October 28, 1904.] 



SCIENCE. 



549 



detail of the more general subject of up- 

 heaval and subsidence, but it exhibits prob- 

 lems of great complexity both from the ex- 

 perimental 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 investi- 

 gation on finite elastic and plastic strain is 

 now under way in mj^ laboratory and has 

 made gratifying progress thus far; but 

 this is not the place for detailed 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 diie 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 

 Avorked out. Meantime observations are 

 being accumulated, but it can be foreseen 

 that these will contribute little to elucida- 

 tion until they include the vertical com- 

 ponents 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. 

 The causes and conditions of earthquakes 

 afford a separate topic of great interest. 

 That some of them are of volcanic origin 

 is evident; others appear to be due to 



paroxysmal faulting, yet there is very 

 possibly a common underlying cause. 



On no subject are opinions more diverg- 

 ent than concerning the origin and mechan- 

 ism of volcanoes. To the ancients they 

 were the mouths of the river Phlegethon. 

 To those who adhere to the Cartesian doc- 

 trine they are communications with the 

 liquid interior of the earth. Most geol- 

 ogists think of them as connected with 

 hypogeal reservoirs of melted matter sub- 

 sisting for indefinitely long periods of time. 

 Finally it is conceivable that the lava may 

 be extruded as soon as the melted mass has 

 accumulated in sufficient quantity, some- 

 what as water may break through an ob- 

 structing dam after its depth reaches a 

 certain value. The continual movements 

 of the rocks show that they must be to 

 some extent in a state of elastic strain, so 

 that a given cubic mile of rock resists sur- 

 rounding pressure in virtue both of its 

 rigidity and of its compressibility. If that 

 cubic mile becomes liquid, its rigidity is 

 gone and the change of shape of surround- 

 ing masses may aid in its expulsion. Of 

 course imprisoned gases, especially the 

 ' juvenile waters ' of Professor Suess, may 

 also play a very important part in expul- 

 sion. But the more I have studied the 

 matter, the less probable it seems to me 

 that considerable bodies of melted lava can 

 remain quiet for long periods of time in 

 the depths of the earth. The influences 

 tending to their expulsion would seem to 

 be at a maximum immediately after the 

 fusion of enough material to supply an 

 eruption. 



Relief of pressure is often invoked to 

 explain fusion of lava, but it is not a 

 wholly satisfactory cause. If a deep crack 

 were to form, the rock at the bottom might 

 melt indeed, but, as the crack filled, the 

 pressure and the solidity of the source 

 would be restored. To me, Mallet's hy- 

 pothesis is more satisfactory, so far as the 



