376 THE RELATIONS OF GEOLOGY. 



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 teon, 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. Accord- 

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

 due to the continual l)reaking down and I'eadjustment of equilibrium; 

 it is at every stage controlled ])y the length and direction of the instan- 

 taneous polar and ecjuatorial diameters of the earth, and by the sum- 

 mational and individual deformations already effected. 



The tendenci«\s of the first theory are to compare all the phenomena 

 of yieldage with those characteristic of solid liodies and to dwell 

 especially ui)<)n 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 li(|uid bodies, and including all 

 types; to })arallel the synunetries with those of wave forms, and to 

 refer the trends to composition, interference, or superposition, as the 

 case maj^ be. 



In the first theory there is inherent the expectation of continuous 

 accretion and discontinuous collapse; in the second the expectation 

 of rh3"thmic recurrence of form in space and of movement in time. 

 According to the first theor}^ the locus of the pole of the land hemi- 

 sphere on or about the forty-fifth ])arall(M is an accident of evolution 

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

 resultant. 



How nmch 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. 



II. — Geology and Practice. 



GEOLOGY AND THE USEFUL ARTS. 



Up to this point I have dealt mainly with the so-called "scientific" 

 aspect of geology, regarding it from the inside point of view as an 

 interpreter of nature and a meml)er of the great family of the 

 sciences. But, as I have already hinted, we are bound also to consider 

 it from the outside or " practical" point of view as being one of the 



