Chapter II: Igneous Agencies (5)

When the dip and surface breadth of a series of strata have been measured, it is a simple problem in trigonometry to determine the true thickness, and the depth below the surface of any particular stratum at any given distance from its outcrop. When the strata are vertical, the surface breadth or traverse measure is equal to the thickness.

By the _outcrop_ of a stratum or formation we ordinarily understand its actual exposure on the surface, where it projects through the soil in ledges or quarries. But the term is also more broadly defined to mean the exposure of the stratum as it would appear if the soil were entirely removed. It is instructive to observe the relations of the outcrop to the form of the surface. Its breadth varies with its inclination to the surface, appearing narrow and showing its true thickness where it is perpendicular to the surface, and broadening out rapidly where the surface cuts it obliquely. The outcrops of horizontal strata form level lines or bands along the sides of hills and valleys, essentially contour lines in the topography; and appear as irregular, sinuous bands bordering the streams and valleys in the map-view of the country. The outcrops of vertical strata, dikes, or veins, on the other hand, are represented by straight lines and bands on the map. While the outcrops of inclined strata are deflected to the right or left in crossing ridges and valleys, according to the direction and amount of their inclination.

A geological map shows the surface distribution of the rocks, _i.e._, gives in one view the forms and arrangement of the outcrops of all the rocks in the district mapped, including the trend or strike of the folded strata. The map may be lithological, each kind of rock, as granite, sandstone, limestone, etc., being represented by a different color; or, it may be historical, each color representing one geological formation, _i.e._, the rocks formed during one period of geological time, without reference to their lithological character. But in the best maps these two methods are combined. The geological section shows the arrangement of the rocks below the surface, revealing the dip of the strata and supplementing the map, both modes of representation, the horizontal and vertical, being required to give a complete idea of the geological structure of a country. For a detailed and satisfactory explanation of the construction and use of geological maps and sections, students are referred to Prof. Geikie’s “Outlines of Field Geology.”

CLEAVAGE STRUCTURE.—This important structure is now known to be, like rock-folds, a direct result of the great horizontal pressure in the earth’s crust. It is entirely distinct in its nature and origin from crystalline cleavage, and may properly be called lithologic cleavage. It is also essentially unlike stratification and joint-structure. It agrees with stratification in dividing the rocks into thin parallel layers, but the cleavage planes are normally vertical instead of horizontal. And the cleavage planes differ from joints in running in only one direction, dividing the rock into layers; while joints, as we shall see, traverse the same mass of rock in various directions, dividing it into blocks.

The principal characteristics of lithologic cleavage are: (1) It is rare, except in fine-grained, soft rocks, having its best development in the slates, roofing slates and school slates affording typical examples. Hence it is commonly known as _slaty cleavage_. (2) The cleavage planes are highly inclined or vertical, very constant in dip and strike, and quite independent of stratification. (3) It is usually associated with folded strata, and often with distorted nodules or fossils. The more important of these characteristics are illustrated by Fig. 27. This represents a block of contorted strata in which the dark layers are slate with very perfect cleavage parallel to the left-hand shaded side of the block; while the white layers are sandstone and quite destitute of cleavage. Many explanations of this interesting structure have been proposed, but that first advanced by Sharpe may be regarded as fully established. He said that _slaty cleavage is always due to powerful pressure at right angles to the planes of cleavage_. All the characteristics of cleavage noted above are in harmony with this theory. Cleavage is limited to fine-grained or soft rocks, because these alone can be modified internally by pressure, without rupture. Harder and more rigid rocks may be bent or broken, but they appear insusceptible of minute wrinkling or other change of structure affecting every particle of the mass. Since the cleavage planes are normally vertical, the pressure, according to the theory, must be horizontal. That this horizontal pressure exists and is adequate in direction and amount, is proved by the folds and contortions of the cleaved strata; for, as shown in Fig. 27, the cleavage planes coincide with the strike of the foldings, and are thus perpendicular to the pressure horizontally as well as vertically. The distortion of the fossils in cleaved slates is plainly due to pressure at right angles to the cleavage, for they are compressed or shortened in that direction, and extended or flattened out in the planes of cleavage. Again, Tyndall has shown that the magnetism of cleaved slate proves that it has been powerfully compressed perpendicularly to the cleavage. And, finally, repeated experiments by Sorby and others have proved that a very perfect cleavage may be developed in clay (unconsolidated slate) by compression, the planes of cleavage being at right angles to the line of pressure. When, however, Sharpe’s theory had been thus fully demonstrated, the question as to _how_ pressure produces cleavage still remained unanswered. Sorby held that clay contains foreign particles with unequal axes, such as mica-scales, etc., and that these are turned by the pressure so as to lie in parallel planes perpendicular to its line of action, thus producing easy splitting or cleavage in those planes. And he proved by experiments that a mixture of clay and mica-scales does behave in this way. But Tyndall showed that the cleavage is more perfect just in proportion as the clay is free from foreign particles, and in such a perfectly homogeneous substance as beeswax, he developed a more perfect cleavage than is possible in clay. His theory, which is now universally accepted, is, that the clay itself is composed of grains which are flattened by pressure, the granular structure with irregular fracture in all directions, changing to a scaly structure with very easy and plane fracture or splitting in one definite direction.

Observations on distorted fossils and nodules have shown that when slaty cleavage is developed, the rock is, on the average, reduced in the direction of the pressure to two-fifths of its original extent, and correspondingly extended in the vertical direction. Thus, whether rocks yield to the horizontal pressure in the earth’s crust, by folding and corrugation, or by the flattening of their constituent particles, they are alike shortened horizontally and extended vertically; and it is impossible to overestimate the importance of these facts in the formation of mountains.

FAULTS OR DISPLACEMENTS.—We may readily conceive that the forces which were adequate to elevate, corrugate, and even crush vast masses of solid rock were also sufficient to crack and break them; and since the fractures indicate that the strains have been applied unequally, it will be seen that unequal movements of the parts must often result. If this unequal movement takes place, _i.e._, if the rocks on opposite sides of a fracture of the earth’s crust do not move together, but slip over each other, a _fault_ is produced. The two sides may move in opposite directions, or in the same direction but unequally, or one side may remain stationary while the other moves up or down. It is simply essential that the movement should be unequal in direction, or amount, or both; that there should be an actual slip, so that strata that were once continuous no longer correspond in position, but lie at different levels on opposite sides of the fracture. The vertical difference in movement is known as the _throw_, _slip_, or _displacement_ of the fault. Fault-fractures rarely approach the horizontal direction, but are usually highly inclined or approximately vertical. When the fault is inclined, as in Fig. 28, the actual slipping in the plane of the fault exceeds the vertical throw, for the movement is then partly horizontal, the beds being pulled apart endwise. The inclination of faults, as of veins and dikes, should be measured from the vertical and called the _hade_. Faults are sometimes hundreds of miles in length; and the throw may vary from a fraction of an inch to thousands of feet.

Transverse sections, such as are represented by Fig. 28 and many specimens and models, do not give the complete plan or idea of a fault; but this is seen more perfectly in Fig. 30. We learn from this that a typical fault is a fracture along which the strata have _sagged_ or settled down unequally. The most important point to be observed here is that the strata do not drop bodily, but are merely bent, the throw being greatest at the middle of the fault and gradually diminishing toward the ends. In other words, every simple fault must die out gradually; for we cannot conceive of a fault as ending abruptly, except where it turns upon itself so as to completely enclose a block of the strata, which may drop down bodily; but the fault is then really endless. A fault may be represented on a map by a line; if a simple fault, by a single straight line. But faults are often compound, and are represented by branching lines; that is, the earth’s crust has been broken irregularly, and the parts adjoining the fracture have sagged or risen unequally.

The rock above an inclined fault, vein, or dike (Fig. 28) is called the _hanging wall_, and that below the _foot wall_. Now inclined faults are divided into two classes, according to the relative movements of the two walls. Usually, the hanging wall slips down and the foot wall slips up, as in Fig. 28. Faults on this plan are so nearly the universal rule that they are called _normal_ faults. They indicate that the strata were in a state of tension, for their broken ends are pulled apart horizontally, so that a vertical line may cross the plane of a stratum without touching it.

A few important faults have been observed, however, in which the foot-wall[**no hyphen before] has fallen and the hanging-wall[**] has risen (Fig. 29). These are known as _reversed_ faults; and they indicate that the strata were in a state of lateral compression, the broken ends of the beds having been pushed horizontally past each other, so that a vertical line or shaft may intersect the same bed twice, as has been actually demonstrated in the case of some beds of coal.

The usual explanation of normal faults is given in Fig. 31. The inclined fractures of the earth’s crust must often be converging, bounding, or enclosing large V-shaped blocks (_A_, _B_). If now, through any cause, as the folding of the strata, they are brought into a state of tension, so that the fractures are widened, the V-shaped masses, being unsupported, settle down, the fractures bounding them becoming normal faults, as is seen by tracing the bed _X_ through the dislocations. The single fracture below the block _A_ is inclined, and the stretching has been accomplished by slipping along it and faulting the bed _Z_ as well as _X_, the entire section to the right of this fracture being part of a much larger V-shaped block the right-hand boundary of which is not seen. But the united fracture below the block _B_ being vertical, any horizontal movement must widen it into a fissure, which is kept open by the great wedge above and may become the seat of a dike or mineral vein. The beds below the V may, in this case, escape dislocation, as is seen by tracing the bed _Z_ across the fissure. These pairs of converging normal faults are called _trough_ faults; and this is the only way in which we can conceive of important faults as terminating at moderate depths below the surface, and not affecting the entire thickness of the earth’s crust.

Important reversed faults are believed to occur chiefly along the axes of overturned anticlines (Fig. 24) where the strata have been broken by the unequal strains, and those on the upper side shoved bodily over those on the lower or inverted side.

An extensive displacement of the strata is sometimes accomplished by short slips along each of a series of parallel fractures, producing a _step_ fault.

Faults cutting inclined or folded strata are divided into two classes, according as they are approximately parallel with the direction of the dip or of the strike. The first are known as _transverse_ or _dip_ faults, and the second as _longitudinal_ or _strike_ faults. The chief interest of either class consists in their effect upon the outcrops of the faulted strata, after erosion has removed the escarpment produced by the dislocation.

Dip faults cause a lateral shift or displacement of the outcrops, as shown in Fig. 32, which represents a plan or map-view of the strata traversed by the fault _b b_, the down throw being on the right and the up throw on the left. The dip of the strata is indicated by the small arrows and the accompanying figures; and it will be observed on tracing the outcrop of any stratum, _a a_, across the fault that it is shifted to the right. If the throw of the fault were reversed, the displacement of the outcrop would be reversed, also. Strike faults are of two kinds, according as they incline in the same direction as the strata, or in the contrary direction. The effect of the first kind is to conceal some of the beds, as shown in Fig. 33, in which beds 5 and 6 do not outcrop, but we pass on the surface abruptly from 4 to 7. The apparent thickness of the section is thus less than the real thickness. When the fault inclines against the strata, on the other hand (Fig. 34), the outcrops of certain strata are repeated on the surface; and a number of parallel faults of this kind, a step fault, will, like a series of closed folds (Fig. 25), cause the apparent thickness of the section to greatly exceed the real thickness. Repetition of the strata by faulting is distinguished from repetition by folding by being in the same instead of the reverse order.

Folds and faults are really closely related. In the former the strata are disturbed and displaced by bending; in the latter by breaking and slipping; and the displacement which is accomplished by a fold may gradually change to a fracture and slip. This relation is especially noticeable with monoclinal folds (Fig. 23), in which the tendency to shear or break the beds is often very marked.

Important faults are rarely simple, well-defined fractures; but, in consequence of the enormous friction, the rocks are usually more or less broken and crushed, sometimes for a breadth of many feet or yards. The fragments of the various beds are then strung along the fault in the direction of the slipping, and this circumstance has been made use of in tracing the continuation of faulted beds of coal. In other cases the direction of the slip is plainly indicated by the bending of the broken ends of the strata (Fig. 35), and the beds are sometimes turned up at a high angle or even overturned in this way.

Since faults are not plane, but undulating and often highly irregular, fractures, the walls will not coincide after slipping; and if the rocks are hard enough to resist the enormous pressure, the cavities or fissures produced in this way may remain open. Now faults are continuous fractures of the earth’s crust, reaching down to an unknown but very great depth; and hence they afford the best outlets for the heated subterranean waters; so that it is common to find an important fault marked on the surface by a line of springs, and these are often thermal. The warm mineral waters on their way to the surface deposit part of the dissolved minerals in the irregular fissures along the fault, which are thus changed to mineral veins. This agrees with the fact that the walls of veins usually show faulting as well as crushed rock, slickensides, and other evidences of slipping.

If the earth’s surface were not subject to erosion, every fault would be marked on the surface by an escarpment equal in height to the throw of the fault; and, notwithstanding the powerful tendency of erosion to obliterate them, these escarpments are sometimes observed, although of diminished height. Thus, according to Gilbert, the Zandia Mountains in New Mexico are due to a fault of 11,000 feet, leaving an escarpment still 7000 feet high. But, as a rule, there is no escarpment or marked inequality of the surface, the fault, like the fold, not being distinctly indicated in the topography. In all such cases we must conclude either that the faults were made a very long time ago, or that they have been formed with extreme slowness, so slowly that erosion has kept pace with the displacement, the escarpments being worn away as fast as formed. These and other considerations make it quite certain that extensive displacements are not produced suddenly, but either grow by a slow, creeping motion, or by small slips many times repeated at long intervals of time.

JOINTS AND JOINT-STRUCTURE.—This is the most universal of all rock-structures, since all hard rocks and many imperfectly consolidated kinds, like clay, are jointed. Joints are cracks or planes of division which are usually approximately vertical and traverse the same mass of rock in several different directions. They are distinguished from stratification planes by being rarely horizontal, and from both stratification and cleavage planes by being actual cracks or fractures, and by dividing the rock into blocks instead of sheets or layers. The art of quarrying consists in removing these natural blocks; and most of the broad flat surfaces of rock exposed in quarries, are the joint-planes (Fig. 36). Some of the most familiar features of rock-scenery are also due to this structure, cliffs, ravines, etc., being largely determined in form and direction by the principal systems of joints; and we have already seen that the same is true of veins and dikes.

Joints are divided by their characteristics and modes of origin into three classes as follows:—

1. _The parallel and intersecting joints._—This is by far the most important class, and has its best development in stratified rocks, such as sandstone, slate, limestone, etc. These joints are straight and continuous cracks which may often be traced for considerable distances on the surface. They usually run in several definite directions, being arranged in sets or systems by their parallelism. Thus in Fig. 36 one set of joints is represented by the broad, flat surfaces in light, and a second set crossing the first nearly at right angles, by the narrower faces in shadow. By the intersections of the different sets of joints the rock is divided into angular blocks.

Although many explanations of this class of joints have been proposed, it has long been the general opinion of geologists that they are due to the contraction of the rocks, _i.e._, that they are shrinkage cracks. We shall soon see, however, that they lack the most important characters of cracks known to be due to shrinkage; and the present writer has advanced the view that movements of the earth’s crust, and especially the swift, vibratory movements known as earthquakes, are a far more adequate and probable cause. It is well known that earthquakes break the rocks; and, if space permitted, it could be shown that the earthquake-fractures must possess all the essential features of parallel and intersecting joints.

2. _Contraction joints or shrinkage cracks._—That many cracks in rocks are due to shrinkage, there can be no doubt. The shrinkage may result from the drying of sedimentary rocks; but more generally from the cooling of eruptive rocks. Every one has noticed in warm weather, the cracks in layers of mud or clay on the shore, or where pools of water have dried up; and we have already seen that these sun-cracks are often preserved in the hard rocks. They have certain characteristic features by which they may be distinguished from the joints of the first class. They divide the clay into irregular, angular blocks, which often show a tendency to be hexagonal instead of quadrangular. The cracks are continually uniting and dividing, but are not parallel, and rarely cross each other. Sun-cracks never affect more than a few feet in thickness of clay, and are an insignificant structural feature of sedimentary rocks. In eruptive rocks, on the other hand, the contraction joints have a very extensive, and, in some cases, a very perfect development, culminating in the prismatic or columnar jointing of the basaltic rocks. This remarkable structure has long excited the interest of geologists, and, although the basalt columns were once regarded as crystals, and later as a species of concretionary structure, it is now generally recognized as the normal result of slow cooling in a homogeneous, brittle mass. The columns are normally hexagonal, and perpendicular to the cooling surface, being vertical in horizontal sheets and lava flows, as in the classic examples of the Giant’s Causeway and Fingal’s Cave, and horizontal in vertical dikes (Fig. 37). They begin to grow on the cooling surface of the mass and gradually extend toward the centre, so that dikes frequently show two independent sets of columns.

3. _The concentric joints of granitic rocks._—In quarries of granite and other massive crystalline rocks, it is often very noticeable that the rock is divided into more or less regular layers by cracks which are approximately parallel with the surface of the ground, some of the granite hills having thus a structure resembling that of an onion. The layers are thin near the surface, become thicker and less distinct downwards, and cannot usually be traced below a depth of fifty or sixty feet. These concentric cracks are of great assistance in quarrying, and are now regarded as due to the expansion of the superficial portions of the granite caused by the heat of the sun. In reference to this view of their origin these may be properly called _expansion joints_.

STRUCTURE OF MOUNTAIN-CHAINS.—Mountains are primarily of two kinds,—volcanic and non-volcanic. The structure of the former belongs properly with the original structures of the volcanic rocks; but the latter—the true mountains—owe their internal structure and altitude or relief almost wholly to the crumpling and mashing together of great zones of the earth’s crust, being, as already pointed out, the culminating points of the plication, cleavage, and faulting of the strata. “A mountain-_chain_ consists of a great plateau or bulge of the earth’s surface, often hundreds of miles wide and thousands of miles long. This is usually more or less distinctly divided by great longitudinal valleys into parallel _ranges_ and _ridges_; and these, again, are serrated along their crests, or divided into _peaks_ by transverse valleys. In many cases this ideal chain is far from realized, but we have instead, a great bulging of the earth’s crust composed on the surface of an inextricable tangle of ridges and valleys of erosion, running in all directions. In all cases, however, the erosion has been immense; for the mountain-chains are the great theatres of erosion as well as of igneous action. As a general fact, all that we see, when we stand on a mountain-chain—every peak and valley, every ridge and cañon, all that constitutes scenery—is wholly due to erosion.”—LE CONTE.

The structure of mountains thus fells under two heads: (1) The internal structure and altitude, which are due to the action of the subterranean agencies. (2) The external forms, the actual relief, which are the product chiefly of the superficial agencies or erosion. The study of mountains has shown that: (1) They are composed of very thick sedimentary formations. Thus the sedimentary rocks have a thickness of 40,000 feet in the Alleghanies; of 50,000 feet in the Alps; and of two to ten miles in all important mountain-chains. Such thick deposits of sediments, as we have already seen, must be formed on a subsiding sea-floor, and in many mountain-chains, as in the Alleghanies, the great bulk of these sediments are still below the level of the sea. Again, thick sedimentary deposits can only be formed in the shallow, marginal portions of the sea; and when such a belt of thick shore deposits yields to the powerful horizontal thrust, and is crumpled and mashed up, it is greatly shortened in the direction of the pressure and thickened vertically, so that its upper surface is lifted high above the level of the sea, and a mountain-chain is formed and added to the edge of the continent. We thus find an explanation of the important fact that on the several continents, but notably on the two Americas, the principal mountain-ranges are near to and parallel with the coast lines.

2. The mountain-forming sediments are usually strongly folded and faulted, and exhibit slaty cleavage wherever they are susceptible of that structure; and the older rocks, especially, in mountains are often highly metamorphosed, and are traversed by numerous veins and dikes, the infallible signs of intense igneous activity.

“In other words, mountain regions have been the great theatres—(1) of sedimentation before the mountains were formed; (2) of plication and upheaval in the formation of the range; and (3) of erosion which determined the present outline. Add to these the metamorphism, the faults, veins, dikes, and volcanic outbursts, and it is seen that all geological agencies concentrate there.”—LE CONTE.

Since mountain-ranges are great up-swellings or bulgings of the strata, their structure is always essentially anticlinal; and they sometimes consist of a single more or less denuded anticline (Fig. 38), the oldest and lowest strata exposed forming the summit of the range. More commonly, however, the single great arch or uplift is modified by a series of longitudinal folds, as shown in the section of the Jura Mountains (Fig. 21). Still more commonly the folds are closely pressed together, overturned, broken, and almost inextricably complicated by smaller folds, contortions, and slips.

The strata on the flanks of the mountains are usually less disturbed than those near the axis of the range, and are sometimes seen to rest unconformably against the latter. In this way it is proved that some ranges are formed by successive upheavals. But we have still more conclusive evidence that mountains are formed with extreme slowness in the fact that rivers sometimes cut directly through important ranges. This proves, first, that the river is older than the mountains; second, that the deepening of its channel has always kept pace with the elevation of the range.

CONCRETIONS AND CONCRETIONARY STRUCTURE.—Folds, cleavage, faults, and joints—all the subsequent structures considered up to this point—are the product of mechanical forces. Chemical agencies, although very efficient in altering the composition and texture of rocks, are almost powerless as regards the development of rock-structures; and the only important structure having a chemical origin is that named above.

Concretions are formed by the segregation of one or more of the constituents of a rock. But there are three distinct kinds of segregation. If the water percolating through or pervading a rock, dissolves a certain mineral and afterwards deposits it in cavities or fissures, _amygdules_, _geodes_, or _veins_ are the result. If the mineral is deposited about particular points in the mass of the rock, it may form _crystals_, the rock becoming _porphyritic_; or it may not crystallize, but build up instead the rounded forms called _concretions_, the texture or structure of the rock becoming _concretionary_. A great variety of minerals occur in the form of concretions, but this mode of occurrence is especially characteristic of certain constituents of rocks, such as calcite, siderite, limonite, hematite, and quartz. Concretions may be classified according to the nature of the segregating minerals; and in each class we may distinguish the _pure_ from the _impure_ concretions. A pure concretion is one entirely composed of the segregating mineral. Most nodules of flint and chert, quartz, geodes, concretions of pyrite, and many hollow iron-balls are good illustrations of this class. In all these cases the segregating mineral has been able in some way to remove the other constituents of the rock, and make room for itself. But in other cases it has lacked this power, and has been deposited between and around the grains of sand, clay, etc.; and the concretions are consequently _impure_, being composed partly of the segregating mineral, and partly of the other constituents of the rock. The calcareous concretions known as clay-stones are a good example of this class, being simply discs of clay, all the minute interstices of which have been filled with segregated calcite. The solid iron-balls are masses of sand filled in a similar manner with iron oxides.

Concretions are of all sizes, from those of microscopic smallness in some oölitic limestones up to those twenty-five feet or more in diameter in some sandstones.

The point of deposition, when a concretion begins to grow, is often determined by some concrete particle, as a grain or crystal of the same or a different mineral, a fragment of a shell, or a bit of vegetation, which thus becomes the nucleus of the concretion. The ideal or typical concretion is spherical; but the form is influenced largely by the structure of the rock. In porous rocks, like sandstone, they are frequently very perfect spheres; but in impervious rocks, like clay, they are flat or disc-shaped, because the water passes much more freely in the direction of the bedding than across it; while the concretions in limestones, the nodules of flint and chert, are often remarkable for the irregularity of their forms. In all sedimentary rocks the concretions are arranged more or less distinctly in layers parallel with the stratification, which usually passes undisturbed through the impure concretions. Many silicious and ferruginous concretions are hollow, apparently in consequence of the contraction of the substance after its segregation; and the shrinkage due to drying is still further indicated by the cracks in the septaria stones. The hollow, silicious concretions are usually lined with crystals (geodes), while the hollow iron-balls frequently enclose a smaller concretion. Rocks often have a concretionary structure when there are no distinct or separable concretions. And the appearance of a concretionary structure (pseudo-concretions) is often the result of the concentric decomposition of the rocks by weathering, as explained on page 13.

SUBSEQUENT STRUCTURES PRODUCED BY THE SUPERFICIAL OR AQUEOUS AGENCIES.—The superficial agencies, as we have seen in the section on dynamical geology, are, in general terms, water, air, and organic matter. Geologically considered, the results which they accomplish, may be summed up under the two heads of deposition and erosion—the formation of new rocks in the sea, and the destruction of old rocks on the land. In the rôle of rock-makers they produce the very important original structures of the stratified rocks; while as agents of erosion they develop the most salient of the subsequent structures of the earth’s crust—the infinitely varied relief of its surface. As a general rule, to which recent volcanoes are one important exception, the original and subterranean structures of rocks are only indirectly, and often very slightly, represented in the topography; for this, as we have seen, is almost wholly the product of erosion. Therefore, what we have chiefly to consider in this section is to what extent and how erosion is influenced by the pre-existing structures of rocks.

Horizontal or very slightly undulating strata, especially if the upper beds are harder than those below, give rise by erosion to flat-topped ridges or table-mountains (Fig. 39). But if the strata be softer and of more uniform texture, erosion yields rounded hills, often very steep, and sometimes passing into pinnacles, as in the Bad Lands of the west. Broad, open folds, as we have seen, give, normally, synclinal hills and anticlinal valleys (Fig. 22), when the erosion is well advanced. But in more strongly, closely folded rocks the ridges and valleys are determined chiefly by the outcrops of harder and softer strata, as shown in Fig. 40, the symmetry of the reliefs depending upon the dip of the strata. This principle of unequal hardness or durability also determines most of the topographic features in regions of metamorphic and crystalline rocks, in which the stratification is obscure or wanting.

The boldness of the topography, and the relation of depth to width in valleys, depends largely upon the altitude above the sea; but partly, also, upon the distribution of the rainfall, the drainage channels or valleys being narrowest and most sharply defined in arid regions traversed by rivers deriving their waters from distant mountains. That these are the conditions most favorable for the formation of cañons is proved by the fact that they are fully realized in the great plateau country traversed by the Colorado and its tributaries, a district which leads the world in the magnitude and grandeur of its cañons. But deep gorges and cañons will be formed wherever a considerable altitude, by increasing the erosive power of the streams, enables them to deepen their channels much more rapidly than the general face of the country is lowered by rain and frost. This is the secret of such cañons as the Yosemite Valley, and the gorge of the Columbia River, and probably of the fiords which fret the north-west coasts of this continent and Europe. For a full description and illustration of the topographic types developed by the action of water and ice upon the surface of the land, and of the various characteristic forms of marine erosion, teachers are referred to the larger works named in the introduction, especially Le Conte’s Elements of Geology, and to the better works on physical geography. We will, in closing this section, merely glance at some of the minor erosion-forms, which are not properly topographic, but may be often illustrated by class-room and museum specimens. Mere weathering, the action of rain and frost, develops very characteristic surfaces upon different classes of rocks, delicately and accurately expressing in relief those slight differences in texture, hardness, and solubility, which must exist even in the most homogeneous rocks. Every one recognizes on sight the hard, smooth surfaces of water-worn rocks. They are exemplified in beach and river pebbles, in sea-worn cliffs, and where rivers flow over the solid ledges. The pot-hole (page 17) is a well-marked and specially interesting rock-form, due to current or river erosion.

Ice has also left highly characteristic traces upon the rocks in all latitudes covered by the great ice-sheet. These consist chiefly of polished, grooved, and scratched or striated surfaces, the grooves and scratches showing the direction in which the ice moved.

The organic agencies, as already noted, accomplish very little in the way of erosion, especially in the hard rocks, but the rock-borings made by certain mollusks and echinoderms may be mentioned as one unimportant but characteristic form due to organic erosion.

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