In terms of value, mineral resources appear in different perspective. The annual world value of mineral production, exclusive of water, is approximately $9,000,000,000. This figure is obtained by dividing the annual value of the United States output of each of the principal minerals by the percentage which the United States output constitutes in the world output, and adding the figures thus obtained. The values here used are mainly selling prices at the mines. It is impossible to reduce the figures absolutely to the value of the mineral as it comes from the ground; there are always some items of transportation included. This method of figuring is of course only the roughest approximation; the values as obtained in the United States cannot be accurately exterpolated for the rest of the world because of locally varying conditions. However, the figures will serve for rough comparative purposes.
Of this total value coal represents roughly 61 per cent, petroleum 12 per cent, iron 6 per cent, copper 5 per cent, and gold 3 per cent.
In terms of value, about 25 per cent of the world's mineral production is available for export beyond the countries of origin. Of this exportable surplus the United States has about 40 per cent, consisting principally of coal, copper, and formerly petroleum.
The value of the United States annual mineral production in recent years has been from about $3,500,000,000 to $5,500,000,000. Annual imports of mineral products into the United States have averaged recently in the general vicinity of $450,000,000, the larger items being copper, tin, fertilizers, petroleum, gems and precious stones, manganese, nickel, and tungsten.
Again the perspective is changed when the value of water resources is considered. As a physiologically indispensable resource, the value of water in one sense is infinite. There is no way of putting an accurate value on the total annual output used for drinking and domestic purposes,—although even here some notion of the magnitude of the figures involved may be obtained by considering the average per capita cost of water in cities where figures are kept, and multiplying this into the world population. This calculation would not imply that any such amount is actually paid for water, because the local use of springs, wells, and streams can hardly be figured on a cash basis; but, if human effort the world over in securing the necessary water is about as efficient as in the average American city, the figures would indicate the total money equivalent of this effort.
The remarkable concentration of the world's mining and smelting around the North Atlantic basin, indicated by the foregoing figures, does not mean that nature has concentrated the mineral deposits here to this extent. It is an expression rather of the localized application of energy to mineral resources by the people of this part of the world. The application of the same amount of energy in other parts of the world would essentially change the distribution of current mineral production. The controlling factor is not the amount of minerals present in the ground; this is known to be large in other parts of the world and more will be found when necessary. Controlling factors must be looked for in historical, ethnological, and environmental conditions. This subject is further discussed in the chapters on the several resources, and particularly in relation to iron and steel.
The extraction of mineral resources on the huge scale above indicated is of comparatively recent date.
From 1880 to the end of 1918 the value of the annual mineral production of the United States has increased from $367,000,000 to more than $5,500,000,000, or nearly fifteen times; measured in another way, it has increased from a little over $7 per capita to more than $52.[10]
More coal has been mined in the United States since 1905 than in all the preceding history of the country. More iron ore has been mined since 1906 than in all the preceding history. The gold production of the United States practically started with the California gold rush in 1849. The great South African gold production began in 1888. Production of diamonds in South Africa began about 1869. The large use of all fertilizer minerals is of comparatively recent date. The world's oil production is greater now each year than it was for any ten years preceding 1891, and more oil has come out of the ground since 1908 than in all the preceding history of the world. The use of bauxite on a large scale as aluminum ore dated practically from the introduction of patented electrolytic methods of reduction in 1889.
In one sense the world has just entered on a gigantic experiment in the use of earth materials.
The most striking feature of this experiment relates to the vast acquisition of power indicated by the accelerating rate of production and consumption of the energy resources—coal, oil, and gas (and water power). Since 1890 the per capita consumption of coal in the United States has trebled and the per capita consumption of oil has become five times as great as it was. If the power from these sources used annually in recent years be translated roughly into man power, it appears that every man, woman, and child in the United States has potential control of the equivalent of thirty laborers,—as against seven in 1890. Energy is being released on a scale never before approximated, with consequences which we can yet hardly ascertain and appraise. This consideration cannot but raise the question as to the ability of modern civilization to control and coödinate the dynamic factors in the situation.
It is impossible to deduce accurately the capital value of mineral resources from values of annual output, but again some approximation may be made. The profit on the extraction of mineral resources on the whole, considering the cost of exploration, is probably no greater than in other industries (p. 330). If we assume a 6 per cent return, which perhaps is somewhere near the world-wide standard of interest rate for money, and capitalize the value of the world's annual output at this rate, we obtain a world capital value for mineral resources, exclusive of water, of 150 billions of dollars. This assumes an indefinitely long life for reserves. This assumption may need some qualifications, but it is the writer's view (Chapter XVII) that it is justified for a sufficiently long period to substantiate the above method of calculation.
The occurrence of a mineral resource within a country does not necessarily mean control by that particular political unit. A citizen of the United States may own a mineral resource in South America. Commercial control of this sort was demonstrated during the war to be of more far-reaching significance than had been supposed, and it became necessary to ascertain, not only the output of the different countries, but the commercial control of this output. Investigation of this subject for twenty-three leading commodities shows that the political and commercial control are by no means the same. These are partly summarized in the accompanying graphs from Spurr.[11] It is to be noted that the graphs show the control of many commodities as it existed in 1913, the last normal year before the war. Changes during and since the war have of course largely altered the situation for certain commodities, notably for iron, coal, and potash. These developments are summarized in the discussion of the individual resources. It is also to be noted that the commercial or financial control of the world's minerals, under the influence of the fostering and protective policies of certain governments discussed in Chapter XVIII, is at present in a state of flux. Considerable changes are taking place today and are to be looked for in the future.
Annual production figures are only to a very partial extent an indication of the distribution of the great reserves of mineral resources. For instance, there are enormous reserves of coal in China which are not yet utilized to any large extent. The minerals of South America and Africa are in a very early stage of development. The total world reserves will of course not be known until exploration and development of the world's resources are complete—a time which will probably never come. Figures of reserves represent only our present partial state of knowledge and are likely to be considerably modified in the future. Furthermore, the quantitative accuracy of knowledge of reserves is so variable in different parts of the world that it is almost impossible to make up world figures which have any great validity. There are, however, certain broad facts ascertainable.
Every country in the globe is deficient in supplies of some minerals. The United States is better off than any other country, but still lacks many mineral commodities (see pp. 396-399.) No single continent has sufficient reserves of all mineral commodities.
For the world, however, it may be stated with reasonable certainty that the reserves of the principal minerals are now known to be ample with the exception of those of oil, tin, and perhaps gold and silver. By ample we mean sufficient to give no cause for worry for the next few decades. For many mineral commodities the amounts now actually in sight will not last long, but the possibilities of extension and discovery are so great that a long future availability of these commodities can be counted upon with reasonable safety.
The present shortages in oil, tin, and other minerals mentioned may be only temporary. There is a large part of the world still to be explored, and the present reserves merely mark a stage in this exploration. Nevertheless, the ratio of reserves and discovery on the one hand to accelerated use on the other gives cause for much concern. Looking forward to the future, the problem of mineral reserves in general is not one of the possible ultimate amount which the earth may contain—presumably in no case is this deficient—but of the success with which the resource may be found and developed to keep up with the rapid acceleration of demand. In the chapter on conservation the suggestion is made that future difficulties are more likely to arise from failure to coödinate the dynamic factors of supply and demand, than from absolute shortage of material in the earth.
[10] Bastin, Edson S., and McCaskey, H. D., The work on mineral resources done by the U. S. Geological Survey: Min. Res. of the United States for 1918, U. S. Geol. Survey, pt. 1, 1920, p. 3a.
[11] Spurr, J. E., Who owns the earth?: Eng. and Min. Jour., vol. 109, 1920, pp. 389-390.
With the solid earth as the special care of geology, it may seem presumptuous for the geologist to claim the waters thereof, but he does not disclaim this inheritance. Water is so all-pervasive that it is more or less taken for granted; and so many and so intricate are its relations that it is not easy to make an objective survey of the water problem in its relation to geology.
The original source of water, as well as of air, is in molten magmas coming from below. These carry water and gases,—some of which are released and some of which are locked up in the rocks on cooling, to be later released during the alterations of the rocks. It is supposed, whatever theory of the origin of the earth we favor, that in its early stages the earth lacked both hydrosphere and atmosphere, and that during the growth of the earth these gradually accumulated on and near the surface in the manner stated.
During alterations at the surface water is added to the mineral constitution of the rocks, and by alterations deep below the surface it may be subtracted. Water is the agent through which most mineral and chemical changes of rocks are accomplished. It is the agent also which is mainly responsible for the segregation of mineral deposits. Water, both as running water and in the solid form of ice, plays an important part in determining the configuration of the earth's surface. Water is the medium in which most sedimentary rocks are formed. It is an important agent in the development of soil and in organic growth. These various influences of water on geological processes touch the economic field at many points, especially in relation to the concentration of ores and to the development of soils and surface forms.
Water comes even more directly into the field of economic geology as a mineral resource. Water supplies, for the greatest variety of purposes, involve geologic considerations at almost every turn.
Finally, water may be an aid or a hindrance to excavation and to a great variety of structural operations, both in war and in peace; and in this relation it again affords geologic problems.
The part played by water in geologic processes, such as that of mineral segregation, is more or less incidentally discussed in other chapters. We may consider more fully in this chapter the application of geology to the general subject of water supplies.
From the geological point of view, water is a mineral,—one of the most important of minerals,—as well as a constituent of other minerals. It becomes a mineral resource when directly used by man. It is ordinarily listed as a mineral resource when shipped and sold as "mineral water," but there is obviously no satisfactory line between waters so named and water supplies in general, for most of them are used for the same purposes and none of them are free from mineral matter. Water which is pumped and piped for municipal water supply is as much a mineral resource as water which is bottled and sold under a trade name. Likewise water which is used for irrigation, water power, and a wide variety of other purposes may logically be considered a mineral resource.
Notwithstanding the immense economic importance of water as a mineral resource its value is more or less taken for granted, and considerations of valuation and taxation are much less in evidence than in the case of other mineral resources. Water must be had, regardless of value, and market considerations are to a much less extent a limiting factor. Economic applications of geology to this resource are rather more confined to matters of exploration, development, total supply, and conservation, than to attempts to fix money value.
Free water exists in the openings in rocks where it is sometimes called hygroscopic water. There is also a large amount of water combined molecularly with many of the minerals of rocks, in which form it is called water of constitution. This water is fixed in the rock so that it is not available for use, though some of the processes of rock alteration liberate it and contribute it to the free water. The immediate source of underground water, both free and combined, is mainly the surface or rain waters. A subordinate amount may come directly from igneous emanations or from destruction of certain hydrous minerals. Ultimately, as already indicated, even the surface water originates from such sources.
The openings in rocks consist of joints and many other fractures, small spaces between the grains of rocks (pore space), and amygdaloidal and other openings characteristic of surface volcanic rocks. Many of these openings are capillary and sub-capillary in size. Most rocks, even dense igneous rocks, are porous in some degree, and certain rocks are porous in a very high degree. The voids in some surface materials may amount to 84 per cent of the total volume. In general the largest and most continuous openings are near the surface,—where rocks on the whole are more largely of the sedimentary type and are more fractured, disintegrated, and decomposed, than they are deep within the earth. The largest supplies of water are in the unconsolidated sediments. The water in igneous and other dense rocks is ordinarily in more limited quantity.
| Material | Volume of water absorbed per 100 of material | |
| Sandy soil2 | 45.4 | |
| Chalk soil2 | 49.5 | |
| Clay2 | 50-52.7 | |
| Loam2 | 45.1-60.1 | |
| Garden earth2 | 69.0 | |
| Coarse sand2 | 39.4 | |
| Peat subsoil2 | 84.0 | |
| Sand | 30-40 | |
| Sandstone | 5-20 | |
| Limestone and dolomite | 1-8 | |
| Chalk | 6-27 | |
| Granite | 03.-.8 | |
| 1 Mead, Daniel W., Hydrology: McGraw-Hill Book Co., New York, 1919, p. 393. | ||
| 2 Woodward, H. B., Geology of soils and substrata: Edward Arnold, London, 1912. | ||
Immediately at the surface, the openings of rocks may not be filled with water; but below the surface, at distances varying with climatic and topographic conditions, the water saturates the openings of the rocks and forms what is sometimes called the zone of saturation or the sea of underground water. The top surface of this zone is called the water table, or the ground-water level. The space between the water table and the earth's surface is sometimes referred to as the vadose zone or the zone of weathering, since it is the belt in which weathering processes are most active. The zone of weathering is not necessarily dry. Water from the surface enters and sinks through it and water also rises through it from below; it may contain suspended pockets of water surrounded by dry rocks; it is not continuously and fully saturated.
The water table or ground-water level may be near or at the surface in low and humid areas, and it may be two thousand feet or more below the surface in arid regions of high topographic relief. Because of the influence of capillarity, the water table is not a horizontal surface. It shows irregularities more or less following the surface contours, though not nearly so sharply accentuated.
The lower limit of the ground-water is more irregular than the upper surface and is less definitely known. In general, openings in rocks tend to diminish with depth, due to cementation and to closing of cavities by pressures which are too great for the rock to withstand. But rocks differ so widely in their original character, and in their response to physical and chemical environment, that it is not unusual to find dense and impervious rocks above, and open and porous rocks below. The lower limit of the zone of abundant underground water varies accordingly. A well may encounter nearly dry rock at a comparatively shallow depth, or it may reach a porous water-bearing stratum at considerable depth. At the greater depths pockets of water are sometimes found which have a composition different from that of the surface water, and which evidently are isolated from the surface water by zones of non-pervious rock.
Attempts have been made to calculate the total volume of underground water by measuring the openings of rocks and making assumptions as to the depth to which such openings may extend. In this manner it has been estimated that, if all the ground-water were assembled in a single body, it would make a shell between eighty and two hundred feet thick (depending on the assumptions) over all the continental areas.
Availability of water supplies is determined by the movement or flow of water as well as by its distribution and amount. The natural flow of water underground is caused by gravity in the larger openings, but in the smaller openings adhesion and capillarity are also important forces.
Of all the water falling on the surface, some may not go below the surface at all but may immediately evaporate or join the runoff—that is, the surface streams. Another part may penetrate a little distance into the zone of weathering and then join the runoff. Of the water which reaches the zone of saturation, a part may soon come to the surface in low areas and join the runoff, and a part may penetrate deeply.
Above the zone of saturation gravity carries the water downward in devious courses until it reaches the water table. Thereafter its course is determined largely by the lowest point of escape from the water table. In other words, the water table is an irregular surface; and under the influence of gravity the water tends to move from the high to the low points of this surface. Between the point of entrance and the point of escape from the water table, the water follows various courses, depending upon the porosity and the openings in the rocks. In general it fills all of the available openings, and uses the entire available cross section in making its progress from one point to another. The difference in height or the "head" between the point of entrance and the point of escape, together with the porosity of the rock and other factors, determine the general speed of its movement (see p. 73). With equal porosity the flow is at a maximum along a line directly connecting the two points, and on more devious courses the flow is less.
The surface water first enters the ground through innumerable small openings. Soon, however, it tends to be concentrated into channels of easiest flow, with the result that in the later part of its underground course it may be much concentrated in large trunk channels. These channels may consist of joints, or frequently of very coarse and pervious beds. The sedimentary rocks as a whole contain the most voids, and therefore the largest flow and largest supply of water is often localized in them. Of the sedimentary rocks, sandstones and limestones usually contain the largest and most continuous openings, and thus afford the freest circulation for water. The voids in fine-grained shales may equal in volume those in sandstones and limestones, but the openings are so small and discontinuous that the water does not flow freely. Regardless of total amount of water, unless there are continuous openings of some size the flow may be small.
The relations of more porous rocks to containing impervious strata also profoundly affect the flow of underground water. Between impervious strata the circulation may be concentrated and vigorous within the porous bed. Where the porous bed is not so contained, the movement may be more dispersed and less vigorous locally. The inclination of the beds, of course, also affects the direction and amount of the flow.
The influence of gravity upon underground water may locally tend toward a state of equilibrium in which there is little movement. In such a case the water is substantially ponded, and moves only when tapped by artificial openings.
Underground water becomes available for use by means of springs and through wells or bore holes. Water rises to the surface in natural springs at points where the pressure or head, due to its entrance into the ground at a higher level, is sufficient to force it to the surface after a longer or shorter underground course. The movement may be all downward and lateral to the point of escape, or it may be downward, lateral, and upward. Ordinarily, the course of spring waters does not carry them far below the surface. Heat and gases may be added beneath the surface by contact with or contributions from cooling igneous rocks. These may accelerate the upward movement of spring waters, and yield thermal and gas-charged waters, as in the springs and geysers of Yellowstone Park.
When a well is sunk to tap the underground water supply, the water may not rise in the artificial opening but may have to be lifted to the surface.
If, however, the water is confined beneath an impervious stratum and is under pressure from the water of higher areas, a well opening may simply allow it to move upward under its own pressure or head. This pressure may carry it upward only a few feet or quite to the surface or beyond, in which latter case the well is called an artesian well. The essential condition for an artesian circulation is a porous zone, inclining downward from the surface beneath an impervious stratum which tends to confine and pond the water. The water at any point in the water-bearing rock is under pressure which is more or less equivalent to the weight of the column of water determined by the difference in height between this point and the point of entrance or feeding area of the water. If the feeding area is higher than the collar of the well, the water will rise quite to the surface; if not, it will rise only part way. Capillary resistance, however, may and usually does lessen the theoretical pressure so figured.
The flow in deep artesian circulations is ordinarily a slow one. For the artesian wells of southern Wisconsin, it has been calculated that waters entering the outcrop of the southward dipping sandstone and limestone layers in the northern part of the state have required two or three hundred years to reach a point in the southern part of the state where they are tapped. Because of this slow movement, a large number of wells in any one spot may exhaust the local supply faster than it is replenished from the remainder of the formation. The drilling of additional wells near at hand in such cases does not increase the total yield, but merely divides it among a larger number of wells.
The porosity of the rocks, and therefore the flow of an artesian circulation, may in some cases be artificially increased by blasting and shattering.
Underground waters are never entirely free from dissolved mineral substances, and seldom are they free from suspended particles. Some waters are desired because they contain very small quantities of dissolved mineral matter. Others are prized because they have an unusually high content of certain mineral substances. In determining the deleterious or beneficial effect of dissolved substances, much depends on the purpose for which the water is to be used,—whether for drinking, washing, steam boilers, or irrigation. Near the surface underground waters may carry bacteria, as well as animal and vegetable refuse, which from a sanitary standpoint are usually objectionable. Deeper waters are more likely to lack this contamination because of filtration through rocks and soils.
The dissolved mineral substances of underground water are derived for the most part from the solution of rocks with which the waters come in contact, particularly at or near the surface. Through the agency of underground water most of the mineral and chemical changes of rocks are produced. The dissolved substances in solution at any time and place may therefore be regarded as by-products of rock alterations. Locally they may to some extent be derived from direct emanations from cooling igneous masses.
The most common mineral substances contained in waters are lime and magnesia. Less common, but abundant locally, are soda, potash, iron, and silica. Waters contain also certain acid and gaseous substances, the most common of which is carbon dioxide; and less widespread, but locally abundant, are chlorine and sulphur dioxide. Where lime and magnesia are abundant the water is ordinarily classed as a hard water. Where absent, or subordinate to soda and potash, the water is ordinarily classed as a soft water. Large amounts of the acid substances like chlorine and sulphur are detrimental for most purposes. Where there are unusual amounts of carbon dioxide or other gases present, they may by expansion cause the water to bubble.
If we were to attempt to describe and define the characteristics, with reference to dissolved mineral content and temperature, which make a given water more desirable than another, we should enter a field of the most amazing complexity and one with many surprising contradictions. For the most widespread use, the most desirable water is a cold water as free from mineral content as possible, and especially one lacking an excess of lime and magnesia which make it hard; also lacking an excess of acid constituents like sulphur dioxide, carbon dioxide, or chlorine, which give the water a taste, or which make impossible its use in boilers. Locally and for special reasons, waters of other qualities are in demand. Waters so excessively carbonated as to bubble, sulphureted waters, chlorine waters, waters high in iron, high in silica, high in potash, high in soda, or high in magnesia, or waters of high temperature, may come to be regarded as desirable. It is an interesting fact that any water with unusual taste, or unusual mineral content, or unusual temperature, is likely to be regarded as having medicinal value. Sometimes this view is based on scientific knowledge; sometimes it is an empirical conclusion based on experience; and again it may be merely superstition. In one case the desirable feature may be the presence of a large amount of carbon dioxide; in another case it may be its absence. In one case the desirable feature may be high temperature; in another case low temperature. The same combination of qualities which in a certain locality may be regarded as highly desirable, may be regarded as highly detrimental somewhere else where certain other types of waters are in vogue.
Proprietary rights and advertising have brought certain waters into use for drinking purposes which are not essentially different from more widely available waters which are not regarded as having special value. Two springs located side by side, or a spring and a deep well, whose waters have exactly the same chemical characteristics, may be used and valued on entirely different scales. Any attempt to classify mineral waters sold to the public in any scientific way discloses a most intricate and confused situation. One can only conclude that the popularity of certain waters is not based alone on objective qualities of composition, but rather on causes which lie in the fields of psychology and commerce.
The part played by sentiment in putting value on water is well illustrated by the general preference for spring waters as compared with well waters. In the public mind, "spring water" denotes water of unusual purity and of more desirable mineral content than well water. Illustrations could be cited of districts in which the surface or spring waters have a composition not different from that of the deeper well waters, and are much more likely to be contaminated because of proximity to the surface; and yet people will pay considerable sums for the spring water in preference to the cheaply available well water.
It is obvious that a knowledge of geology is helpful in locating an underground water supply. Locally the facts may become so well known empirically that the well driller is able to get satisfactory results without using anything but the crudest geologic knowledge; but in general, attention to geologic considerations tends to eliminate failures in well drilling and to insure a more certain and satisfactory water supply.
In drilling for water, it is essential to know the nature, succession, and structure of the rocks beneath the surface in order to be able to identify and correlate them from drill samples. The mere identification of samples is often sufficient to determine whether a well has been drilled far enough or too far to secure the maximum results. In order to arrive at any advance approximation of results for a given locality, a knowledge of the general geology of the entire region may be necessary. Especially for expensive deep artesian wells it is necessary to work out the geologic possibilities well in advance. It is useless, for instance, to look for artesian water in a granite; but in an area of gently inclined strata, with alternations of porous and impervious layers, the expert may often figure with a considerable degree of certainty the depth at which a given porous stratum will be found, and the pressure under which the water will be in this particular stratum at a given point. Even the mineral content of the water may in some cases be predicted from geologic study.
One of the most obvious and immediately useful services of the geologist in most localities is the collection and preservation of well samples for purposes of identification and correlation of rock formations, and as a guide to further drilling. Failure to preserve samples has often led to useless and expensive duplication of work.
The problem of water supply in some localities is comparatively simple and easy. In other areas there is an infinite variety of geologic conditions which affect the problem, and the geologist finds it necessary to bring to bear all the scientific knowledge of any sort which can be used,—particularly knowledge in relation to the type of rock, the stratigraphy and the structure.
Where underground water is not abundant or not cheaply available, or where larger amounts of water are needed, as in large cities or for irrigation purposes, surface water is used. In general, surface waters are more likely to be contaminated by vegetable and animal matter and to require purification for drinking purposes.
Surface waters are also used for irrigation, water power, drainage, the carrying of sewage, etc. This great variety of uses brings the consideration of surface waters into many fields other than geology, but an understanding and interpretation of the geological conditions is none the less fundamental. This is evidenced by the inclusion of geologic discussions in most textbooks of hydrology, and in the reports of the Hydrographic Branch of the U. S. Geological Survey. The very fact that this important branch of governmental investigation is in a charge of the U. S. Geological Survey indicates its close relation to geology.
The principles of geology used in the study of surface waters relate chiefly to physiography (see Chapter I). It is usually necessary to know the total quantity of flow, its annual and seasonal variation, and the possible methods of equalization or concentration; the maximum quantity of flow, the variation during periods of flood, and the possibilities of reduction or control; the minimum flow and its possible modification by storage or an auxiliary supply. These questions are obviously related to the size and shape of the catchment area, the topography, the rock structure, the relation between underground flow or absorption and the runoff, and other physiographic factors. Quoting from D. W. Mead:[12]
Geological conditions are frequently of great importance in their influence on the quantity and regularity of runoff. If the geological deposits of the drainage area are highly impervious, the surface flow will receive and transmit the water into the mass only through the cracks and fissures in the rock. Pervious materials, such as sandstones, sands, gravels, and cracked or fissured rocks, induce seepage, retard runoff, and, if such deposits are underlaid with an impervious bed, provide underground storage which impounds water away from the conditions which permit evaporation, and hence tends to increase runoff and equalize flow. On the other hand, if such pervious deposits possess other outlets outside of the stream channel and drainage area, they may result in the withdrawal of more or less of the seepage waters entirely from the ultimate flow of the stream. Coarse sands and gravels will rapidly imbibe the rainfall into their structure. Fine and loose beds of sand also rapidly receive and transmit the rainfall unless the precipitation is exceedingly heavy under which conditions some of it may flow away on the surface.
Many of the highly pervious indurated formations receive water slowly and require a considerable time of contact in order to receive and remove the maximum amount.
In flat, pervious areas, rainfalls of a certain intensity are frequently essential to the production of any resulting stream flow. In a certain Colorado drainage area, the drainage channel is normally dry except after a rainfall of one-half inch or more. A less rainfall, except under the condition of a previously saturated area, evaporates and sinks through the soil and into the deep lying pervious sand rock under the surface which transmits it beyond the drainage area. Such results are frequently greatly obscured by the interference of other factors, such as temperature, vegetation, etc.
The natural storage of any drainage area and the possibilities of artificial storage depend principally upon its topography and geology. Storage equalizes flow, although the withdrawal of precipitation by snow or ice storage in northern areas often reduces winter flow to the minimum for the year. Both surface and sub-surface storage sometimes hold the water from the streams at times when it might be advantageously used. Storage, while essential to regulation, is not always an advantage to immediate flow conditions.
Scarcely more than a mention of this subject is necessary. In mining, the pumping charge is one of the great factors of cost. A forecast of the amount and flow of water to be encountered in mining is based on the geologic conditions. The same is true in excavating tunnels, canals, and deep foundations. Detailed study of the amount and nature of water in the rock and soil of the Panama Canal has been vital to a knowledge of the cause and possibilities of prevention of slides. Rock slides in general are closely related to the amount and distribution of the water content.
The importance of ground-water as a detriment in military operations was shown during the recent war in trenching and other field works. At the outset, with the possible exception of the German army, a lack of scientific study of ground-water conditions led to much unnecessary difficulty. It soon became necessary to study and map the water conditions in great detail in advance of operations. Much of this work was done by geologists (see Chapter XIX).
Geological considerations are involved in a great variety of engineering undertakings related to river and harbor improvements, dam sites, etc., mentioned in Chapter XX.
[12] Mead, Daniel W., Hydrology: McGraw-Hill Book Co., New York, 1919, pp. 447-448, 456.
Under the general heading of common rocks are included the ordinary igneous, sedimentary, and "metamorphic" rocks, and the unconsolidated clays, sands, and gravels characteristic of surface conditions, which are mined and quarried for commercial use. Soils are closely related to this group; but since they present special problems of their own, they are discussed under a separate heading at the end of the chapter. Names of the common rocks will be used with the general commercial significance given them by the United States Geological Survey in its mineral resource reports.
Because of their inexhaustible quantity and ready availability, the value of the common rock products is not large per unit of weight; but in the aggregate it ranks high among mineral products. In respect to tonnage, common rocks constitute perhaps 10 per cent of the world annual output of all mineral commodities (exclusive of water).
The greater tonnage of the common rocks is used commercially in crushed or comminuted forms for road material, for railroad ballast, and for cement, brick, concrete, and flux. In blocks and structural shapes, of less aggregate tonnage, they are used as building stone, monumental stone, paving blocks, curbing, flagging, roofing, refractory stone, and for many other building and manufacturing purposes.
The common rocks are commodities in which most countries of the globe are self-sufficing. International trade in these commodities is insignificant, being confined to small quantities of materials for special purposes, or to local movements of short distances, allowed by good transportation facilities.
The common rocks are so abundant and widespread that the conservation of raw materials is not ordinarily a vital problem. Conservational principles do apply, however, to the human energy factor required for their efficient use. In the valuation of common rocks, also, the more important factors are not the intrinsic qualities of the stones, but rather the conditions of their availability for use.
Because of bulk and comparatively low intrinsic value, the principal commercial factors in the availability of the common rocks are transportation and ease of quarrying, but these are by no means the only factors determining availability. Their mineral and chemical composition, their texture and structure, their durability, their behavior under pressure and temperature changes, and other factors enter in to important degrees. The weighting and integration of these factors, for the purpose of reaching conclusions as to the availability of particular rock materials, depend also on the purposes for which these materials are to be used. The problem is anything but simple. The search for a particular rock to meet a certain demand within certain limits of cost is often a long and arduous one. On account of the abundance and widespread distribution of common rocks and their variety of uses, there is a good deal of popular misapprehension as to their availability. Many building and manufacturing enterprises have met disastrous checks, because of a tendency to assume availability of stone without making the fullest technical investigation. Many quarrying ventures have come to grief for the same reason. It is easy to assume that, because a granite in a certain locality is profitably quarried and used, some other granite in the same locality has equal chances. However, minor differences in structure, texture, and composition, or in costs of quarrying and transportation, may make all the difference between profit and loss. Even though all these conditions are satisfactorily met, builders and users are often so conservative that a new product finds difficulty in breaking into the market. A well-established building or ornamental stone, or a limestone used for flux, may hold the market for years in the face of competition from equally good and cheaper supplies. The very size of a quarry undertaking may determine its success or failure.
The term granite, as used commercially, includes true granite and such allied rocks as syenite and gneiss. In fact even quartzite is sometimes called granite in commerce, as in the case of the Baraboo quartzites of Wisconsin, but this is going too far. For statistical purposes, the United States Geological Survey has also included small quantities of diorite and gabbro. The principal uses of granite are, roughly in order of importance, for monumental stone, building stone, crushed stone, paving, curbing, riprap and rubble. Thirty states in the United States produce granite, the leaders being Vermont, Massachusetts, North Carolina, Maine, Wisconsin, Minnesota, and California.
Basalt and related rocks are sometimes included under the name "trap rock," which comprises,—besides typical basalt and diabase,—fine-grained diorite, gabbro, and other basic rocks, which are less common in occurrence and are similar in chemical and physical properties. The principal use of these rocks is as crushed stone for road and ballast purposes and for concrete. They are produced in some fifteen states, the leaders being New Jersey, Pennsylvania, California, and Connecticut.
In the United States limestone is used principally as crushed stone for road material, railroad ballast, concrete, and cement, as fluxing stone for metallurgical purposes, and in the manufacture of lime. Minor uses are as building stones, paving blocks, curbing, flagging, rubble, and riprap; in alkali works, sugar factories, paper mills, and glass works; and for agricultural purposes. For the making of cement, in metallurgical fluxes, and in most of the manufacturing and agricultural uses, both limestone and lime (limestone with the CO_2 driven out by heating) are used. Lime is also extensively used in the making of mortar for building operations, in tanning leather, and in a great variety of chemical industries. The total quantity of limestone used for all purposes in the United States nearly equals that of iron ore. Nearly every state in the union produces limestone, but the more important producers are Pennsylvania (where a large amount is used for fluxing), Ohio, Indiana, New York, Michigan, and Illinois.
Closely associated with limestone in commercial uses, as well as in chemical composition, is calcareous marl, which is used extensively in the manufacture of Portland cement.
Chalk is a soft amorphous substance of the same composition as limestone. The main uses of chalk are as a filler in rubber, and as a component of paint and putty. It is also used for polishing. The principal producers of this commodity are England, Denmark, and France, and the chief consumer is the United States. The United States depends upon imports for its supply of chalk for the manufacture of whiting. Before the war two-thirds came from England and a third from France. During the war importation was confined to England, with a small tonnage from Denmark. No deposits of domestic chalk have been exploited commercially. A somewhat inferior whiting, but one capable of being substituted for chalk in most cases, is manufactured from the waste fine material of limestone and marble quarries.
Marble is limestone which has been coarsely recrystallized by metamorphism. The marble of commerce includes a small quantity of serpentine as quarried and sold in Massachusetts, California, Maryland, Pennsylvania, and Vermont, and also a small amount of so-called onyx marble or travertine obtained from caves and other deposits in Kentucky and other states. The principal uses of marble are for building and monumental stones. Of the twenty-two states producing marble, the leaders are Vermont, Georgia, and Tennessee.
A small amount of marble of special beauty, adapted to ornamental purposes, is imported from European countries, especially from Italy. Marble imports from Italy constitute about two-thirds, both in tonnage and value, of all stone imported into the United States.
Sand is composed mainly of particles of quartz or silica, though sometimes feldspar and other minerals are present. Sandstones are partially cemented sands. Quartzites are completely cemented sands. To some extent these substances are used interchangeably for the same purposes.
The principal uses of sand in order of commercial totals are for building purposes—for mortar, concrete, sand-lime brick, etc.,—as molding sand in foundries, as a constituent of glass, in grinding and polishing, in paving, as engine sand, as fire or furnace sand, in the manufacture of ferrosilicon (a steel alloy), and in filters. Reference is made to sand as an abrasive and in the manufacture of steel in Chapters XIII and IX. Almost every state produces some sand, but for some of the more specialized uses, such as glass sand, molding sand, and fire or furnace sand, the distribution is more or less limited. The United States Geological Survey has collected information concerning the distribution of various kinds of sand and gravel, and serves a very useful function in furnishing data as to supplies of material for particular purposes. Fine molding sands have been imported from France, but during the war domestic sources in New York and Ohio were developed sufficiently to meet any requirements.
The sandstone of commerce includes the quartzites of Minnesota, South Dakota, and Wisconsin, and the fine-grained sandstones of New York, Pennsylvania, and elsewhere, known to the trade as "bluestone." In Kentucky most of the sandstone quarried is known locally as "freestone." The principal uses of sandstone are for building stone, crushed stone, and ganister (for silica brick and furnace-linings). Other uses are for paving blocks, curbing, flagging, riprap, rubble, grindstones, whetstones, and pulpstones (see also Chapter XIII). Sandstone is sometimes crushed into sand and is used in the manufacture of glass and as molding-sand. Most of the states of the union produce sandstone, the principal producers being Pennsylvania, Ohio, and New York.
Where sand is coarse and impure and mixed with pebbles, it is Ordinarily referred to as "sand and gravel." For sand and gravel the principal uses are for railroad ballast, for road building, and for concrete. Sand and gravel are produced in almost every state in the union, the largest producers being Pennsylvania, Ohio, Illinois, New Jersey, and North Carolina.
Shale is consolidated clay, usually with a fine lamination due to bedding. Slate is a more dense and crystalline rock, produced usually by the anamorphism of clay or shale under pressure, and characterized by a fine cleavage which is usually inclined to the sedimentary bedding.
Clays are used principally for building and paving brick and tile, sewer-pipe, railroad ballast, road material, puddle, Portland cement, and pottery. Clay is mined in almost every state. Ohio, Pennsylvania, New Jersey, and Illinois have the largest production. There has been a considerable importation of high-grade clays, principally from England, for special purposes—such as the filling and coating of paper; the manufacture of china, of porcelain for electrical purposes, and of crucibles; and for use in ultramarine pigments, in sanitary ware, in oilcloth, and as fillers in cotton bleacheries. War experience showed the possibility of substitution of domestic clays for most of these uses; but results were not in all cases satisfactory, and the United States will doubtless continue to use imported clays for some of these special purposes.
Shales, because of their thinly bedded character and softness, are of no value as building stones, but are used in the manufacture of brick, tile, pottery, and Portland cement.
Slates owe their commercial value primarily to their cleavage, which gives well-defined planes of splitting. The principal uses are for roofing and, in the form of so-called mill stock for sanitary, structural, and electrical purposes. Small amounts are used for tombstones, roads, slate granules for patent roofing, school slates, blackboard material, billiard table material, etc. The color, fineness of the cleavage, and size of the flakes are the principal features determining the use of any particular slate. Ten states produce slate, the principal production coming from Pennsylvania and Vermont.
Feldspars are minerals, not rocks, but mention of them is made here because, with quartz, they make up such an overwhelming percentage of earth materials. It is estimated that the feldspars make up 50 per cent of all the igneous rocks and 16 per cent of the sedimentary rocks. As the igneous rocks are so much more abundant than the sedimentary rocks, the percentage of feldspars in the earth approaches the former rather than the latter figure. In most rocks feldspar is in too small grains and is too intimately associated with other minerals to be of commercial importance; in only one type of rock, pegmatite, which is an igneous rock of extremely coarse and irregular texture, are the feldspar crystals sufficiently large and concentrated to be commercially available.
Feldspar is used principally in the manufacture of pottery, china ware, porcelain, enamel ware, and enamel brick and tile. In the body of these products it is used to lower the fusing point of the other ingredients and to form a firm bond between their particles. Its use in forming the glaze of ceramic products is also due to its low melting point. A less widespread use of feldspar is as an abrasive (Chapter XIII). One of the varieties of feldspar carries about 15 per cent of potash, and because of the abundance of the mineral there has been much experimental work to ascertain the possibility of separating potash for fertilizer purposes; but, because of cost, this source of potash is not likely for a long time to compete with the potash salts already concentrated by nature.
Feldspar is mined in eleven states, but the important production comes from North Carolina and Maine. The United States also imports some feldspar from Canada.
Cement is a manufactured product made from limestone (or marl) and clay (or shale). Sometimes these two kinds of substances are so combined in nature (as in certain clayey limestones) that they are available for cement manufacture without artificial mixing. It is not our purpose in this volume to discuss manufactured products; but the cement industry involves such a simple transformation of raw materials, and is so closely localized by the distribution of the raw materials, that a mention of some of its outstanding features seems desirable.
Hydraulic cement is used almost exclusively as a structural material. It is an essential ingredient of concrete. Originally used chiefly for the bonding of brick and stone masonry and for foundation work, its uses have grown rapidly, especially with the introduction of reinforced concrete. It is being used in the construction of roads, and its latest use is in ship construction.
With the exception of satisfactory fuels, the raw materials required for the manufacture of cement are found quite generally throughout the world. While practically all countries produce some cement, much of it of natural grade, only the largest producers make enough for their own requirements and as a result there is a large world movement of this commodity. The world trade is chiefly in Portland cement.
Next to the United States, the producing countries having the largest exportable surplus of cement in normal times are Germany and Great Britain. France and Belgium were both large producers and exporters before the war, but the war greatly reduced their capacity to produce for the time being. Sweden, Denmark, Austria, Japan, and Switzerland all produce less extensively but have considerable surplus available for export. Italy and Spain have large productions, which are about sufficient for their own requirements. Holland and Russia import large amounts from the other European countries. The far eastern trade absorbs the excess production of Japan. In South Africa and Australasia, production nearly equals demand. In Canada, although the industry has been growing very rapidly, the demand still exceeds production. In South and Central America, Mexico and the West Indies, the demand is considerable and will probably increase; production has thus far been insufficient. Several modern mills are either recently completed or under construction in these countries, and concessions have been granted for several others. These new mills are largely financed by American capital.
The United States is the largest single producer of cement in the world, its annual production being about 45 per cent of the world's total. Domestic consumption has always been nearly as great as the production, and exports have usually not exceeded 4 per cent of the total shipments from the mills. South and Central America offer fields for exportation of cement from the United States.