May, 1922 dynamics of the lithosphere 199 



This theory leaves no place for a "crust" conception as in the 

 La Placian theory. But it does make place for enough variation 

 in the composition and fusibility of earth materials even at the 

 same depth that given a certain temperature in a zone only parts 

 will become fluid. Fluid parts might tend to rise toward the 

 surface. Parts remaining solid must then collapse leaving a solid 

 zone. This would tend to produce a zonal arrangement of the 

 earth materials. That such a gigantic banding is not perfected is 

 shown by the persistence of volcanic intrusion and probably by 

 diastrophism. 



Geographic Distribution of Igneous Intrusion. — A third line of 

 evidence is that afforded by the extent and distribution of igne- 

 ous intrusion." Were a molten globe surrounded by a thin crust, 

 strains and stresses within the sphere would be in large measure 

 relieved by igneous extrusion. The extent of igneous extrusion 

 does not seem in any way commensurate with the" stresses that 

 are found. The localization of its geographic extension to certain 

 well-defined areas would hardly be expected of lavas extruded 

 from a molten interior ; and it is therefore unlikely that igneous 

 phenomena have more than a local and perhaps rather shallow 

 source. 



Seismo graphic Records. — The invention of the seismograph, by 

 which earth tremors are automatically recorded with the exact 

 time of their occurrence, has contributed much to the solution of 

 our problem. From every seismic focus there proceed three sets 

 of waves : those that travel along the surface and those that pass 

 through the earth, of which there are two kinds — compressional 

 and distortional."" Those which pass through the interior 

 increase in velocity with depth below the surface, and increase 

 more rapidly than the density, showing that the earth is rigid, 

 and that its rigidity increases. toward the center. Waves have 

 never been recorded along a diameter, but as far as they have 

 passed into the earth's interior, geologists may be sure that the 

 earth is solid, for transverse waves cannot exist in a liquid and 

 they are always recorded with the longitudinal (compressional) 

 waves. 



These records have made it possible to get at the depth of 

 the seismic focus. In the case of the Neapolitan earthquake 

 103^ kilometers was the calculated depth. -^ The mean depth 



sointerior of Earth: Oldham, Q. J., G. vS. Lon., v. G2, 1906. 

 -^Davidson: Earthquakes. 



