PRESENT PROBLEMS OF GEOPHYSICS 519 



precisely the intrinsic elastic moduluses of the earth along the paths 

 of earthquake-waves. 



It will also be practicable to examine critically the possible rup- 

 ture of the globe as a consequence of change of figure and to study 

 intelligently the simpler cases of the crumpling of strata, fissuring, 

 and other problems in the mechanics of orogeny. 



The science of elasticity has had a very disappointing history. 

 Simple as is the assumption ut tensio, sic vis, the attempt to solve 

 even such seemingly elementary problems as the flexure of a uni- 

 formly loaded rectangular bar leads to insoluble equations; so that 

 the science has been relatively unfruitful. It remains to be seen 

 whether a truer relation between load and strain will not simplify 

 formulas and increase the applicability of algebra to concrete cases. 



From an astrophysical point of view the dialytic action of mineral 

 septa is unimportant, but it is very interesting in its bearing on 

 metamorphism and ore deposition, and may readily contribute to 

 economic technology. 



The relations of viscosity to the diffusion of matter have not yet 

 been elucidated, even for ordinary temperatures. This subject is one 

 of much importance in connection with the genesis of rock species, 

 and of course it should be studied at 10 before undertaking researches 

 at 1000. 



High temperature work is essential even to the investigation of the 

 elastic problem, and it is almost a virgin field. Even thermometry is 

 very imperfect above the melting-point of gold, though it is destined 

 soon to become exact at least as high as 2000, a range which will 

 probably suffice for geophysics. But we are also in almost total 

 ignorance of the extent to which the laws of physics, studied at 

 ordinary temperatures, prevail at 1000 or 2000 degrees. One of the 

 less difficult problems of this group is that of thermal conductivity 

 and specific heat of solid bodies at high temperatures. For the 

 principal metals this is already known as far as 100, but not for 

 rocks or minerals. It would be especially desirable to have such 

 determinations for granite, basalt, and andesite, the last representing 

 the average composition of the accessible part of the lithosphere. 



It seems to me that when the thermal diffusivities are known for 

 these rocks, over a range of a thousand degrees, the question of 

 upheaval and subsidence can be attacked with a good prospect of 

 success. A cooling sphere is conceivable in which the distribution 

 of thermal diffusivity is such that the flow of heat would be " steady," 

 in Fourier's sense, and thus accompanied by no superficial deforma- 

 tion. With any other distribution of diffusivities, deformation would 

 occur, and the globe would act as an imperfect heat-engine, the work 

 done being that of upheaval or subsidence. Now when the assuredly 

 variable value of diffusivity for the materials of the globe is known, 



