THE GEOLOGICAL EVIDENCES. 55 



If the lithosphere could be supposed to have acted under the forces of 

 gravity and rotation so nearly as though it were a perfect fluid that its form 

 would be at all times perfectly adapted in all its parts to its rate of rota- 

 tion, however much that may have changed, the argument here introduced 

 would have Httle or no force. If this assumption is made here, it must 

 of course be carried consistently throughout the whole range of deforma- 

 tive interpretation. If this is done faithfully, very grave difficulties will 

 be encountered, so grave that, for myself, I have found them insuperable. 

 It is indeed commonly thought consistent with experiments and geological 

 observations, to regard the lithosphere as a solid which acts rigidly toward 

 stresses of short period, and quasi-fluidly towards those of long period. 

 Under this proposition it is possible to assume that the accommodation of 

 the earth to a steady change of rotation might be so nearly perfect that 

 variations would escape detection by even so delicate a registration as 

 that of the sea-surface. But if this is done, it should be with the full con- 

 sciousness that this is not a deduction from the proposition, but merely 

 an assumption under it; for the general proposition that the lithosphere 

 will yield under stress applied for a sufficient time does not in itself carry 

 the conclusion that it will yield under the given stress in the given time. 

 A quartz crystal is under self-gravitative stress and may have also been 

 under terrestrial gravitative stress for eons, and yet it shows no signs of 

 becoming a gravitative spheroid. Mountains and continents are under 

 gravitative stresses and they probably yield to these, but at what rate is a 

 practical question of much geological importance. The postulate of quasi- 

 fiuidal accommodation is not a solution; it is only a broad generalization 

 under which a solution may be sought by specific evidence. 



The shell of the earth is chiefly an aggregate of interlocking crystals 

 which are possessed of specific elasticities of form, and the whole aggregate 

 clearly has elasticity of form. If the great mass of the earth or even the 

 deep outer portion be similarly an elastico-rigid soHd, deformations will 

 only take place when the stress-differences rise to equality with the elastic 

 resistances, except in the limited form of strain, and to the limited degree 

 permitted by the individual transfer of molecules from one rigid attach- 

 ment to another. Deformations in this case await a certain accumula- 

 tion of stress-difference. As the crux of the whole deformative problem 

 lies largely in these basal conceptions, we may do well to turn to geological 

 phenomena to ascertain, if possible, whether the earth does habitually 

 yield concurrently with the accumulating stress-differences and thus con- 

 stantly accommodate itself to stress-demands, or whether stress-differences 

 do actually accumulate until the elastic limit is reached when deformation 

 proceeds with relative rapidity until an approximate equilibrium is reached. 

 This is but stating in dynamic terms the question of periodicity in geo- 

 logical deformation. On this question, a consensus of geological opinion 

 can not now be cited without qualification. Apparently views differ and 

 reserve predominates among cautious geologists. It appears to me, how- 

 ever, that strong evidence is steadily accumulating, from various quarters 

 of the globe, that there were great periods of base-leveling of essentially 

 world-wide prevalence, with concurrent sea-transgi-ession, separated by 



