I9IS-] EARTH FROM THE VIEWPOINT OF GEOLOGY. 285 



its deep interior, is negligible in general studies, and enters into 

 general terrestrial mechanics only as a subsidiary feature. It seems 

 necessary to limit liquid and viscous lacunae— if there are lacunae in 

 any proper sense at all — to such moderate dimensions that they do 

 not seriously kill out distortional waves passing through the outer 

 half of the globe in various directions; for seismic instruments show 

 that these waves retain their integrity with surprising tenacity 

 through long traverses. It seems equally necessary to limit the 

 liquid and viscous factor rather severely if the interior structure is 

 to be consistent with so prompt a response of the earth to twelve- 

 hour stress-pulses as to imply almost complete elastic iidelity. 



In the light of these determinations, strengthened not a little by 

 their concurrence with the later geological determinations, the work- 

 ing hypotheses of the earth-student can scarcely fail to give pre- 

 cedence to dynamic tenets founded on a rigid earth. 



The limitation of liquid and viscous matter, thus imposed, quite 

 radically conditions all tenable views of magmas and of vulcanism, 

 and thus bears upon the igneous nature of the interior. No small 

 part of petrologic effort in past decades has been spent on the dif- 

 ferentiation of magmas. To a notable degree these efforts have pro- 

 ceeded on the assumption, conscious or unconscious, that differen- 

 tiation took its departure from an original homogeneous magma 

 such as might arise from residual portions of a molten earth. In- 

 definite lapses of time, and such conditions of quiet as are naturally 

 assignable to residual reservoirs of lava, have been freely assumed 

 as working conditions without much question as to their reality. 

 Under the hypothesis of a molten earth passing slowly into a par- 

 tially solid earth, and retaining residual lacunae of molten matter 

 as an incident of the change, these assumptions are quite natural. 

 On the other hand, under the hypothesis of a pervasively rigid 

 earth, affected by stress-conditions that are constantly varymg in 

 intensity and in distribution — and subject to more radical changes 

 at times of periodic readjustment — the existence of such residual 

 magmas becomes at least questionable, perhaps improbable. Still 

 more questionable is the assumption that the multitude of little 

 liquid spots supposed to arise within the elastico-rigid mass, always 

 have conformed to one type or set of types. The inherent proba- 



