246 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1916. 



First study the modifying effects of changes of temperature. 

 Wherever viscous flov^^ takes place in the quasisolid 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 Aveakening 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 

 that about 1° C. for each 32 meters of depth of erosion or 

 deposition. These temperature changes tend ultimately to 

 lower areas of recent erosion and to raise areas 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 

 one-fiftieth part as great as the thickness of the material eroded or 

 deposited. This is a change so large as to have considerable effects 

 in modifying or magnifying the actions which would otherwise 

 occur. Possibly this elastic change is much larger than the esti- 

 mate here given. Of course if the erosion or deposition takes place 

 in a small area only, such elastic response will be largely inhibited 

 by surrounding material on which the load has not been directly 

 changed. But under large areas of erosion or deposition such action 

 must take place and extend to depths possibly as great as 122 

 kilometers. 



Study next the modifying effects, on the phenomena already pic- 

 tured, of chemical changes which are probably produced in the earth 

 by changes of pressure. The expression " chemical changes " is 

 here used in the broadest possible sense. A relief of pressure at 



