518 GEOPHYSICS 



pressures and very high temperatures exist near the earth's centre, 

 while the quality of matter which constitutes the interior cannot 

 be satisfactorily determined until we know how substances would 

 behave under extreme pressures and at temperatures approaching 

 2000 C. There is every reason to suppose that under purely cubical 

 compression, dense, undeformed solids are perfectly elastic. Hence 

 the basal problem of geophysics is to find the law of elastic compressi- 

 bility. This cannot be accomplished by direct means, but the task is, 

 nevertheless, as pointed out above, not a hopeless one, and has been 

 taken in hand. Should success be achieved, researches will follow on 

 the variation of elasticity with temperature. This feature of the 

 investigation will present very great experimental and theoretical 

 difficulties, but there is no good reason to despair of success. 



When the law of resistance of solid bodies becomes known as a 

 function of both temperature and pressure, even for isotropic sub- 

 stances with only two moduluses of elasticity, the way will be opened 

 to various important investigations, largely mathematical in char- 

 acter. It is true that thoroughly isotropic bodies are seldom met 

 with, yet geological masses must, nevertheless, often approach closely 

 to this ideal. Many of the most important rocks are chiefly com- 

 posed of triclinic feldspars, which, indeed, occur about as abundantly 

 as all other minerals found at the surface of the earth put together. 

 A triclinic feldspar crystal rejoices in the full possible number of 

 elastic moduluses, 21. Yet a large spherical mass of small, fortui- 

 tously oriented feldspars will behave to external forces of given in- 

 tensity and direction in the same way no matter how the sphere may 

 be turned about its centre, and it will, therefore, act as an isotropic 

 body. This fact is enough to show that an infinite variety of intimate 

 molecular structures are compatible with molar isotropy. 



Thus a knowledge of isotropic elasticity will suffice as a basis for 

 testing reasonable hypotheses of the constitution of the earth's 

 interior, taking into account its known rigidity and density. Still 

 greater light can be thrown on this subject by including in the inves- 

 tigation the moon and Mars; for their masses and dimensions are 

 known, and there seems every probability that they are composed 

 of the same materials as the earth, though in different proportions. 

 If a given hypothesis as to the chief constituents satisfies the known 

 conditions of all three planets, it will doubtless find acceptance. 

 Such a result would open the way to fresh advances in geodesy and 

 terrestrial magnetism, and cast backward through the vista of time 

 a ray of light on the nebular hypothesis. 



Again, when the law of elasticity and the approximate constitution 

 of the globe are known, it will be possible to work out a satisfactory 

 theory of the simpler modes of vibration in a terrestrial sphere, and 

 then seismological observations can be applied to determining more 



