907.1 AND MOUNTAIN FORMATION. 407 



to the agitation and to the movement of streams beneath the crust. 

 These seem to be the most probable causes of the changes in the 

 earth's magnetic field, often noticed to disturb the needle at the 

 time of earthquakes and volcanic eruptions ; but our knowledge of 

 the subject is still in its infancy and we must be cautious in draw- 

 ing conclusions. Some of the disturbances of course are purely 

 mechanical. 



The comparatively slow propagation (868 miles per hour) of 

 the magnetic disturbance at the time of the Krakatoa outbreak, 

 August 2.J, 1883, might be ascribed to the undulatory commotion 

 in the upper air, since sound so propagated would have about this 

 velocity. It would seem to depend more upon the indirect effects 

 of a disturbance of the air than upon the forces exerted by a direct 

 magnetic charge. Both causes are no doubt at work in certain 

 disturbances, and time must decide just how the effects are 

 propagated. 



Conclusion. 



At the close of the two former papers on the physics of the 

 earth, the chief results at which we arrived are briefly summarized. 

 In the same way we may here add a recapitulation of the results 

 which are more fully established in this 1?hird paper. 



1. It has long been customary to refer mountain making to 

 catastrophes in remote geological ages, and to suppose that the 

 formation of mountains is not going on upon the earth at the 

 present time. Such supposed spasmodic activity is contrary to 

 the whole order of nature and to the doctrine of evolution by 

 which we labor to interpret animate and inanimate phenomena. 

 On the other hand the discovery of mountains in the depths of the 

 sea has excited the surprise without satisfying the wonder of the 

 naturalist, so that the whole subject has become involved in 

 darkness. 



2. The doctrine of continuity, as respects both time and space, 

 permits us to view earthquakes, mountain formation, and kindred 

 phenomena, merely as a part of an unbroken whole which has 

 come about by the steady action of natural laws operating not only 

 along the borders of the continents, but also in the depths of the 

 ocean. A theory which unites and harmonizes these phenomena. 



