GEOPHYSICAL LABORATORY. 129 



igneous rocks; gabbros occupy an intermediate position; and when we note 

 that the rigidity of an ultra-basic rock, e. g., a peridotite, is two-thirds as 

 great as that of steel, we no longer find it so difficult to see how the crust 

 of the earth can be nearly as rigid as steel. How temperature influences 

 these values has not yet been determined. This is probably a small effect, 

 but we hope that we may be able to measure the effect of high temperature on 

 volume change and on elasticity in general. 



With a knowledge of the elasticity of a specified rock at a specified depth, 

 we have in our hands a powerful instrument for determining the character 

 of the rocks at each level in the earth's crust. For, from the velocity of 

 transmission of earthquake waves, we obtain a value for the elasticity of the 

 rocks at a specified depth, and by combining the two sets of data we can 

 state the nature of the rocks not only near the surface but to a certain extent 

 in the very interior of the earth. The velocity of transmission of earthquake 

 waves, especially for the shallower depths, is known none too well; but 

 taking the best data available, the initial velocity of the faster waves is about 

 7 km. per second. Now, from our compressibility measurements, the velocity 

 of such a wave in a typical granite is 5.6 km. /sec. and in a typical 

 gabbro is 6.9 km./sec. Hence, if we place any confidence in the seismic data 

 for near-by earthquakes, it is evident that the granitic layer is a mere film 

 on the earth's surface — so thin, indeed, that where considerable depths are 

 involved it may be neglected.^ The main part of the so-called crust of the 

 earth is, therefore, at least as basic as a gabbro, and possibly more like a 

 peridotite. Thus, very indirectly, and somewhat adventitiously, our infor- 

 mation about the interior of the earth is gradually widened. 



The compressibility of a substance is connected with the forces between 

 the atoms and will eventually be of value in the study of atomic structure. 

 Already the close correspondence between our value for the compressibility 

 of diamond and the value calculated by Sir J. J. Thomson in accordance with 

 his theory of atomic structure is of considerable interest. 



It is our purpose, if the opportunity can be provided (Year Book 20, p. 1 1 ; 

 Year Book 21, p. 12), to undertake a much more thorough study of these 

 elastic constants of the rocks through as great a range of temperatures as 

 possible, and so to determine, with as much certainty as we may, the con- 

 stitution of the earth beneath our feet. Pressures equivalent to a depth of 

 25 miles and temperatures equivalent to a depth perhaps of 50 miles are now 

 within the reach of laboratory experiment. 



It is also a part of the program of the Advisory Committee in Seismology, 

 of which a report is submitted on a later page, to develop apparatus for a 

 much more critical study of the manner and speed of transmission of local 

 earthquake shocks than has heretofore been possible. These two under- 

 takings together offer great promise of yielding real information about the 

 interior of the earth through experimental study and measurement. Should 

 it prove practicable to define such measurements explicitly through trans- 

 mission of artificial earthquakes set in motion by high explosives in known 

 rock masses, the method would seem to be both direct and quantitative. 



^ Recent calculations from data obtained from the Oppau explosions indicate a granite layer 

 Bome 15 km. thick (Dorothy Wrinch and H. Jeffreys, Roy. Astr. Soc, M. N. Geophys. Supple- 

 ment 15-22, 1923). 



