IxXX PKOCEEDINGS OF THE GEOLOGICAL SOCIETY. [May I903, 



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 orogeny, crust- warping, 

 and secular elevation and depression, we are still very 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 definiteness and breadth of grasp to afford matter 

 from which anything more than suggestive deductions may be 

 drawn by the higher physics and mathematics. 



But although our materials are as yet too heterogeneous and too 

 complicated to admit satisfactorily of such outside analysis, yet 

 among geologists themselves there is being developed a' tendency 

 to assort and interpret them from two extreme points of view, 

 which may perhaps 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 (con- 

 traction) 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 intensely 

 heated (and probably 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 deformations due to the con- 

 traction and collapse, these deformations being grouped about the 

 remains of the chief irregularities proper to the crust at the time of 

 its original consolidation. 



The working theory employed by the few at the other extreme 

 is the fold-theory, founded essentially on the (undulation) hypothesis 

 that the deformation may be largely due to tidal movements and to 

 the constant redistribution of load and resistances. It starts from 

 the known modes of deformation of the rock-sheets which make up 

 the present supererust and of those of its superposed coverings of 

 water and of air. It regards the earth-crust as a spheroidal shell 

 or bridge surrounding and balanced upon a fluid nucleus (probably 

 gas-like), the shell being in a state of general vibration and its parts 

 in a state of regional and local stress. This shell yields harmonically 

 'as a whole ; and its various parts yield in groups or individually to 



