

In arid regions the deposition of substances other than alkali is common. The percolating waters dissolve whatever is soluble, and when they evaporate their mineral content is left. The pebbles and stones of the arid plains have in many places become heavily coated with mineral matter deposited in this way, and not infrequently cemented into conglomerate. One of the commonest mineral substances found in such situations is lime carbonate. In some cases it was doubtless derived by solution from limestone beds beneath the surface, but this is not always the case. It often encrusts the bits of lava on lava plains where it can hardly have been derived from limestone. The faces of cliffs of granite or gneiss, hundreds and even thousands of feet above all other sorts of rock, are sometimes spotted with patches of lime carbonate. In the first case the lime carbonate was derived by chemical change from the lava, and in the second, from the granite or gneiss (see Carbonation below), but its present position is the result of evaporation.

=Oxidation.=—In the presence of moisture the oxygen of the air enters into combination with various elements of the soil and rocks. This is oxidation. No other common mineral substance shows the results of oxidation so quickly and so distinctly as iron. The oxidized portion is loose and friable, and a mass of iron exposed to a moist atmosphere will ultimately crumble away. This change is comparable to other less obvious changes taking place in many minerals at and below the surface. Oxidation generally involves the disintegration of the rock concerned. Its effects in this direction will be referred to in other connections.

=Carbonation.=—The production of lime carbonate from rock containing calcium compounds, but not in the form of carbonates, is known as carbonation, and is one of the important chemical changes effected by the carbon dioxide of the atmosphere in coöperation with water. In the process of carbonation the original minerals of complex composition are decomposed and simpler ones usually formed. Volumetric changes are involved, which often lead to the disruption of the rock (see Ground water). Furthermore, carbonates are among the more soluble minerals, and their production therefore brings some of the rock materials into a soluble condition, and their extraction through solution tends still further to disintegrate the rock. The carbonation of crystalline rocks is therefore a disintegrating process, and will be considered further in its many concrete applications.

=Other chemical changes.=—A third chemical process which often accompanies oxidation and carbonation is hydration. This is effected by water rather than by air, and will be considered in that connection. In general it leads to the disintegration of the minerals and rocks affected. The chemical effects of nitric acid, etc., developed through the agency of atmospheric electricity, and the corresponding effects of the gases and vapors which issue from volcanoes, many of them chemically active, are to be mentioned in this connection.

=Conditions favorable for chemical changes.=—Conditions are not everywhere equally favorable for the chemical work of the atmosphere. In general, high temperatures facilitate chemical action, and, other things being equal, rocks are more readily decomposed by atmospheric action in warm than in cold regions. Chemical activity is probably greater where the climate is continuously warm than where there are great changes of temperature. Changes of temperature, on the other hand, tend to disrupt rock, and thus increase the amount of surface exposed to chemical change. Since nearly all the chemical changes worked by the atmosphere on the rocks are increased by the presence of moisture, the chemical activity of the atmosphere is greater in moist than in dry regions.

B. THE ATMOSPHERE AS A CONDITIONING AGENCY.

The most obvious mechanical work of the atmosphere is effected by the wind, but mechanical results of great importance, conditioned by the atmosphere, are also effected when the air is still.

I. Temperature Effects.

When the sun shines on bare rock its surface is heated and expanded, and the expanded particles crowd one another with great force. Since rock is a poor conductor of heat its surface is heated and expanded notably more than parts beneath the surface. It follows that strains are set up between the expanded outer portion and the cooler and less expanded parts within. In the cooling of the same rock mass it is the outermost portion which cools first and fastest, and, contracting as it cools, strains are again set up between the outer part, which is cooled more, and the inner part, which is cooled less. The result may be illustrated by the effect of cold water on hot glass, or of hot water on cold glass. In either case the fracture is the result of the sudden and considerable differential expansion or contraction. Since the heating and cooling of rock are much slower than the heating and cooling of glass under the conditions mentioned, the rupturing effects are less conspicuous, but none the less real. The actual effects of temperature changes are illustrated by familiar phenomena. The surface portions of bowlders exposed to the sun are frequently seen to be shelling off (Fig. 26). The loosened concentric shells may be a fraction of an inch, or sometimes even several inches in thickness. This process of exfoliation affects not only bowlders, but bare rock surfaces wherever exposed to the sun (Figs. 27, 28). It is often conspicuous on the faces of cliffs.

Several conditions, some of which are connected with the atmosphere and some with the rock, determine the efficiency of this process. Since the breaking of the rock results from the expansion and contraction due to its changes of temperature, it follows that, other things being equal, the greater the change, the greater the breaking; but the suddenness of the temperature change is even more important than its amount. It follows that great daily, rather than great annual, changes of temperature favor rock-breaking, though with changes of a given frequency their effectiveness is greater the greater their range. A partial exception to this generalization should be noted. If abundant moisture is present in the pores and cracks of the rock a change of temperature from 45° to 35° (Fahr.) might be far less effective in breaking the rock than a change from 35° to 25° in the same time, for in the latter case the sudden and very considerable expansion (about one-tenth) which water undergoes on freezing is brought into play. This may be called the wedge-work of ice. The daily range of temperature is influenced especially by latitude, altitude, and humidity. Other things being equal, the greatest daily ranges of temperature occur in high-temperate latitudes, though to this general statement there are local exceptions, depending on other conditions. High altitudes favor great daily ranges of temperature, so far as the rock surface is concerned (see Figs. 29, 30), for though the rock becomes heated during the sunny day, the thinness and dryness of the atmosphere allow its heat to radiate rapidly at night. Here, too, the daily range of temperature is likely to bring the wedge-work of ice into play. Since the south side of a mountain (in the northern hemisphere) is heated more than the north, it is subject to the greater daily range of temperature, and the rock on this side suffers the greater disruption. Similarly, rock surfaces on which the sun shines daily are subject to greater disruption than those much shielded by clouds. Isolated peaks, because of their greater exposure, are subject to rather greater daily ranges of temperature than plateaus of the same elevation.

The daily range of temperature is also influenced by humidity. Because of the effect of water vapor in the atmosphere on insolation and radiation, a rock surface becomes hotter in the day and cooler at night beneath a dry atmosphere than beneath a moist one. Aridity therefore favors the disruption of rock by changing temperatures.

Turning from the conditions of the atmosphere which affect the disruption of rock to the conditions of the rock which influence the same process, several points are to be noted. In the first place, the disrupting effects of changes of temperature are slight or nil where the solid rock is protected by soil, clay, sand, gravel, snow, or other incoherent material. If the constituent parts of the loose material are coarse, like bowlders, their surfaces are affected like those of larger bodies of rock. The color of rock, its texture and its composition, also influence its range of daily temperature by influencing absorption and conduction. Dark-colored rocks absorb more heat than light-colored ones, and compact rocks are better conductors than porous ones. Great absorption and slow conduction favor disruption. A given range of temperature is unequally effective on rocks of different mineral composition. In general crystalline rocks (igneous and metamorphic) are more subject to disruption by this means than sedimentary rocks, partly because they are more compact, but especially because they are made up of aggregates of crystals of different minerals which, under changes of temperature, expand and contract at different rates, while the common sedimentary rocks are made up largely of numerous particles of one mineral.

The freezing of water in the pores of rock is effective in disrupting them only when the pores are essentially full at the time of freezing. Otherwise there is room for the expansion attending the freezing. If the pores of the rock are large, the expansion on freezing may force out sufficient water to balance the increase of volume, even though the rock was completely saturated. If the pores be very small the water passes out less readily, and if the rock is saturated, freezing is more likely to be attended with disruption.

In view of these considerations the breaking of rock by changes of temperature should be greatest on the bare slopes of isolated elevations of crystalline rock, where the temperature conditions of temperate latitudes prevail, and where the atmosphere is relatively free from moisture. All these conditions are not often found in one place, but the disrupting effects of changing temperatures are best seen where several of them are associated (Figs. 29, 30, and 31).

The importance of this method of rock-breaking has rarely been appreciated except by those who have worked in high and dry regions. Climbers of high mountains know that almost every high peak is covered with broken rock to such an extent as to make its ascent dangerous to the uninitiated. High serrate peaks, especially of crystalline rock, are, as a rule, literally crumbling to pieces (Fig. 31). The piles of talus which lie at the bases of steep mountain slopes are often hundreds of feet in height, and their materials are often in large part the result of the process here under discussion. In mountain regions where atmospheric conditions favor sudden changes of temperature, the sharp reports of the disruption of rock masses are often heard. Masses of rock, scores and even hundreds of pounds in weight, are frequently thus detached and started on their downward course. Small pieces of rock are of course much more commonly broken off than large ones. The disruption of rock by changes of temperature is not usually the result of a single change of temperature, but rather of many successive expansions and contractions.

The sharp needle-like peaks which mark the summits of most high mountain ranges (Fig. 32) are largely developed by the process here outlined. The altitude at which the serrate topography appears varies with the latitude, being, as a rule, higher in low latitudes and lower in high. But even in the same latitude it varies notably with the isolation of the mountains and with the aridity of the climate. Thus within the United States the sharply serrate summits appear in some places, as in Washington and Oregon, at altitudes of 6000 to 10,000 feet, while in the isolated Wichita range of Oklahoma, much farther south, but in a much drier climate, the same sort of topography is developed at altitudes of 2500 to 3000 feet.

Even in low latitudes and moist climates the effects of temperature changes are often seen. Thin beds of limestone at the bottom of quarries are sometimes so expanded by the heat of the sun as to arch up and break. In desert and arid regions, whatever the altitude, the effects of temperature changes are often striking.

The disruption of rock by changes of temperature is one phase of weathering. It tends to the formation of a mantle of rock waste, which, were it not removed, would soon completely cover the solid rock beneath and protect it from further disruption by heating and cooling; but the loose material thus produced becomes an easy prey to running water, so that the work of the atmosphere prepares the way for that of other eroding agencies.

II. Evaporation and Precipitation.

Perhaps the most important work of the atmosphere as a dynamic agent lies in its function as the medium for the circulation and distribution of water. Atmospheric temperature is the primary factor governing evaporation, an important factor in the circulation of the vapor after it is formed, and controls its condensation and precipitation.

The average amount of annual precipitation on the land is variously estimated at from forty to sixty inches, the lesser figure being probably more nearly correct. Since much of this water falls at high altitudes, the work which it accomplishes in getting back to the sea is great. The water which falls on the land, if withdrawn wholly from the ocean, would exhaust that body of water in 10,000 to 15,000 years if none of it returned. The work of evaporation is of course not done by the atmosphere, though the atmosphere determines the effect of the solar energy which does the work.

The precipitation is distributed with great inequality, and this inequality affects both the rain and the snow. Some regions have heavy precipitation and some light; some regions have much rain and little snow; others have much snow and little rain; others have rain and no snow, and still others have snow and little or no rain. The amount and distribution of rain and snow determine the size and distribution of streams and glaciers, and streams and glaciers are the most important agencies modifying the surface of the land.

It is impossible to separate sharply the geologic work of the water of the atmosphere from that of other waters; but so long as moisture is in the atmosphere (including the time of its precipitation) its effects are best considered in connection with the atmosphere.

=The mechanical work of the rain.=—In falling the rain washes the atmosphere, taking from it much of the dust, spores, etc., which the winds have lifted from the surface of the dry land. Not only this, but in passing through the atmosphere the water dissolves some of its gases, and perhaps particles of soluble solid matter. When therefore the falling water reaches the surface of the land it is no longer pure, and some of the gases it has taken up in its descent enable it to dissolve various mineral matters on which pure water has little effect.

As it falls on the surface of the land the rain produces various effects of a mechanical nature. In the first place, it leaves on the surface the solid matter taken from the air. The amount of material, thus added to any given region in any particular shower is trivial, but in the course of long periods of time the total amount of material washed out of the air must be very great.

Every rain-drop strikes a blow. If the drops fall on vegetation, they have little effect, but if they fall on sand or unprotected earthy matter they cause movements of the particles on one another, and this movement involves friction and wear. While the results thus effected are inconsiderable in any brief period of time, they are not so insignificant when the long periods of the earth’s history are considered.

Clayey soils contract and often crack on drying. Falling on such a soil when it is dry the rain causes it to expand, and the cracks are healed by lateral swelling. The same soils are baked under the influence of the sun, and when in this condition are softened and made more mobile by the falling of rain. Under the influence of the expansion and contraction occasioned by wetting and drying, the soils and earths on slopes creep slowly downward. When rain falls on dry sand or dust the cohesion is at once increased, and shifting by the wind is temporarily stopped.

After the water has fallen on the land its further work cannot be looked upon as a part of the work of the atmosphere; but any conception of the geological work of the atmosphere which did not recognize the fact that the waters of the land have come through the atmosphere would be inadequate. The work of the water after it has been precipitated from the atmosphere must be considered in another chapter.

III. Effects of Electricity.

Another dynamic effect conditioned by the atmosphere is that produced by lightning. In the aggregate this result is inconsequential; yet instances are known where large bodies of rock have been fractured by a stroke of lightning, and masses many tons in weight have sometimes been moved appreciable distances. Incipient fusion in very limited spots is also known to have been induced by lightning. Where it strikes sand it often fuses the sand for a short distance, and, on cooling, the partially fused material is consolidated, forming a little tube or irregular rod (a fulgurite) of partially glassy matter. Fulgurites are usually only a few inches in length, and more commonly than otherwise a fraction of an inch in diameter. Strictly speaking these results are the effect of the electricity of the atmosphere rather than of the atmosphere itself, but they are best mentioned in this connection.

Allusion has already been made to the chemical changes in the atmosphere occasioned by electric discharges.

SUMMARY.

=Weathering.=—The result of all atmospheric processes, whether physical or chemical, by which surface rock is disrupted, decomposed or in any way loosened, is weathering. This convenient term also includes similar results effected by ground water, plants, etc. The tendency of weathering is to produce a mantle of residuary earth over solid rock. Weathering by mechanical means tends to produce material which, though in a finer state of division, is still like the original rock in chemical composition. Weathering by chemical means tends to produce a mantle made up chiefly of the less soluble parts of the rock from which it was derived. All processes of weathering prepare material for transportation by wind and water.

Many considerations determine the thickness which the mantle of weathered rock (mantle rock) attains. Some of these considerations have to do with the atmosphere, and some with drainage. Since the latter are, on the whole, more important, this matter will be discussed in connection with the work of water (Chapters III and IV).