306 HAYFORD— THE EARTH FROM [April 24, 



So much for the direct effects of gravity which it seems im- 

 portant to picture clearly. Next study other effects, some of which 

 are indirectly produced by gravity. 



First study the modifying effects of changes of temperature. 

 Wherever viscous flow takes place in the quasi-solid portions of the 

 earth there heat is necessarily developed in amount equivalent to the 

 mechanical energy expended in overcoming the resistance to flow. 

 This will tend to increase the volume of the material, to increase 

 the pressure, and to raise the surface above the region of viscous 

 flow. It is probable also that the increase of temperature will tend 

 to weaken the material, thus emphasizing the weakening produced 

 by the damaging mechanical effects of the flow. 



This temperature effect is probably locally important. 



Beneath areas of recent deposition the temperature of a given 

 part of the buried material will slowly increase for long periods of 

 time, on account of heat conducted up from below and prevented by 

 the new blanket of deposited material from rising to the surface so 

 freely as before. Conversely, beneath the areas of recent erosion 

 the temperature of a given portion of material will decrease. The 

 ultimate limit of change will tend to be in each case not greater than 

 about one degree Centigrade for each thirty-two meters of depth of 

 erosion or deposition. These temperature changes tend ultimately 

 to lower areas of recent erosion and to raise area's of recent deposi- 

 tion, possibly as much as one thirtieth of the thickness of the erosion 

 or deposition, — the temperature effect taking place much later than 

 the erosion or deposition which initiated it. 



Study next the effects which may be computed from the bulk 

 modulus of elasticity. Beneath areas of erosion a given particle of 

 matter tends to rise by an amount which may be computed from the 

 bulk modulus of material, and similarly a particle tends to fall be- 

 neath an area of deposition. If the depth to which the elastic phe- 

 nomena extend is as great as 122 kilometers and the bulk modulus 

 is 500,000 kilograms per square centimeter (corresponding to 

 granite) the rise or fall of a particle near the surface will tend to 

 be at least i/50th part as great as the thickness of the material 

 eroded or deposited. This is a change so large as to have consid- 

 erable effects in modifying or magnifying the actions which would 



