THE RELATIONS OF GEOLOGY. 375 



been reco^-nized and agreed upon, but the mechanical modes and means 

 of their deformation have, many of them, j^et to be identitied and 

 established. Jn the matters of cleavage, jointing, and foliation we 

 have advanced, and in the modes and effects of faulting we have 

 already made some headway. But in the grander problems of orogen}', 

 crust warping, and secular elevation and depression we are still ver3'^ 

 much in the dark. In spite of all the brilliant work which has been 

 done of recent years, we are forced to acknowledge that we are still 

 busied in collecting data upon which to found a philosophic system of 

 crust deformation. Nothing yet formulated in this direction is of 

 sufficient detiniteness and breadth of grasp to atford matter from 

 which anything more than suggestive deductions may ])e drawn b\' 

 the higlier physics and mathematics. 



But although our materials are as yet too heterogeneous and too 

 complicated to admit satisfactorily of such butside analysis, yet among 

 geologists themselves there is being developed a tendency to assort 

 and interpret them from two extreme points of view, which may per- 

 haps be distinguished as the astronomical and the geonomical. 



The working theory employed by the many at the one extreme is 

 the collapse theory, which is founded essentially upon the (contraction) 

 hypothesis of the gradual loss of heat of the earth's interior. This 

 theory starts from the original covering of our globe, and regards the 

 present state of that covering as that of a solid and more or less cooled 

 crust, which warps, folds, and fractures as it follows down upon the 

 slowly contracting, but still intenseh" heated (and probal)ly solid), 

 nucleus. This crust shows in its structure and in the major forms of 

 the outer surface the combined effects of the radial and tangential 

 deformafions due to the contraction and collapse, these deformations 

 being grou})ed about the remains of the chief irregularities proper to 

 the crust at the time of its original const)lidation. 



The working theory employed by the few at the othin- (>xti-eme is 

 the fold theory, founded essentially on the (undulation) liy})othesisthat 

 the deformation may be largely due to tidal movements and to the con- 

 stant redistribution of load and resistances. It starts from the known 

 modes of deformation of the rock sheets which make up the present 

 supercrust and of those of its superposed coverings of water and of air. 

 It regards the earth crust as a spherical shell or bridge surrounding 

 and l)alanced upon a fluid nucleus (prol)a))ly gas-like), the shell being 

 in a staie of general \il)ration and its parts in a state of regional and 

 local strain. This shell yields harmonically as a whole, and its various 

 parts yield in groups or individually to the several stresses, but always 

 in theoretic units (duacls), each made up of two moieties w hich an^ the 

 positive and negative e(|uivalents of each other. 



According to both theories, the type of defoiination mav t)e that of 

 undulation, \varping, folding, gliding, fracture, or tiow, according as 

 SM 1903^—25 



