support considerable structures against isostatic tendencies; that it is not essentially molten or fluidal in the ordinary sense; that molten magmas are probably local and incidental.

As to depth and distribution of the movements, and as to the manner of movement, whether by fracture or plastic flow or by some unknown process, there is wide divergence of opinion. Likewise, there is doubt as to the laws or control under which stresses may be transmitted. We may refer briefly to these questions.

Does a Zone of Weakness or Mobility Exist in the Unseen Depth?-A common conception of the distribution of movement deep below our zone of observation confines it to a single spherical zone of weakness or mobility surrounding the centrosphere and surrounded in turn by a rigid shell. This zone is supposed to be marked by a capacity to yield readily to long enduring strains. It may be in part the generating zone of magmas, which may be a factor in its supposed easy yielding. The conception of the existence of a weak and mobile zone has found expression in several ways.

The widely held belief in the existence of a zone of rock flowage below a surficial zone of fracture has commonly carried with it an assumption of the relative weakness and mobility of this zone. In fact zone of rock flowage " and "zone of weakness" have come to be almost synonymous in discussion of this problem. Doubt as to this correlation is expressed later. Even if the existence of a single zone of rock flowage were proved, it does not necessarily follow that this is a zone of weakness.

Van Hise assigned a depth of only six miles to the top of this zone, though with the important reservation that increased rigidity under containing pressures would greatly increase this figure.

Adams and Bancroft, on the basis of experiments with rock failure under great containing pressures, conclude that the amount of tangential thrust required to produce movements increases so rapidly below the surface Loc. cit., p. 635.

:

Hayford assumed concentration of mo ment within the lower part of the zone of i static compensation, that is within 75 mi of the surface.

Willis concludes that there is a zone of justment below 40 miles and extending to base of the asthenosphere, and that the justments necessary to isostatic undertow ta place mainly between 45 and 100 miles fr the surface.

In contrast to these conceptions of a de mobile zone, are the views of T. C. Chamber and R. T. Chamberlin, who postulate multipl ity and irregularity of movement zones.

R. T. Chamberlin concludes that mounta making diastrophism affects wedge shap masses and implies steeply inclined zones movement.

T. C. Chamberlin emphasizes the superfic nature of diastrophic movements of the mo tain making kind, whether these are tang tially compressive or the result of creep continental masses under gravity. In rega to deeper, so-called massive, movements of

• Loc. cit.

66

kind reflected in major features of continental and oceanic relief, he does not assume any mobile substratum, but rather steeply inclined zones of movement. As he states it:10 Inherited inequalities of specific gravity are, perhaps more than any other agency, the governing power in shaping if not actuating diastrophic movements "—but that "the normal mode of isostatic adjustment in such an earth is thought to be wedging action in the form of movements on the part of its constituent tapering prisms, conical, pyramidal, or otherwise, in response to the varying stresses imposed on them. . . . They should reach to whatever depths may be seriously affected by differential stresses of an order requiring readjustment. No undertow in a hypothetical mobile substratum is necessarily involved and none is postulated."

These are only a few of the views that might be cited to indicate the wide range of hypotheses possible as to depth, number, and attitude of deep mobile zones. The very diversity of these views emphasizes the restricted range of known facts. The requirement of proof naturally rests most heavily on hypotheses which most precisely restrict the locus of movement. So many assumptions must enter into this proof that in our present state of knowledge it can not be rigorous. The safest scientific attitude for the time being would seem to be one of rigid adherence to the known facts, and the recognition of the possibility of more than one hypothesis to explain them. This is not incompatible with a sympathetic attitude toward the efforts of those attempting proof of a single hypothesis.

Until the time comes when it is possible to furnish definite proof of any specific localization of movement, my own inclination is to keep clearly in mind the distribution of movements within the zone of observation, already summarized, as perhaps the best guide to the condition that may be assumed at least for some distance below our lowest observations. This measuring stick is short, but there are 10 Chamberlin, T. C., "Diastrophism and the Formative Processes, "Jour. Geol., Vol. 21, 1913, p. 520; Vol. 26, 1918, p. 197.

some reasons for believing tha any yet available to meas through the complex of hypot the deep zone. Especially is keep in mind the fact that cl ing rock flowage, as observed part of our zone of observati general have an attitude requi ception of tangential shearing i This does not disprove a differ low, but it does eliminate an a ing on the question which has implied.

Ac

Are Deep Movements Rock Flowage Rather than b ture?-It remains to consider processes through which deep accomplished, whether by pla fracture or by some combina kinds of deformation. The v hypothesis is that deformation zone is mainly by rock flow formed rocks have not been se the environmental conditions be measured; yet there are weigh tions favoring this view:

Experimental work has shov flowage requires containing press least to the crushing strength these pressures surely exist in t Within the zone of observati strongest rocks have locally flowage and hence have locally, shallow depth, been under con sures sufficiently in excess of th strength to permit flowage. Wit creased pressures at greater depth to argue that conditions for flow improved. Under these condition ance to deformation is a func internal friction or viscosity c This property does not of necess relation to the compressive stren petency of the rock-qualities ▾ mine its behavior in the absence taining pressures. Quartzite or far as we know, may have no grea than marble or slate. Adams' show diabase and marble in a

specimen behaving similarly. In fact marble actually penetrated the harder diabase. Likewise, gypsum penetrated steel. While there are probably differences in the internal friction or viscosity of different rocks under these conditions, the results are nevertheless homogeneous in approximating rock flowage-in contrast to the heterogeneous results under less containing pressures where competency and strength of rocks play a part.

Earth temperatures increase with depth. Increase in temperature aids and accelerates rock flowage. This is evidenced by flowage of hard rocks at moderate depths at batholithic contacts. Also facts of physical chemistry show that increase of temperature increases molecular activity, hastens endothermic reactions (anamorphic reactions are largely endothermic), increases solution, both liquid and solid, and hence recrystallization, and decreases viscosity or internal friction.

Notwithstanding these and other considerations, any conclusions as to the existence of a deep zone in which all rocks flow when deformed is hypothesis, not proved fact, and perhaps will always remain so. The environmental conditions are not accurately known; and even if each of the factors were measured, their conjoint effect is still speculative. Variations in the time factor alone may determine whether a rock flows or fractures. Rock flowage which has occurred in rocks now accessible to our observation fails to indicate increase with depth with sufficient clearness and definiteness to warrant confident downward projection.

evidence of rock flow. Presumably with longer time and proper conditions of temperature and mineralizers, parallelism of newly developed minerals, characteristic of rock flow, would result. So far as the experimental results go, however, they fail to exhibit structures which in ordinary geologic field interpretation would be classed as typical rock flowage. They would be called fracture or combined fracture and flowage. They would be described as shear planes and faults. They might suggest rupture of the kind that originates earthquake shocks.

Experimental evidence has been construed to indicate that under great containing pressures, of the kind probably existing at depth, the movement under thrust or shear is of the nature of rock flowage, but this is partly a matter of definition. The rock breaks and granulates, often along definite planes, but the parts are still held together; it really flows. Displacements along these planes may partake of the nature of faults, and there is no development of true flow cleavage determined by a parallel arrangement of minerals under recrystallization, the common geologic

Rock flowage has been widely assumed to indicate weakness and mobility. The correlation of rock flowage with weakness may arise from the fact that certain soft rocks such as shales, which are inherently weak, may often be observed to have undergone rock flowage, while adjacent strong rocks have been unaffected. Or, a zone of flowage passing through a homogeneous formation unquestionably indicates movement along the flowage zone, and, therefore, indicates the weakness of this zone relative to adjacent undeformed parts of the mass. But it would be equally valid to argue that where fracturing has been concentrated along a zone between unde formed rocks it too indicates movement, and therefore relative weakness. It is a long step from this to the conclusion that flowage indi cates greater weakness than fracture. It i entirely conceivable that it might requir more energy to make rock flow than to mak it fracture. Indeed there is some reason fo believing, both from experimental work an from observations in areas of combined frac ture and flowage, that relief actually take place first and most easily by fracture an that flowage occurs only when it is possibl to concentrate much more energy into th rock. Both structures show weakness relativ to adjacent undeformed masses, but in rela tion to each other degree of weakness is much more complicated problem.

Our question, then, as to the extent which deep movements are accomplished 1 rock flowage can not be simply and definite answered in the present state of knowledg

The preponderance of environmental evidence seems to indicate that rock flowage is the distinctive kind of movement, but so many qualifications, definitions and assumptions enter into this conclusion that my present inclination is to keep firmly in mind the complex facts of deformation in our zone of observation as a possible key to the interpretation of unseen movements. This attitude will require us to pay more attention than heretofore to the possibilities of heterogeneous structural behavior at great depths. Particularly should we keep in mind the fact that

the kind of rock flowage accomplished experimentally produces structures which in the earth have sometimes been called fracture or combined fracture and flowage. We may assume a downward extension of combined fracture and flowage, as observed in the field, and still meet the conditions of flow implied by experiment.

How Are Stresses Transmitted in the Deep Zone?-In our zone of observation stresses are clearly transmitted by the competent members of the lithosphere. In any area of deformation evidence may usually be found of the control of the structure by one or more competent members. When the notion was widely held that the interior of the earth was molten or fluidal, hydrostatic stress conditions were naturally assumed. With the later knowledge that the earth acts essentially as a solid throughout, this view was largely abandoned in favor of the view that rocks in the deep zone act as rigid competent members capable of transmitting stresses in definite directions. The vector properties of cleavage and other structures supposed to develop in this zone were cited to indicate the definite orientation of stresses. It does not follow from this, however, that pressure conditions were or are not hydrostatic, especially under slow movements. Rocks under compression from all sides greater than their crushing strength seem to transmit stresses in a manner suggesting approach to hydrostatic conditions of pressure. When the stress differ

ences

are such as to requ movement in the direction The stress as reflected by would seem to have been t definite direction, and yet th have remained hydrostatic. imagine a volume of liquid surface subjected to differen cient to deform its containing that the movement would be of easiest relief, notwithstan static conditions within the li ity of movement is possible ception. Rock structures ind

only, not necessarily the in

Movement of rocks under the posed to obtain deep below th likely to be at least in part a of materials so contained betw bers that the direction of esca oriented. Of course this supp that on some scale, small or 1 units of mass competent to a walls for materials acting un pressure. If all the mass in were under hydrostatic pressure walls might be regarded as above, inequalities in the compe would control the movements i of easiest relief. However, r such as cleavage and folds, w rangement of the sort observed face and of the sort supposed tell us only of the direction of fail to indicate whether the stre static or otherwise.

CONCLUSION

Within the zone accessible t movements of rock masses are by fracture and flowage. These be distinct and separate, or so i to make definition difficult. movement are many, their posit tudes diverse. In general they i ing or grinding movements 1 masses, accomplished both by flowage, and caused by stresse

MARCH 4, 1921]

This conception is the zones of movement. taken to afford the best initial basis for the interpretation and correlation of observed rock structures. There is no certain evidence of increase or decrease of movement toward the bottom of this zone. Beyond a shallow surface zone, there is no certain evidence of increase of rock flowage and decrease of rock fracture with depth. There is no certain evidence that rock flowage means greater weakness than rock fracture. There is no certain evidence in rock flowage that pressures are dominantly hydrostatic or dominantly those of competent solid bodies.

Movements are known to occur in the zone below our range of observation, but their nature and distribution are the subjects of varied hypotheses based on a few known conditions. Much of the sharper diastrophism seems to be confined to a thin surficial zone. Deeper movements, of a more massive type, periodic, and possibly slower, seem to be implied by the relative movement of great earth segments as represented by continents and ocean basins. Their depth is unknown. Most of the current hypotheses agree in assuming a single mobile zone in which rocks move dominantly by rock flowage. The basic requirements of reasonable hypothesis, however, may be equally well met by a conception of movement much like that of the zone of ob

servation. This does not require or postulate the conception of the existence of any single mobile zone, or zone of slipping, or zone of flowage, or of an asthenosphere. It supposes movement irregularly distributed in many zones, with any inclination, and accomplished by both fracture and flowage as far below the surface as movement extendsalways remembering that some of the structures geologically described as fractures, may be expressions of mass movement of the kind defined as flow in experimental results.

Conditions of temperature and pressure and vulcanism become more intense with depth, but it remains to be shown that their conjoint action results in a uniform environ

SCIENTIFIC EVENTS

DINNER IN HONOR OF THE RETIRING
SECRETARY OF AGRICULTURE

THE success of Secretary E. T. Meredith interesting the public in the investigatic work of the U. S. Department of Agricul has been unique. His prompt recognition the needs of the department and his acti in behalf of the investigators there, H attracted the attention of scientific throughout the country. Coming to the se taryship at a time when the morale of scientists in many government departm was being seriously impaired through couragement as to the possibility of secu adequate support for investigation, his paign of education had the effect bot awakening the public to the extent and portance of the work, and of heartening workers.