200 SEE— FURTHER RESEARCHES ON [April 24, 



Now earthquake disturbances are often complex, and consist in 

 horizontal and vertical movements combined. We have seen that in 

 the long run the uplifting tendency predominates, because it is in 

 this way that the mountains and plateaus have arisen. Nevertheless 

 there are numerous cases in which subsidences take place, and these 

 settlements often seem to be somewhat gradual, as if the substratum 

 was slowly yielding and flowing under the stresses to which it is 

 subjected. These gradual subsidences, of the class that was observed 

 by Darwin and Fitzroy at Conception in 1835, seem to afford con- 

 vincing evidence that the layer beneath the crust is certainly plastic, 

 perhaps viscous.* The yielding of the layer beneath the crust is 

 shown not only in movements noticed in earthquakes, when lava is 

 expelled from under the sea and pushed under the land ; but also in 

 the subsidences which the sea trenches experience after earthquakes. 

 These subsidences have folded the rocks seen in mountain ranges 

 now on land ; and although most of such subsidence is due to the 

 undermining of the troughs by the expulsion of lava, it seems likely 

 that some very gradual yielding also takes place. The layer under 

 the crust is therefore certainly plastic, when partially undermined, 

 and probably so, independent of the undermining, if it is subjected 

 to great forces, as in world-shaking earthquakes, where mountains 

 are in process of upheaval. If the matter is also viscous, the viscosity 

 must be very high. With the matter imprisoned beneath the earth's 

 crust it is difficult if not impossible to distinguish between plasticity 

 and true viscosity, because, if the fluid is very stiff, it would behave 

 almost as a solid. And the tests heretofore afforded by earthquakes 

 are not decisive. This view of the substratum just beneath the crust 

 is not essentially different from the theory held by Arrhenius with re- 

 gard to the interior of the earth as a whole. But this layer is the 

 only part of the interior in which movements may be observed, and 

 even here movements would not take place but for the steam de- 

 veloped beneath the crust by the secular leakage of the oceans. It 

 may be that the future study of these movements will some day 



* We follow Sir George Darwin in "distinguishing viscositj', in which 

 flow is caused by infinitesimal forces, from plasticity in which permanent 

 distortion or flow sets in when the stresses exceed a certain limit." (Letter 

 to Sir A. Geikie, January 9, 1884.) 



