512 GEOPHYSICS 



the shape of the globe, and the only known disturbance of this de- 

 scription is the ehange of figure referred to. On the other hand, were 

 the earth homogeneous, such ruptures would be expected to have as 

 envelopes small circles in latitude 45 instead of at about latitude 

 70. But since the earth is not homogeneous, this discordance does 

 not invalidate the suggestion. 



Be this as it may, upheavals, subsidences, and attendant contrac- 

 tions have been in progress throughout the whole of historical geology, 

 or the period within which fossils afford a guide to the succession of 

 strata. The so-called contractional theory has shown itself wholly 

 inadequate to account for the amount of deformation traceable in the 

 rocks of the globe, nor has the extravasation of igneous rock been 

 sufficient to account for the phenomena. To me the earth appears to 

 be a somewhat imperfect heat-engine in which the escape of thermal 

 energy is attended by the conversion of a part of the supply into the 

 vast amount of molar energy manifested in the upthrust and crump- 

 ling of continents. The subject will probably turn out to be access- 

 ible mathematically after certain experimental determinations have 

 been made, and I shall return to it presently. 



Orogeny, or mountain-building, is a mere detail of the more general 

 subject of upheaval and subsidence, but it exhibits problems of great 

 complexity both from the experimental and from the theoretical 

 points of view. There is no question that unit-strains are often reached 

 or even surpassed in contorted strata and in belts of slate, but the 

 theories of elasticity and plasticity as yet developed are inadequate 

 to deal with these strains in complex cases. An investigation on finite 

 elastic and plastic strain is now under way in my laboratory and has 

 made gratifying progress thus far; but this is not the place for de- 

 tailed results. Something also has been done in the way of working 

 out homogeneous finite strains in rocks, so that the general nature of 

 joints, faults, and systems of fissures, and the mechanism of faulting 

 is now fairly clear. The theory of slaty cleavage is a subject of dispute 

 in which I have taken part. Few colleagues appear to agree with 

 me that this cleavage is due to weakening of cohesion on planes of 

 maximum slide, but I am not hopeless that my view will make its 

 way to favor in time. 



Seismology is a vast subject by itself, but one almost totally lacking 

 in theoretical foundation. Seismological observations should afford 

 the means of exploring the elastic properties of the earth throughout 

 its interior, but the theory of the vibrations of a spheroid like the 

 earth is not yet worked out. Meantime observations are being accu- 

 mulated, but it can be foreseen that these will contribute little to eluci- 

 dation until they include the vertical components of the vibrations as 

 well as the horizontal ones. In other words, we must know the angle 

 at which the wave emerges from the surface, as well as its azimuth. 



