tively, with what results-whether rising or falling average tempera ture, increasing or decreasing volume, etc.-it is impossible now to say.

The preceding considerations apply also to the larger units of structure of the interior-although what these units may be we can only guess-and also to the earth itself as a unit. Complex mathematical analysis is necessary in applying the data to the units of structure as well as to the whole of a body as large as the earth, where the force of gravitation is itself variable with depth. These applications can in many cases be best made by those familiar with the properties in question as measured in the laboratory. The course of earthquake waves, to take an example of interest to the Section of Seismology, is dependent both upon the elastic constants of the earth's materials and the possible reflection and refraction of waves at the boundaries of internal structural features.

In this connection, the members of this Section could do a service to the other Sections by making clearer the real meaning of some of the physical concepts and physical constants involved in geophysical problems. For instance, much confusion has been caused by the fact that there is more than one kind of "rigidity." The geologist to whom the statement that "the earth has the rigidity of steel" is rather vague may take temporary comfort from the fact that the statement also needs much qualification and explanation to the physicist.3

General geophysics.-The physics of the earth considered as a unit (the classical "geophysics") is for the most part either covered by other Sections of the Geophysical Union or is customarily considered as a part of geology. Certain phases of geophysics, however, are not thus assignable and may be mentioned here in order that all of the groups of problems of the science may receive attention in this assembly of surveys.

The form and gravitation relations of our suspended spheroid, its magnetic and electrical properties, its properties as a vibrating body, and the physics of its air and water envelopes, are the obvious fields of appropriate Sections. Hypotheses of its origin and the logical deductions there from may confidently be left to the geologists, among whose faults that of narrow-mindedness and lack of a broad outlook in time and space have seldom been numbered. Its properties as an absorbing and reflecting body for external radiation are being well handled by the astrophysicists and meteorologists.

See Lambert, Journ. Wash. Acad. Sci., 10, 1920 (122–143).

In this connection it is important that the study of the composition and probable sources of the matter now being received by the earth, in the form of stony and metallic meteorites, be continued and extended.

The problem of the earth as a body radiating its own heat into space, however, is an example of a problem that may fail of treatment as a unit problem by any one of the above-mentioned groups acting alone. We are dealing not with a solid homogeneous spheroid of uniform surface temperature radiating freely into space, but with a rather heterogeneous body blanketed with several kinds of heat insulators whose composition varies both with depth and with time. Factors in the problem are: the production of heat by shrinkage; the contributions of heat from radioactive sources; the earth's present thermal conductivity and thermal gradient; the effect of varying carbon dioxide, water, ozone, inorganic dust, and clouds, upon the heat loss; and the effects of the possibly very different atmospheres with which the earth has been blanketed in past ages. The temperature at a given time and at a given distance from the center, as for instance at the solid surface of the land, depends upon a complex set of factors, and may well have been periodic in its variations.

The earth's volume and shape may have been similarly variable. In addition to the variation of temperature, already mentioned, the following are among the factors to be considered: the tides in the solid earth (on which extensive experimental work has recently been in progress) and the earth's properties as an elastic body; the viscosity of the earth as a whole, with relation to long-continued forces, and the existing state and method of maintenance of isostatic equilibrium in its surface layer; its breaking strength and form of rupture under forces changing too rapidly to cause flow; and the lag of elastic and viscous responses to changing forces, as in the case of the addition or removal of continental ice sheets.

Limitations of space forbid more than a sketchy outline of the problems set before the Section of Geophysical-chemistry, but it is hoped that the outline may have been sufficient to indicate the very fundamental character of those problems.

THE YIELDING OF THE EARTH'S CRUST.1

By WILLIAM BOWIE,

Chief, Division of Geodesy, U. S. Coast and Geodetic Survey.

The geological evidence is such as to justify anyone in concluding that the material of at least the outer portion of the earth has moved from place to place during past geological periods. Material that must have been laid down in the form of sediments in shallow oceanic waters, as is evidenced by the presence of sea-shell fossils, is now above sea level, in some cases to the extent of many thousands of feet. Sedimentary rocks, which must have been laid down in horizontal or nearly horizontal strata, are now much curved and distorted. There could be cited many cases to show that strata have not only moved in elevation but also horizontally. What has caused these movements is one of the outstanding problems in geophysics and geology.

To the superficial view, erosion of a mountain region and consequent sedimentation of a river delta present nothing but a tendency to smooth out the earth by cutting down its elevations to fill up its hollows. In short, the matter may appear as simple as the operation of a steam shovel and a line of carts grading a new city section. Comparatively recent geodetic investigations referred to in what follows lead us, however, to regard the matter as much more complex. Even yet a great deal more evidence is required, but there seems to be reason to think that the secondary consequences of erosion and sedimentation are most far-reaching and significant, leading to profound modification of the views of mountain building which formerly prevailed.

ISOSTASY AND ISOSTATIC COMPENSATION.

During the last half of the nineteenth century scientists dealing with geodetic and geological problems advanced the idea that land masses were higher in elevation than the bottoms of the oceans because of lighter material under the former than under the latter.

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1 Paper delivered at meeting of Philosophical Society of Washington, Mar. 11, 1922.

This balancing of the earth's crust was termed by C. E. Dutton, isostasy, or equal pressure. The early writers on the subject of isostasy developed the qualitative side of the theory, but it was only during the past 30 years that any quantitative values were made available. John F. Hayford, as chief of the Division of Geodesy of the U. S. Coast and Geodetic Survey, conducted a very elaborate investigation which showed that blocks of the earth's crust, of the same cross section at their bases, were in equilibrium; that is, that each block had the same mass as any other block of the crust. The base of each of these blocks was assumed to be at a definite depth below sea level, which depth was termed the depth of compensation. The compensation is defined as the deficiency of mass in the block under any particular area and exactly balances the mass which appears at the top of the block above sea level. It is also the excess in mass in a block under an ocean, and in this case it exactly offsets or balances the deficiency of material in the space occupied by the water of the ocean above the block.

FIG. 1.-A simple case of isostatic equilibrium.

If equal masses of different metals, each lighter than mercury, are moulded to the same cross section they will sink to the same depth when placed in mercury. Their lower surfaces will form a plane while their upper surfaces will be irregular. There will be equal pressure at the base of the different blocks. This is isostatic equilibrium in its simplest form.

The results of the investigations of Hayford were reported in two publications of the U. S. Coast and Geodetic Survey. The investigations started by Hayford have been continued by the U. S. Coast and Geodetic Survey up to the present time. Later results have

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2 The earliest attempt to prove the theory of isostasy was made by G. R. Putnam. (See Report U. S. Coast and Geodetic Survey for 1894, pt. 2, app. 1, Relative determinations of gravity with half-second pendulums, and other pendulum investigations.)

The figure of the earth and isostasy from measurements in the United States, by John F. Hayford.

Supplemental investigations in 1909 of the figure of the earth and isostasy, by John F. Hayford.

Effect of topography and isostatic compensation on the intensity of gravity, U. S. Coast and Geodetic Survey Special Publication No. 10, by John F. Hayford and William Bowie.

Same, U. S. Coast and Geodetic Survey Special Publication No. 12, by William Bowie. Investigations of gravity and isostasy, U. S. Coast and Geodetic Survey Special Publication No. 40, by William Bowie.

dealt especially with the relation between the theory of isostasy and the value of the intensity of gravity.

FIG. 2.-Isostatic equilibrium in the earth's crust.

The isostatic investigations show that blocks of the earth's crust of different lengths have the same mass; therefore the pressure of the blocks at a certain distance below sea level where the material changes from a resisting to a yielding solid will be the same throughout. The longer blocks are composed of less dense material than the shorter ones.

While it is impossible to determine definitely the depth within which the isostatic compensation occurs, the investigations have

US COAST AND GEOGETIC SURVEY

E.LESTER JONES
DIRECTOR

DISTRIBUTION OF GRAVITY STATIONS

1921

FIG. 3.-Gravity stations in the United States.

There are now 286 gravity stations in the United States, located as shown on the diagram. The values of gravity are used in the isostatic investigation.

shown that the depth is of the order of magnitude of 60 miles. At or just below that depth the material of the earth is assumed to change from a solid which resists stresses acting in a horizontal direction to one which will yield plastically to those stresses.