262 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



other rocks, and on the common rock-forming minerals. Plate 2 is a 

 photograph of a portion of the specimens that have been investigated. 

 The immediate conclusions from the results were first, that typical 

 rocks had a much lower compressibility, and hence a much liigher 

 wave velocity, than had previously been supposed; and second, that 

 except for very low pressures the compressibility of a given rock was 

 merely the average of the compressibilities of its component minerals. 



The above-mentioned results were obtained at or near room temper- 

 ature. Quite recently Birch and Dow at Harvard have been able to 

 carry out measurements at elevated temperatures and thus to supply 

 information concerning the effect of temperature on elasticity and 

 wave speed in rocks.* 



Brief mention will be made of one other expeiimental research, 

 wliich is pertinent to the general subject before us. This is an investi- 

 gation of the effect of high pressure on the critical temperature at 

 which iron loses its magnetism. It was carried out as a joint effort of 

 two branches of the institution, the department of terrestrial magnet- 

 ism and the geophysical laboratory. At or above 768° C, iron loses 

 its strong magnetic properties. Any masses of metallic iron within 

 the earth are subjected to the combined effect of high pressures and 

 temperatures. In order to learn something as to the possibility that 

 deep-seated motalhc iron, despite a presumably high temperature, 

 should still be strongly magnetic, measurements of critical temper- 

 ature of magnetization of iron and other ferro-magnetic materials at 

 pressures up to 4,000 atmospheres were carried out. The final result, 

 that the effect of pressure on the critical temperature was practically 

 ml, or probably no greater than 0.001° C. per atmosphere (for pure 

 iron),' will be referred to presently in connection with the core of 

 the earth. 



THE INTERIOR OF THE EARTH 



The observations and experiments that have been described have 

 not been intended merely as a partial Hst of unrelated facts in the field 

 of geophysics. They are the main clues by which we are enabled to 

 solve, at least partially, the problem of the structure of the earth's 

 interior. We are now quite certain that the earth consists of three 

 principal regions or zones, the core or central region, the crust or 

 superficial layer, and the intermediate zone. 



That the material of the earth near its center must be very heavy 

 was one of the earhest conclusions and an excellent example of deduc- 

 tions concerning the interior. Since the average density of the earth 

 as a whole is 5.5, as determined from the measured constant of gravi- 

 tation, and smce the average density of rocks found at the surface is 



8 Birch, Francis, and Dow, Richard B., Bull. Geol. Soc. Amer., vol. 47, pp. 1236-1255, 1936. 

 • Adams, L. H., and Green, J. W., Phil. Mag., vol. 12, pp. 361-380, 1931. 



