i9o8.] THE PHYSICS OF THE EARTH. 257 



way. We owe the establishment of these effects of loading chiefly 

 to the researches of Professor Sir G. H. Darwin, whose labors 

 have so greatly advanced our knowledge of the physics of the earth. 

 They have an extremely high importance in the theory of bodies 

 approximating elastic solids. The undisturbed crust of the globe 

 fulfills these conditions quite perfectly. 



But to suppose that any of these small surface effects could give 

 rise to world-shaking earthquakes which would shake down cities, 

 raise sea coasts, and uplift mountains and islands in the sea, is too 

 severe a test of credulity to be entertained. The class of minute 

 movements, due to surface yielding under varying loads depending 

 on sediments, tides and meteorological causes, and the class of great 

 moz'cmcnts, due to the expulsion of lava from under the bed of the 

 sea, are quite distinct. One class of these phenomena is micro- 

 seismic, the other magaseismic. Previous investigators have gen- 

 erally confounded the two classes of phenomena, and hence they 

 have been unable to recognize the true cause of earthquakes and 

 mountain formation. For that reason it was necessary to restrict 

 our investigation to the great disturbances, in the first search for the 

 cause of the great movements of the earth's crust. 



We repeat that both classes of phenomena are important in a 

 complete theory of the physics of the earth ; but the small yieldings 

 of microscopic dimensions must be kept distinct from the great 

 movements which have shaped the surface of the globe. Many of 

 the small effects depend on the greater movements of the earth, 

 while few of the great movements are influenced by surface forces— 

 indeed none at all, except where accumulation of subterranean 

 stresses has already rendered the conditions highly unstable. In 

 this latter case small surface forces may occasionally accelerate the 

 outbreak of an earthquake, just as a spark discharges a loaded gun, 

 or a shock explodes a charge of dynamite. 



On a par with the theory that slight inequalities of surface load- 

 ing produce earthquakes is another equally untenable view that 

 the earth is a failing structure. Such a doctrine might have been 

 entertained a quarter of a century ago, when the theory of secular 

 cooling was generally accepted, but to-day such a view is anti- 

 quated and utterly indefensible. Ozving to the demonstrated de- 



