MECHANICAL STATE OF THE ISOSTATIC SHELL 261 



Hence it follows that, however small the ratio which expresses the relation 

 of strength to load may be, it will always be a finite quantity. 



Pressure, which increases internal friction, can not destroy solidity or 

 promote mobility. 



EFFECT OF HEAT 



The preceding discussion deals with the problem of the strength of a 

 cold lithosphere. The lithosphere being hot, the concept of rigidity and 

 great strength which has been suggested must be somewhat qualified by 

 the weakening effects of heat. 



The surprisingly high values of rigidity and strength which Adams has 

 demonstrated are effects of exceedingly great pressures. In the litho- 

 sphere there are proportionately high temperatures, which oppose ex- 

 pansion to compression, and appeal is commonly made to this effect of 

 heat to justify the assumption of flow of solid rock under stresses which 

 would otherwise be incompetent to produce movement. Comparison and 

 expansion are thus placed in the balance, and it is a question which out- 

 weighs the other. 



The question is rigidly conditioned by the rates of increase of pressure 

 and temperature with depth in the lithosphere, which we may accept with 

 the values given by Adams^^ as being in accord with the most reliable 

 available data. They are expressed by the straight lines marked pressure 

 and temperature in plate 9. 



The moduli of cubical compression of rocks, as determined by Adams, 

 vary from four to ten million, granites being the most compressible and 

 basic intrusives the least so. Adams says : 



"The cubic compressibility, D, of the earth's crust must lie between the 

 values given for the granites and the basic intrusives, approaching one or the 

 other of these values according to the proportion in it of one or the other of 

 these classes of rocks. If we take the average of the values obtained from 

 these two classes of rocks as represented by the seven granites and the five 

 basic intrusives, the value obtained for D is 6,353,500. This, as will be seen, 

 differs little from the value of D obtained for plate glass, which is 6,448,000." 



A search for the moduli of thermal expansion of rocks has not resulted 

 in finding satisfactory data. The modulus of cubical expansion of quartz 

 (SiOs) was determined by Kopp as .000039 to .000042, by Pfaff as 

 .00003840 for 1 to 100 degrees centigrade, and by Fizeau as .00003619 

 for 40 degrees centigrade. Hornblende, according to Pfaff, has a modulus 

 of .00002845 for 1 to 100 degrees centigrade.^^ These, however, are indi- 



« F. D. Adams : Op. cit., 1912, p. 99. 



" F. W. Clarke : Constants of nature. Smithsonian Misc. Collections, vol. 14, no. 289, 

 1876. 



