DIASTROPHISM AND THE FORMATIVE PROCESSES 501 



class known as zonal harmonics of the second order, such as the 

 bulging of an equator and flattening of the poles, or the pulling out 

 of polar cones and the flattening of the equatorial belt between, 

 give rise to stresses much greater in the central parts than in the 

 outer parts. Sir George Darwin' and others have computed these 

 for an incompressible homogeneous earth and for certain com- 

 pressible variations from this. In an incompressible homogeneous 

 earth Darwin gives the differential stresses as bearing the ratios 8 

 at the center, 3 at the equatorial surface, and i at the poles. In a 

 compressible earth the surficial stresses are relatively lower and 

 those at the center relatively higher. For a certain compressibility 

 the surficial stresses disappear and the central stresses rise \ in 

 value. ^ 



Now the main tidal stresses come and go every twelve hours and 

 the subordinate tidal stresses at other and generally longer intervals. 

 While relatively small, they are constantly acting in a given 

 direction, and this presumably has a certain kind of cumulative 

 effect. This efTect is doubtless chiefly felt by such molecules of 

 the interior as are under stress and are about ready to change their 

 attachments and so are responsive to the influence of even small 

 strains. It is coming to be recognized that such individual molec- 

 ular activities constitute a notable factor in rock metamorphism, 

 glacial motion, and other geological changes of a very intimate 

 sort. This has been set forth by Leith^ and other close students 

 of the intimate nature of geological phenomena. Such persistent 

 rhythmical oscillations of stress and strain as those of the tides 

 seem well suited to aid effectually these individual molecular 

 changes. The nutations of the poles represent pulsatory action 

 whose periods are longer, but whose chief effects are probably of 

 the same intimate sort. 



Changes of rotation, however, represent action of a much higher 

 order of power and much greater length of period. In deformative 

 potency, rotation has a competency of the fi.rst order. Changes of 

 rate of rotation were probably most active and effective while 



' Scientific Papers, by Sir George Darwin, Vol. II (1908), pp. 476-81. 



= Ibid., p. 505. 



3 Leith and Mead, Metamorphic Geology (1905), pp. 173-76, and elsewhere. 



