Popular Science Monthly/Volume 54/February 1899/The Science of Observation
|THE SCIENCE OF OBSERVATION.|
By CHARLES LIVY WHITTLE.
THIS is an era of observation; in many fields and in divers countries the study of Nature from a strictly scientific standpoint is being prosecuted with results which are rapidly increasing our knowledge of the universe. This modern growth has come about as the natural rebound of the suppressed energy that has been held forcibly under subjugation during the last two thousand years, at a time when the closing echoes of the warfare between the literal interpretation of the Scriptures and science have ceased.
A review of this long battle with the forces of the Catholic and Protestant churches on the one hand, arrayed against a relatively few investigators, scattered through the last ten centuries, on the other hand, shows a record on which none can look without regret. As far as we are able to learn, there was little opposition to the study of science before the collection and translation of the old manuscripts now constituting the Alexandrian version of the Bible and the consequent upbuilding of the Jewish church. The remains of ancient Egyptian civilization show that science prior to that period, as measured by the discoveries in physics and astronomy, had attained no inconsiderable prominence; and had this people endured until the present time, uninfluenced by the strife that for many centuries racked the inhabitants of the eastern hemisphere, we should to-day be far more advanced in our understanding of the universe.
In the more progressive countries, at least, the breaking of the shackles in which the investigating mind had been imprisoned for so long has led not only to a greater number of scientific workers, but also to an increase in the fields of observation. The methods of investigation have likewise undergone a transformation. In place of deductive reasoning, even as late as a few decades in the past, conclusions and generalizations are now founded on lines of thought more largely inductive. Men of middle age are able to recall the time when even our leading institutions of learning required instruction in several branches of science to be given by one teacher. It was possible twenty-five years ago for a man of great ability to master the essentials of the leading sciences and to teach them, but under the present stimulus for investigation no one can hope to excel in more than one subject. It has thus come about that in place of the many-sided teacher of science we now have in our larger universities specialists in every subject. As the work of research progresses, the specialist—for example, in geology—is compelled by the increased scope of the information on his subject to select one branch of geology of which lie shall be master. The chair of geology is now split up into economic, glacial, and mining geology, paleontology, etc., and specialists are required in each division. This breaking up is true of most other sciences. In this labyrinth of specialized subjects, and the maze of technical terms rendered necessary thereby, the people as a whole can only grope in darkness; but out of this bewildering condition of affairs, from the mass of facts collected, and the resulting generalizations and theories, there may be culled the kernel of one important principle by means of which these facts are ascertained and the generalizations made. The growth of science and its ever-ramifying divisions, and the gradual establishment of new methods of investigation, have brought forth what may be termed the science of observation; and it is through an application of the above principle that the people may be taught correctly to interpret Nature, and, by their new habit of thought, to free the brain from the tangle of superstition which is still present with most of us.A knowledge of how to observe natural phenomena and to draw correct inferences therefrom has been the product of slow growth, while through long custom, in matters closely pertaining to our daily life, there has been observation on strictly scientific principles for centuries. Stated succinctly, natural phenomena are due to causes, one or more, simple or complex. These causes are the laws of the universe, and to arrive at an understanding of them we must free our minds of any bias and study phenomena experimentally in the laboratory, or in our daily contact with Nature. In this way a mass of facts will be gathered by the systematic observer which will be found to fall into natural groups, and by inductive reasoning the laws governing each group may be learned. It is not possible for mankind as a whole to investigate in this exhaustive manner; but it is important that the method of arriving at the laws of Nature be understood. Many and, in fact, most phenomena met with in some of the sciences, particularly those having to deal with the earth, are susceptible of correct interpretation without attempting broad generalizations, if the principles of scientific observation are brought to bear upon their solution, and it is our purpose to show by practical examples drawn from Nature how elementary students may attack and solve some of the simple problems met with on every side. It is proposed to use for illustration simple phenomena pertaining to the earth, drawn from geology and its newly constituted sister science, physical geography. These two sciences perhaps afford the greatest range of phenomena, which are accessible to every one, in whatsoever part of the earth he may reside. No part of the land surface is wanting in problems which demand explanation, and which may be
Fig. 1.—Quarry showing Fresh and Weathered Rocks.
One of the most pronounced departures taking place in preparatory-school education at the present time is to be found in the prominence given to these subjects, not only in the schoolroom, but by practical experience in the laboratory of Nature, among the hills and mountains, as well. The object of this departure is twofold: the first and mostis to train the young early to observe phenomena and to interpret them; the second, in a narrower sense, is purely educational. The one inculcates a habit of thought that will be of inestimable advantage in pursuing future study; the other, without taking into consideration the element of mental training, constitutes instruction in concrete things that are matters of general education.
Before the student in the introductory schools is brought in contact with problems in the field, it is essential that he receive textbook or oral instruction in some of the geological processes giving rise to the phenomena to be studied later out of doors. In practical teaching the student is taken on excursions into the region not far removed from the school. At first some simple geological facts are shown him, often on a very small scale, but embodying principles which, when understood, lead to a ready interpretation of larger problems. Step by step the first principles are amplified by a larger and more varied class of examples, until the student is able logically to apply the reasoning in explanation of simple problems to the solution of the greater problems in physical geography and geology. In the absence of such excursions, I shall introduce a series of photographs carefully arranged to lead the reader along the same line of reasoning up to similar broad conclusions—a method which, if not so satisfactory and instructive, will at least have an educative value.
Our first excursion will be to a locality where an open cut has been made for the purpose of carrying on quarrying operations. The accompanying photograph has been so taken as to include both the top and the bottom of the quarry (Fig. 1). Let us first inspect the rock in the lower part of the quarry. The existence of planes of fracture, or joints, crossing the rock in various directions, dividing it into blocks, early attracts our attention. The stone appears dark-colored, tough, and is seen to be made up of two or three different minerals: one is black, cleaves readily into thin plates of a translucent nature, and we easily recognize it as an iron-bearing mica, or isinglass. Another is white, and cleaves or breaks in two directions, making angles of about ninety degrees; this we know as common feldspar. The third is less easily recognized as pyroxene, another of the many minerals containing iron. Having tested our knowledge of mineralogy, we will look about and see if all the rock exposed is like that at the bottom of the quarry. As we ascend from the point indicated by the lower hammer, we notice that the dark-blue rock gradually takes on a rusty hue, and its toughness has become less. Going still higher, the rusty character increases, and along joints the rock is so lacking in coherency as to fall to pieces when struck a light blow with a hammer. The central portions of the blocks, however, after we have removed the outer shell of rusty
Fig. 2.—Detailed View of a Portion of Quarry showing Weathered Rock.
material, are seen to be like the lower rock. In the middle foreground of the picture there are shown several bowlders derived from above, which are merely these residual cores, and are known as bowlders of disintegration. These are also shown in place near the top of the picture at the extreme left. Near the top of the quarry, at a point marked by the upper hammer, the solid rock gives place to a rusty mass of loose material, traversing which the cracks may still be seen, and in which there are few indications of the solid rock (see Fig. 2). This loose material when carefully examined is found to be made up of exactly the same minerals as the dense rock below, but we notice that the mica and pyroxene are rusty and that the feldspar is stained yellowish brown. The pyroxene in particular is very much changed, and quickly crumbles away in the hand. It is clear that there is every stage between the solid rock, and the incoherent powder at the surface of the ground. The joint planes crossing the solid rock below may still be observed traversing the decayed portion, and also many rounded areas of rock, which are seen to be identical with the stone at the bottom of the quarry.
How shall the facts before us be explained? It has been shown that the dense rock and the loose material are the same mineralogically, and grade from one into the other, and it is certainly rational to suppose that the latter is merely a changed form of the first. Some force must have been at work on the solid rock, destroying its coherency and converting it into loose sand. If we inspect the powdered rock, it will become apparent that this change has been brought about mainly by the process of weathering: surface water, with its ever-present acid impurities, has brought about the partial decay of the pyroxene and mica and caused the disintegration of the upper part of the rock. Water has not only attacked the rock from the upper surface, but has penetrated to considerable depths along the joint planes, working inward toward the center of each block until the mass becomes completely disintegrated. This process explains the concentric shells about cores of unaltered rock, each representing original joint blocks, which are seen in the second photograph. All our excursions into the field will show that this is not an isolated case, for wherever a ledge is exposed to our view there will be found a zone of weathered rock, varying in thickness from mere films to many feet.
By this process the greatest part of the materials constituting soils is formed, and the flora and fauna of the earth are rendered possible. Upon such products of decay the food supply of running water manifestly depends in a large measure, as will be pointed out on our next excursion; and were the scope of this article somewhat larger, it would be easy to show that the rock decay seen in our photograph has taken place in a length of time measured by something like ten thousand years. If all rock decayed as easily, and if the rate of decomposition, as determined here, held good for great distances from the surface, mountains two miles in height would become a prey to the force of chemical action in six and a half million years. We can not, however, give a time equivalent for the destruction of a mountain range, since decay, and consequent disintegration, is only one of the many forces acting to sap the strength of solid rocks and to tear them asunder. The above figures are given merely to make plain that the time necessary to accomplish the leveling of a mountain chain is but a small part of the earth's existence as such, great as this period may seem from the standpoint of human history.
We shall, if possible, time the second excursion immediately after a heavy rain, and we shall select for our objective point a place
Fig. 3.—Temporary Wet-weather Delta.
where the rain water, in its efforts to reach a stream, is forced to run down some steep declivity. Under such circumstances, the carrying power of the water will be very great, and we shall hope to find evidence of its work in transporting the products of rock weathering and other material broken up by the action of frost. A little diligence will soon reward us with the evidence which we seek. A local inequality of the ground, perhaps only a few feet across, is found filled with water—a minute, temporary lake caused by the recent heavy rainfall. Such little water bodies are extremely common, but the accompanying geological phenomena are, notwithstanding, none the less interesting, and the conclusions to be drawn from the evidence thus presented are none the less valuable.
If we examine the pool critically, it will be noticed that its shore line is cut by a little channel along which the overflow makes its escape. Further investigation will show that at another point along the shore, especially if we are fortunate enough to visit the locality very soon after a rain, there is a small rivulet entering the pool; and also that the entering stream is discolored with mud and carries more or less sand, while the escaping stream is nearly clear, and is free from all traces of coarse, sandy material. It is therefore evident that the sediment brought in by the stream has been left behind in the pool, and of course will be found deposited at its bottom, and it will appear that the only explanation of the inability of the water further to transport its burden is to be found in the fact that water loses nearly all its motion, and therefore its transporting power, on entering a stagnant pool. These are elementary truths, but an amplification of such simple phenomena is often fully capable of accounting for the most stupendous results.
Having made these observations, let us look at the form assumed by the sediment when it is forced to fall to the bottom. At the point where the stream enters the pool there is seen an accumulation of material having a nearly level upper surface, presenting a scalloped or lobe-shaped outer margin, upon which the stream may be seen flowing and entering the water at one of the lobes. Other channels, though unoccupied by water, also lead to similar lobes. If We watch closely, we may be able to witness the growth of this body of sand, called a delta, as the falling sediment rapidly increases the size of the lobe; and also to perceive that as soon as the lobe is built out considerably in advance of the main body of sand, it will be easier for the stream to enter the water on one side of the scallop, thus abandoning its old mouth. In this manner the stream moves from one place to another, successively building the little scallops and continually carving new channels for itself. Fig. 3 is a photograph of such a delta, some three feet across, taken after the water had been drained away, and reveals its form in a characteristic manner. As we watch its growth, it will become evident that only the coarsest material transported by the stream goes to make up the delta, and that the clay and finest sand are deposited farther The sand grains of which the deposit is largely composed are seen to be arranged in layers nearly horizontal, and these layers are found to be due to alternations of sediment varying in fineness. This phenomenon is called stratification, and is what we should expect of the action of gravity operating on material of different sizes and densities suspended in a body of water. It has been found inexpedient to attempt to show a photograph of this section, owing to the smallness of the subject, but the same phenomena may be observed on a much larger scale in Fig. 5, which will be described below., where the water is more quiet, or else pass out in the stream draining the pool. Let us look about a little. Not far from our miniature lake there are several others. In some the size of the delta is much larger in proportion to the area of the pool than is the case with the one first studied. We find in some cases that the stream has progressively built its delta completely across the old water surface. Taking a thin piece of board or a large knife, we can easily cut vertically through this sand deposit, thus exposing what is called a geological section.
A few rods away the stream that feeds the pool has its origin. The sediment carried by the water and going to build up its delta has its source in part in a neighboring bank made up of material derived from solid rock by weathering, similar to that shown on our first excursion, and partly from older water deposits. Steep channels exist in the disintegrated rock, which represent the material removed by the fast-flowing rain water.
Now what geological phenomena have we observed at this locality? In the first place, it has become clear that running water possesses the power of transporting sediment. In the second place, this sediment has been deposited wherever the velocity of the water has been materially checked. The sediment has been laid down in horizontal layers under the influence of gravity. Furthermore, the material of which the delta is composed has been shown, in part at least, to have been derived from a solid rock such as forms our mountains. In our first excursion we saw that chemical change promoted disintegration; in our second, running water is observed seizing upon these products of decay, transporting them and building them into stratified deposits in the first convenient pool. A level-topped delta is first formed, which may or may not grow to fill the pool in which it is born. Some of the pools have become filled, while the delta as such has disappeared; it has grown into a tiny sand plain.
Let us see if the work performed by these temporary rivulets is typical of running water in general. For this purpose we shall visit a spot where a river enters some considerable body of water such as a lake. Let us inspect the river. Its water is sluggish, discolored by organic matter derived from decaying vegetation, and for some distance up stream from its mouth it meanders slowly across a flat, marshy area or meadow. If we also visit the spot at a time when the river is swollen by heavy rains or melting snows, the presence of this organic matter will be masked by the turbidity of the water; we shall learn that only in the freshet seasons does the water attain sufficient velocity to carry any visible load of sand and clay. The upper end of the lake will be found to be shallow, muddy, and water lilies will have discovered congenial surroundings. At another part of the lake the outflowing water appears clear as crystal; the sediment brought in by the river has manifestly been deposited in the lake, as was the case in our little pool. The marsh at the upper end, of course, is merely another delta, slow growing in this instance, grass-covered, but as surely encroaching on the water area as in the earlier examples. When an entering stream is normally of great transporting power, owing to steep slopes down which it rushes, the form of its delta is not unlike the one first described.
With the data already gathered, we can not escape from the conclusion that the growth going on at the head of the lake will in time,
Fig. 4.—A Common Form of Large Delta.
if present conditions continue to exist, push its way forward until it has occupied the whole water area. The sediment which is now deposited therein will then be transported across the plain, and will be carried along until another body of water is reached. Further search will bring to light the fact that there are plenty of examples showing all stages between the simple delta and the completely filled lake. The innumerable marshes and meadows which characterize the northern part of the United States are fine examples of lakes which have perished in this manner.
Our next excursion will be made to the locality shown in Fig. 4, which is a sketch of a large delta occurring at a considerable height above the general level of the country, although at the present time the delta is not in vicinity of water. It will be evident to the reader that it differs in no important particular, excepting size, from our little type specimen formed in a pool. Its level top and frontal lobes are to-day nearly as strongly marked as at the time it was made. The reader will have little difficulty in picturing the original conditions of its formation in some ancient lake. This old lake did not endure until the inflowing streams had filled it to a level plain, but for some reason, which it is unnecessary for us to consider, the water was permitted to escape, leaving the delta perched on the valley side. Such deltas are very common, and we find them in all stages, from simple beginnings, as above, to the completed sand plain.The sand of which our first delta was composed has already been referred to as arranged in horizontal layers. In order to verify our conclusions regarding the origin of this delta, let us seek for an opportunity to observe its internal structure, and to compare it with that observed in the first example. It may happen that the opportunity does not exist at this immediate locality, but a little way off a similar deposit occurs, and a beautiful section has been uncovered by the vigorous attacks of a steam shovel. This section has already been referred to on page 464, as illustrating the structure of the sand layers making up the tiny delta, as well as water deposits in general, and is reproduced here as Fig. 5. The reader will observe in this picture many familiar features common to railroad excavations. The upper part of the geological section thus exposed is somewhat masked by a downfall of sand and loam, and the lower part is also hidden by the same materials. Along the central part, however, the sand and gravel may be seen arranged in horizontal layers of a varying thickness. A close inspection of the uppermost layers will detect a variation in coarseness among the different strata. Such alternations of layers of coarse and fine material are due to differences in the transporting power of the running water that brought the sand and pebbles to their present resting place; the coarse gravel and pebbles were carried by fast-flowing rivers, and the fine sand by streams of less rapidity and consequently less transporting power. Beds of this character ordinarily correspond closely in time with alternating periods of great rainfall or snow melting and the summer seasons. The pebbles of which the coarse layers are composed, as we should expect, are far from spherical, and the operation of gravity on such bodies, as they fall to the floor of a lake or ocean, is to cause them to arrange themselves with their flat surfaces horizontal and parallel to one another. In the
Fig. 5.—Geological Cross Section of a Delta.
example before us this fact is apparent, and affords the basis for another line of reasoning by which all such stratified deposits, however great their magnitude, are to be referred to the same source—namely, stream-transported materials derived from a decaying and wasting land surface, laid down in water under the influence of gravity.
We have now arrived at a most important and far-reaching generalization so far as the work performed by running water is concerned, and its action in filling our lakes and ponds; and we have learned by observation on a small scale the means by which such deposits may be recognized. Let us apply these means of recognition to the phenomena shown by our large rivers and the more enduring oceans into which they drain. In the same manner that we have studied the little pool and larger lake, we will look into the work done by the great waterways of our continents, selecting as a type of such streams the mighty Mississippi. Careful measurement has shown that this river annually transports two hundred million tons of sediment mechanically suspended. What becomes of this enormous quantity of sand and clay, equal to a cubic mile in a little over a century, as it is swept into the waters of the Gulf of Mexico? For this purpose we have only to visit the region about its mouth to become acquainted with the almost impotent struggles that have been made by our Government during the last fifty years in an effort to keep the river below New Orleans, in part at least, confined to its present channels; and to study the chart of that portion of the Gulf coast prepared by the United States Coast and Geodetic Survey (see Fig. 6). We have not forgotten the little lobes; their method of growth, and the general form of our first-seen delta, shown in Fig. 3. In viewing the phenomena at the mouth of the Mississippi, it is no longer necessary for our present purposes to make a detailed study, since it will become apparent at once that the river is doing the work on a larger scale typified by the performance of the tiny stream flowing into its temporary pool. In place of the little delta with its still smaller lobes, the Mississippi has deposited at its mouth an enormous delta, thousands of square miles in area, and its bifurcating arms may be seen building out several scallops for miles into the waters of the gulf. For centuries these long lobes have been building in advance of the delta front. The arms gradually become clogged with sediment, a new passage to the ocean is opened on the sides, where deposition will begin at a new point, producing a lobe as before. Situated many miles up the river, it is to-day the great fear of New Orleans that its only navigable arm to the sea will thus be closed to that commerce upon which the life of the city depends.
Only a portion of the sediment brought in by the river goes to form its delta; a large part of the finest material, such as clay, is transported by temporary and permanent currents thousands of miles away, where it is deposited in the more quiet waters of the ocean. In this manner the Mississippi has been shown to deposit a cubic mile of mechanically transported material in a little over a century. What shall we say of the effects produced on the continents and oceans by thousands of rivers, each doing its proportionate share of work and acting through millions of years? Two main results must follow, unless interruptions occur: the lower elevations and the magnificent mountain ranges, which rear their lofty heads above the permanent snow line, will be divided into minor peaks; valleys will be carved
Fig. 6.—The Delta of the Mississippi.
out; the whole land surface will slowly waste away, at first rapidly, at last slowly, and be transported to the oceans, where it will form great horizontal beds differing in no essential particular, excepting size, from those shown in Fig. 5—great deposits that are merely deltas on a large scale. The geologist, however, finds no evidence to indicate that at any time in the earth's history have these theoretical results taken place. Land masses, of continental dimensions, have not been allowed thus to waste entirely away to a general flatness on account of the interruptions caused by elevation—the bodily lifting of great areas of rock, even out of the ocean floor, to become mountains or plateaus, in some cases higher than any point in this country. If our observations thus far and those yet to be made serve to make this clear, one of the objects of this article will have been accomplished. It is to be hoped that our observations have made plain the processes of rock disintegration and water transportation; that in the oceans all these materials are eventually deposited in beds horizontally arranged, composed of such products of decay in the condition of sand and mud. We have only to point out the proof that great land masses, composed of water-deposited materials, have been lifted from the ocean to become continents and mountain ranges.
As the ocean deposits slowly accumulate in layers to beds of many thousands of feet in thickness, the lower parts are gradually subjected to greatly increased pressure produced by the overlying beds. During this time waters of a varying temperature, carrying, chemically dissolved, great quantities of lime, silica, and iron oxide, are allowed free circulation through them. These conditions promote chemical change: much silica (the mineral quartz), lesser amounts of carbonate of lime (the mineral calcite), and iron oxide are precipitated about the loose sand grains, firmly cementing them together into a solid rock. A cycle has thus been completed; the dense rocks composing a continent have passed by the process of weathering into incoherent sand and clay, which, when transported to the ocean floor, become again converted into solid rock.
Historical records prove that during the last three thousand years there have taken place many changes in the ocean's level. Old islands have disappeared; new ones have emerged above the surface of the water. Great stretches of seacoast exist at the present time which within the historical period have been covered by the ocean. Even at the present writing we are witnessing the gradual submergence of some parts of the earth and the rising of others; terraces on the northern Atlantic coast may be seen along the hillsides many feet above the present level of the ocean—all of which go to show that the relationship of the land to the water is an unstable one. These are the evidences of continental growth and depressions from the historical standpoint, and the validity of the data upon which the belief is founded can not be shaken. The evidence from the geological side is -overwhelming, but before we speak of this it will be well once more to say a word as to the causes of continental uplift.
From an original fluid globe possessing a high temperature, the earth has now cooled down to a degree sufficiently low to permit the formation of a thick rock crust. Underneath this crust an approach to the old surface temperatures is still maintained, and the existence of a certain degree of fluidity is demonstrated to us from time to time by the phenomenon of volcanism. Successive zones of cooling took place. The outer part could only conform to a shrinking interior by wrinkling, folding, or bodily lifting considerable areas above the general level. An adjustment of strains thus set up would take place either with or without folding of the strata. These initial wrinkles pave rise to our first mountains, and the continuation of these conditions at the present time is as surely nourishing mountain growth as at any time in the past. In this way the fluctuations of the ocean's
Fig. 7.—Mountain showing Rock Folding.
level, above referred to, alone are to be explained, and such form but temporary rises and falls in the history of a continent.
The rate at which an ocean bed is raised to form a mountain range is, no doubt, a variable one; always slow, often interrupted, but seldom or never violent. During this time the strata usually undergo crushing and folding; stretching takes place, and displacements of the rocks, or faulting, are not uncommon. As an example of the wrinkling that the strata may suffer under these conditions, the reader is referred to the beautiful symmetrical fold shown on the side of a mountain in the Appalachians (Fig. 7). Similar folding is the rule, but often immense areas are raised to great heights above the •ocean without disturbing the horizontal position of the beds (see Fig.-8). Coincident with the emergence of the rocks from beneath the water, there begin the attacks of the forces operating to destroy them. Hand in hand there go on growth and destruction. The two may keep an even pace; either may obtain the mastery. In the one case, lack of considerable elevation and flatness result; in the other, great altitudes may be attained. The rivers may cut their valleys downward as fast as the land rises, or the down-cutting may be relatively slower. In any case, after a given land mass has attained its greatest height above the sea, the larger rivers soon cut their channels down as far as river cutting is possible-—namely, to within a few feet of
Fig. 8.—Horizontal Rocks, Grand Canon of the Colorado.
sea level. With relatively rapid elevation, soft rocks, and large rivers, the resultant valley takes the form of a canon, examples of which are found along the courses of the Colorado and the Yellowstone Rivers (see Fig. 8). Valleys of this nature soon lose their steep sides by the action of weathering and all that this implies, and pass into a more open state, like that shown in Fig. 9.These views have been selected in order that a comparison of this type of mountain structure may be made with that shown in Fig.
Fig. 9.—Mount Stephen, showing its Horizontal Rocks.
6. The points of resemblance between the two sections exposed, one by a steam shovel, the other by river action, are the horizontal position of the strata and the alternations of beds of unlike character. The differences are mainly that the beds making up the mountain show that they are built up of alternating layers of sand (now converted into a sandstone) and clay (now in the condition of a slate). Are not these the products of a decayed continent? Is their position to be explained otherwise than along the lines already stated? Our only difficulty in readily accepting this conclusion is founded on a hereditary belief, born in ignorance and nourished to maturity by superstition, that the earth came into existence as we see it to-day, the surface dissected by valleys in which the rivers find established courses to the sea; possessing a multiplicity of highland and lowland, granite mountains and marble hills, as a result of some plan carried into effect as a creative act. Science has revealed the impossibility of this interpretation. Considered in the light of evolution, acting through an immense period of time, by means of the processes already enumerated, the diversity of land form is made plain to us, and the ever-varying characters of rock structure and composition are in the main made easy of comprehension. Viewed in the light of the foregoing pages, and illustrating as they do land form and the greater part of the earth's crust, the rock structures revealed on the sides of the mountains and canons, as well as the broader valley itself, take on a new and more intelligent interest. High and enduring as the mountains may appear, resistant as their solid rocks may seem, they are doomed as mountains to the same fate that their own structure and composition-prove to have overtaken earlier mountains before them.
The earth has known no cessation in this cycle of decay, deposition, and elevation; again and again have continental masses been raised from the ocean floor only to become a prey to the forces that destroy them. These cycles will continue—mountain ranges will fade away and new ones will be born. A more permanent relationship between the lowland, the upland, and the ocean level will never be attained until the forces that warp and wrinkle the earth's crust shall have ceased forever.
- The position of the solid rock is shown by the hammer at the extreme right, standing vertically.
- This photograph represents a more detailed view of the quarry wall seen in Fig. 1. The relation of the two views will be understood by observing the positions of the hammers, which are in the same place in both photographs. These photographs, as well as some of the others that follow, were taken by Mr. John L. Gardner, 2d, for the purpose of illustrating these pages.
- In order to obtain this sketch, a survey was made of the delta, and from the information thus gathered a model was constructed out of clay. The dimensions of the delta are about one thousand by seven hundred feet.
- The bottom of the canon at this point is between four and five thousand feet below the flat surfaces in the foreground—a sheer descent of nearly a mile.