376 A. KEITH OUTLINES OF APPALACHIAN STRUCTURE 



The heat derived from the fall and acc-iimulation of meteorites is ex- 

 ceedingly small in comparison with the mass of the earth, and it is very 

 difficult to show that a concentration of it is probable either in time or 

 in space so as to answer the requirements of mountain-building. Any 

 hyj^othesis of a special accumulation of meteorites to account for a period 

 of diastrophism must necessarily be purely speculative. If there were 

 such an accumulation, which could be caused only by passing through 

 an unusual swarm of meteorites, such a passage would be rhythmic and 

 repeated at similar intervals in the earth's history. There is no evidence 

 of such rhythm, but rather the contrary. Still less is there evidence of 

 special concentration of meteorites in certain parts of the earth, and it 

 is difficult to see how such could be the case, on account of the rapid 

 revolution of the earth and the exposure of all of its parts to the fall of 

 meteorites. Finally, it is questionable if any meteorites are warmer than 

 outer space until they reach the atmosphere. Therefore, it is not easy 

 to understand how they could heat the earth's interior to temperatures 

 higher than their own. 



Owing to the existing difficulty of demonstrating the i^rimal source 

 of the internal heat of the earth, we can at present do hardly more than 

 accept its presence and its great downward increase and adjust to that 

 condition our theories of mountain-building. 



GRAVITY 



The other great fact of the world, gravity, must of course be the major 

 part of the theory, just as it is the most compelling force in the universe. 

 The gravity may be that inherent in the mass of the earth itself or be- 

 tween different parts of that mass, or it may be the gravitative influence 

 exerted by the planets or other heavenly bodies. There is no motion of 

 the crust in which gravity is not concerned in some way, even if not as 

 the principal cause. All investigators agree that its power is great 

 enough to cause any of the effects that we see in mountain-building, and 

 both through theory and observation we know that the rocks and minerals 

 of the earth's crust are unable to resist gravity indefinitely. Its power 

 is tangibly shown in a multitude of phenomena : the falling bank of 

 earth, the rocks breaking from the cliff, the landslide covering miles, the 

 toppling of mountain peaks, and the sliding of thousands of cubic miles 

 of glaciers. The greater the masses involved the greater are the forces 

 exerted. 



The chief difficulty in establishing the connection between gravity and 

 mountain-building lies in the facts that the pull of gravity is chiefly 

 downward toward the center of tlie earth and that the pull must be 



