448 ANNUAL EEPOKT SMITHSONIAN INSTITUTION, 1908. 



crust, but that it is interrupted by solidified portions and thus 

 divided into provinces. Various phenomena of vulcanism would 

 seem to agree better with that assumption. I will readily grant that 

 such inferences may correspond to the reality; but yet it seems to 

 me very remarkable that the observations on earthquakes furnish no 

 indication of a liquid layer. While I must admit that the mass of 

 existing observational material is as yet small, and that this fact 

 may explain the circumstance just mentioned, yet while awaiting 

 further developments, I am inclined to subscribe to the view that 

 really liquid areas are found only in isolated parts of the earth's 

 crust, and that on the whole the substratum of the crust is not 

 " liquid," but merely " plastic." It is well known that many ap- 

 parently solid bodies gradually yield even to very slight deforming 

 forces, if these act long enough. A stick of sealing wax supported 

 at both ends will bend, according to the temperature, in a few min- 

 utes, days, or years ; pitch flows readily and quickly. Glass acts like 

 sealing wax even under a slight rise of temperature, and the same 

 has been proved to be the case with many minerals. There is reason 

 to believe that by far the greater part of the substances surrounding 

 us, perhaps all, are plastic, provided sufficient time is available for 

 them to show it. Thus it seems possible that the earth's crust in 

 the deeper, hot parts may indeed act as a solid toward the quickly 

 passing earthquake waves, but as a plastic body toward the geologic 

 forces acting through millions of years. 



One more point has to be considered, which is of great importance, 

 in many respects of decisive importance, for the shaping of the earth's 

 crust and of the visible surface of the earth. A notable part of the 

 rocks forming the earth's surface possesses the property, when sub- 

 jected to high temperature and high pressure, of absorbing water and 

 forming with it more liquid and less dense compounds. Thus we may 

 well assume beneath the earth's surface a hot rock layer formed of 

 these water-impregnated compounds ; it is usually called the " magma 

 layer." It probably represents the plastic factor which, by yielding 

 to increased pressure, effects the equalization of mass in the earth's 

 crust. During the gradual cooling of the earth the water is liberated 

 from the magma. It has often been suggested, and I must confess 

 that I am disposed to w^elcome the suggestion, that in this way the 

 origin of the entire ocean may be explained, and that the hot springs 

 show us at least some of the paths by which the water liberated from 

 the depths of the earth by cooling and forced out by the pressure 

 of the earth, is conveyed to the ocean. 



One more remark. The plasticity of the earth's crust gives rise to 

 a peculiar instability of the surface. If, in the case of differences of 

 level, material is washed away from one part of the surface by water 

 and deposited on another part, the earth's crust at the points of 



