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highly successful. The various structures resulting from rock failure have usually been explained on the simple conception of the application of a non-rotational stress-either tension, causing elongation in the direction of pull, or simple compression, producing a shortening parallel to the principal stress and elongation at right angles to it. A fold, for instance, is assumed to indicate application of stress normal to its axial plane; a set of compressive joints is taken to indicate application of stress at 45° to the fractures; cleavage is taken to indicate application of pressure normal to its plane. Experimental work on rock deformation has been conducted mainly with the same limited assumptions, and the results have been widely quoted and applied to the interpretation of rock structures in the field. These conceptions may be correct as far as the immediate feature is concerned, but the forces are only minor constituents of the major causal movement and give no clue to its direction.

Much less attention has been paid to the conception that the compressional forces may be rotational, that is, that they may be applied in the form of a couple. Under this conception, the net result is a shearing between the heterogeneous rock units along planes ranging from parallel to 45° to the principal axis of stress, the shearing usually accompanied by local tension-in other words, no matter what the origin of compressional stresses and their angle of application, when applied to the heterogeneous rock masses constituting the earth they tend as a whole to act in couples and are resolved into components usually acting in directions inclined to the resulting planes of movement. A mountain making movement under this conception is a shear of certain rock masses over others, resulting in faults, joints, folds, and cleavage. Tensional stresses may be minor consequences of such shear. Field observations within the range of my own experience favor this view of the dominance of shear. It is the view also which geologists have commonly applied to an assumed shear of a thin brittle crust over a thin mobile zone below, though curi

ously enough not to the loca can be observed.

Illustration of Shear Stru trate the prevalence of shear folds are not symmetrical and inclination of their axial plan structural unit past another. tion is conspicuous they may folds." A fold has usually indicating direct shortening axial plane, and therefore app normal to this plane. The Ap for instance, have been ordina indicating pressure from the southeast. The same results equally well be produced by a shearing movement acting in clined to the trend of the fold mountain range as a whole. reproduction of Appalachian shearing stresses gives more sults than experiments with r ing.2 The folds indicate the shortening or elongation, in the nature of the strain, but n application of the stress.

The interpretation of roc schistosity, a common though evidence of rock flowage, affor good illustration of the danger row assumptions as to its rela stresses. Cleavage is a capacit parallel surfaces determined 1 dimensional arrangement of mi There is abundant proof tha rock has been elongated parall age surface, and cleavage thu dence of elongation. It does n ever, that the stress producing applied normal to it.

The elongation may well under a shearing stress of t exists when a mass of dough i the table by the application of to the table surface. Field stud seem to indicate that in the m

2 Mead, W. J., "Notes on the M logic Structures," Jour. Geol., V 521-523.

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the cleavage is merely the expression of the yielding of a weaker formation or weaker part of a formation by a slipping or differential movement between harder members. Even in areas with regional cleavage, the same interpretation may be applied on a large scale when the harder units in adjacent terranes are taken into account. My own observations in old pre-Cambrian terranes tend to the conclusion that cleavage, indicating rock flowage, has been confined to comparatively narrow mesh-like zones between large massifs. The evidence leading to this conclusion that cleavage is the result of slipping between rock masses may usually be checked by drag folds which develop simultaneously in the softer rocks, and by fissures and faults which develop simultaneously in the harder rocks.

The zones of movement marked by cleavage may have almost any inclination or direction, but the plane of the cleavage itself has a strong tendency toward steep inclination or verticality. Both in strike and dip the cleavage is more uniform than the movement zone of which it is a part. The relation is not unlike that between folds and cleavage, to be presently discussed. This steep inclination of cleavage does not necessarily indicate prevailing horizontality of stresses on the assumption that cleavage must develop normal to stress. In part it may have this relationship, but when considered in relation to folds and relative movement of adjacent massifs it more often indicates shearing stresses inclined to the cleavage. So far as any general inference is possible, the tendency of cleavage to show uniform strike and steep inclination over great areas suggests differential movement in vertical or steeply inclined planes, the movements in these planes ranging from vertical to horizontal. It can not be explained by movement along planes tangential to the earth, which would require prevalence of flat or gently inclined cleavage. In short the attitude of cleavage, so far as it may be generalized, does not correspond to the conception of the tangential shearing of a competent surface zone over a mobile zone below.

Cleavage has a definite relationship to folds which is of great usefulness in interpretation of rock structures, and which affords valuable suggestions as to the general relations of cleavage to the great zones of flowage of which it is often an expression. Cleavage is approximately parallel to the axial planes of the folds. It therefore usually stands more steeply than bedding and is more uniform in dip and strike than bedding. Where cleavage is noted in a rock outcrop, the direction and inclination of the axial planes of the folds are thereby indicated-not only for the folds within the rock observed, but also, usually, for the folds in the adjacent rocks as well.

As a consequence of the fact that cleavage is roughly parallel to the axial planes of folds, it follows that the trace of any bedding plane on the cleavage surface indicates approximately the direction and degree of pitch of the fold, that is, the inclination of the axial line of the fold to the horizontal. A single fragment of cleavable rock appearing in an outcrop may be sufficient to establish the pitch for a considerable area.

The inclination of bedding to cleavage-always remembering that the latter indicates the attitude of the axial plane of the fold-indicates faithfully the position of the observed bedding on the fold, whether the fold be upright inclined or overturned. This principle is useful in determining whether a bed is right side up or overturned. Inferences of the same sort may be drawn from strike observations on bedding and cleavage in deformed areas.

Still further, cleavage is a phenomenon of rock flowage. The very existence of cleavage therefore, means the rock has been deformed under the conditions of rock flowage, wher the folds are likely to be of a rather intricat type, with much interior thinning and thicken ing of the beds. Even though evidences of thi folding are not immediately at hand, the ver existence of cleavage on a considerable scal indicates with reasonable certainty the exist ence not only of folds, but folds of the roc flowage type.

All of these inferences may be made induc

tively from a surprisingly narrow range of observation.

These remarks on cleavage apply to the structure ordinarily associated with the deformation of rocks which is almost without exception inclined to bedding or other primary structures. They do not apply to cleavage developed solely by load or gravity, which might reasonably be expected to be horizontal. The latter type of cleavage has been described for certain terranes and districts, as for instance in the Belt series of the Canadian boundary; but within my own observation of deformed areas it is a phenomenon of such local and special character as not to invalidate the generalizations above made. So far as 'load cleavage is assumed to develop under static conditions of load, without movement, I doubt its existence. Cleavage usually indicates movement, not static pressures.

The interpretation of jointing and faulting has likewise suffered from far too narrow and simple assumptions of the mechanical conditions. Quoting from a recent paper by Mead,3 such a simple structure as an open fissure or joint "obviously due to tensional stresses (so far as the fissure itself is concerned) may be an incident in simple elongation, shear, crossbending, compression or shortening, or torsional warping. A reverse fault implies conditions of shortening or compression but may in addition to this possibly be an incident in a general shearing movement, or a phenomenon of simple cross bending, or may be due to torsional warping." In my own field of experience I have been impressed with the frequency of joints and faults developed as incidents in differential or shearing movements. There is rapidly accumulating evidence of the existence of great thrust faults with low dips as prominent features of diastrophism.

When the shearing movements have been determined by the study of a single type of structure like folds, important corroborative evidence may be obtained from other structures. Instead of regarding structures as independent units, each with its own set of mechanical conditions, they may be viewed as a group expres3 Loc. cit., pp. 505–506.

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sion of some major move viewed the shearing nature often becomes obvious.

Distribution of Moveme zone of observation, it is diff tively whether or not ther movement or less movem Neither is it possible with an gree of definiteness to discer tude or pattern in the comp zones. The zones range from zontal, are parallel or interse horizontal position of stratifie suggests dominance of the h in movements affecting them, lution of forces along bedding ness, but the beds soon becom tical when deformed and dist be anything but horizontal. T masses between may have al Locally they may be discoidal, oval or rod-shaped, or rhomboi attempts have been made to d trolling pattern, both in larg structural features, but subje enter to so large an extent th the pattern presented is often to others.

Possible Increase of Rock Depth. Within a few hundr few thousand feet of the sur much of it open, is clearly the ess, though even here soft roc flowage. In the lower part of servation combined fracture ai

rule. Fractures are more cc closed shearing type. It has sume that this combination i tional to a zone of flowage b that rocks which have been de

often highly schistose as a res age has been cited as indicatin flowage with depth. I have view. From some familiarity formerly deeply buried terrane however, but that a careful in‹ field sections requires conside tions of this generalization. may be cited of rock flowage

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in the geologic section and rock fracture below. On the whole, the oldest rocks undoubtedly show greater evidences of rock flowage, though even here such evidences are localized in relatively narrow and numerous zones. These rocks have suffered more periods of deformation, some near the surface and some deep below, than the younger rocks. The present evidences of flow do not necessarily indicate that all the flowage occurred at great depths. Plutonic intrusions of great mass often, not always, cause rock flowage in the adjacent beds, and so far as such intrusions are more numerous with depth, rock flowage may increase. On the other hand, some plutonic intrusions in younger series which have not been very deeply buried likewise cause rock flowage. Certain it is that shearing movements, resulting in displacements which we call faults, have extended down to the bottom of our zone of observation. These partake of the nature of rock fracture in their confinement to planes and in their relations to stresses, but whether the processes be called flow or fracture is partly a matter of definition to which we shall presently make further allusion.

II. THE UNSEEN ZONE BELOW

Below the zone where the evidences of structural failure can be observed, conceptions of the structural behavior of rocks are based on such a variety of assumptions that the layman, and for that matter the geologist, has much difficulty in understanding and reconciling the various views. It is certain that rocks fail in this zone; there is evidence which permits of no other conclusion; but the manner, distribution, and causes of this failure are by no means clear. There are certain fundamental facts upon which any hypothesis must be built.

Known Facts.-Tidal experiments have shown that the earth as a whole is stronger than steel and acts almost as an ideally rigid substance.

The behavior of earthquake waves indicates that the earth behaves as a solid throughout; and for the outer quarter of the earth, at least,

the waves increase in velocity of transmissio with depth, showing that elasticity and rigidit increase faster than density.

Under surface conditions a dome of th strongest rock, corresponding to the sphericit of the earth, has a calculated supportin strength equal only to a very small fraction o the dome's own weight; but experimental wor on deformation of rocks has shown that, wit increase of containing pressures or cubica compression, the rock takes on a rigidity ca pable of resisting enormous stress difference The range of experimental evidence is not ye sufficient to show the magnitude of these di ferential stresses necessary to produce defor mation under the conditions of pressure whic might be reasonably inferred below our zone o observation; but quoting from Adams "th experiments seem to indicate that with a con taining pressure of about 10,000 atmosphere which would be equivalent to a depth of abou twenty-two miles below the surface, it woul be impossible to make the marble flow, excep under a pressure which would be simply colo sal." Geologic evidence seems to indicate supporting strength in the deep zone fa greater than that of surface rocks.

1

The rocks in the deep zone are under high temperature and greater pressure than in t zone of observation. Some notion of t quantitative values of these factors is afford by downward extrapolation of observed grac

ents nearer the surface.

The density of rocks within the zone of o servation averages about 2.7; the density the earth as a whole as determined astrono ically is in round number 5. It follows, ther fore, that the density of part of the earth mu be higher than 5, and that the density of t deep zone must be higher than at the surfac but beyond this the distribution of density the deep zone, both vertically and horizontal are unknown.

By means of the plumb line and pendulu ♦ Adams, Frank D. and Bancroft, J. Aust "On the Amount of Internal Friction Develo] in Rocks during Deformation and on the R tive Plasticity of Different Types of Rocks," Jo Geol., Vol. 25, 1917, p. 635.

it is known that the horizontal distribution of densities is heterogeneous. The density is low in the earth protuberances and high in earth depressions, as if the earth masses were in flotational equilibrium. The subcrustal densities are balanced against topographic relief. This is called isostatic equilibrium. Certain parts of the earth, called negative elements by Willis, seem to have been subjected during geologic history to long-continued deposition. Other parts, called positive elements, have been more commonly subjected to erosion than deposition. Negative elements are heavy and positive elements are light. Loading and unloading is not necessarily the primary cause of movement, but may serve to accentuate an inherent and prevailing tendency to isostatic adjustment between masses of differing density.

Isostatic balance is not complete. Some parts of the earth vary from this condition, suggesting that they have sufficient strength to sustain themselves in opposition to isostatic tendencies.

The observed relations between density and relief may be explained on the assumption that the differences in density extend uniformly to a depth of about 75 miles, called the depth of isostatic compensation. This figure is favored by geodesists. No one knows, however, the density gradients deep below the surface, or the extent to which there is heterogeneous vertical distribution of density. If instead of assuming the uniform downward extension of densities observed at the surface, assumptions are made of other vertical distributions of density, various other depths of compensation may be calculated, ranging up to several hundred miles. So far as geologic evidence goes, it seems to favor the view that depth of compensation is not uniform.

A comparison of the up-lift of mountain masses with their horizontal shortening indicates how deep the mountain making movements have extended. In general sharp close Willis, Bailey, "Discoidal Structure of the Lithosphere," Bull. Geol. Soc. of Am., Vol. 31, 1920, p. 277.

Chamberlin, R. T., "The Appalachian Folds of Central Pennsylvania," Jour. Geol., Vol. 18, 1910,

folding indicates a comp depth, whereas broad open f the plateau type of deform plained only by movements o ing to great depths. Major tinental and oceanic relief al the latter inference. If the ening observed in some mour to extend downward indefir much higher than those actua have resulted; hence the con erable movements of a shall equivalent movement below, the conception of mobility o layer, though at widely differe ferent localities.

Geologic evidence points t earth movements, indicating ment to stress is not uniform

Finally, magmas originate zone of observation and presu in the mechanical easements. conditions of rigidity alread seems certain that liquid cond intermittent. Quoting from

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The continuous or secular rela temperature, and density in the gion from which liquid rock rise be assumed to be such that mo condition either induces liquef lowers the density of rock alre render it eruptible; and such a ditions implies some sort of mol

From these facts it is clear ments extend to considerable zone of observation, that the periodic, that the earth as a rigid than steel when subje stresses like earthquake shock that it yields slowly and peri continued stress; that as a v ciently weak to allow a large static adjustment, but still st pp. 228-251; "The Building of t ies," Jour. Geol., Vol. 27, 1919, 251.

7 Gilbert, G. K., "Interpretat of Gravity," Prof. Paper 85, U 1914, p. 34.

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