Vol. 59.] ANNIVERSARY ADDRESS OF THE PRESIDENT. lxxxi 



the several stresses, but always in theoretic units (duads) each 

 made up of two moieties which are the positive and negative 

 equivalents of each other. 



According to both theories, the type of deformation may be that 

 of undulation, warping, folding, gliding, fracture, or flow, according 

 as the magnitude of the stress, the speed of the action, or the 

 relative elasticity of the material may determine : its development 

 may range in time from that of an instant to that of an seon ; and 

 its extent from microscopic to hemispheric. 



According to the first theory, however, the deformation is not 

 theoretically symmetrical, but is consequent upon and has ever been 

 controlled by the salient features of the original earth-crust. According 

 to the second, the deformation is theoretically symmetrical, and is 

 due to the continual breaking-down and readjustment of equilibrium; 

 it is at every stage controlled by the length and direction of the 

 instantaneous polar and equatorial diameters of the earth, and by 

 the summational and individual deformations already effected. 



The tendencies of the first theory are to compare all the phenomena 

 of yieldage with those characteristic of solid bodies, and to dwell 

 especially upon the proofs of fracture (with the fault as the central 

 type) ; to parallel such signs of symmetry as are apparent with that 

 of crystals, and the loxodromic trend-lines of the earth's surface 

 with those of crystalline cleavage. The tendencies of the second 

 theory are to compare the yieldage-phenomena with those of flexible 

 bodies (with the fold as the central type), grading on the one hand 

 into those of rigid, and on the other into those of liquid bodies, and 

 including all types ; to parallel the symmetries with those of wave- 

 forms, and to refer the trends to composition, interference, or super- 

 position as the case may be. 



In the first theory there is inherent the expectation of continuous 

 accretion and discontinuous collapse ; in the second the expectation 

 of rhythmic recurrence of form in space and of movement in time. 

 According to the first theory the locus of the pole of the land- 

 hemisphere on or about the 45th parallel is an accident of evolution 

 and a survival ; according to the second it is a theoretic necessity 

 and a resultant. 



How much of each of these views is a mere mental expedient, and 

 how much is an expression of fact, must be left for future research 

 to determine. The discovery of the true path lying between the 

 two extremes will form one of the tasks which await the geologists 

 of the coming era. 



