128 GEOLOGY. 



(3) in tracts that have been weakened by previous folding or other cau es. To 

 initiate folding, the lateral thrust of the shell must not only become superior to 

 the shearing resistance at its base, but also to the resistance of the beds in the 

 weak tract to folding. 



It must be confessed that the extent to which a common lateral shear appears 

 to have taken place is greater than might have been anticipated from theoretical 

 considerations. Shears for short distances only and moderate buckling at many 

 points might have been expected, rather than shears of many miles affecting 

 vast tracts by a common movement which concentrated their surplus dimensions 

 in a single folded tract. It is perhaps not strange that the crust beneath each 

 ocean basin should have thrust laterally from its center to its borders, where it 

 was already flexed and weak, but the great concentration of the folding in most 

 cases, and the fewness of the folded tracts in any given folding-period, seem to 

 indicate that a conjoint movement of even greater extent took place. 



Periodicity. — While the ulterior causes of folding may be supposed to be in 

 continuous action, and hence folding liable to take place at various irregular 

 times in different parts of the earth, and more or less continuously, the evidence 

 of geologic history seems to indicate that there were long periods of accumulating 

 stresses, followed by shorter periods in which the stresses relieved themselves 

 by a rather general deformation of the several kinds indicated above. This view 

 cannot be said to be held universally, however. 



Suppossd combined action. — From the foregoing considerations, the following 

 conception of a typical deformation of the major order is deduced (Fig. 32c). 

 Presuming, in accordance with the sketch of the stages of growth, that the oceanic 

 basins and continental platforms have been determined previously, it is assumed 

 that the transfer of material and heat in the body of the earth, the loss of heat 

 from the surface, and, probably most important of all, the molecular re-arrange- 

 ment of matter in the interest of greater density, gradually developed and ac- 

 cumulated stress enough to overcome the effective rigidity of the great mass of 

 the earth, and to cause a general shrinkage. For reasons previously assigned, 

 this is supposed to have been inaugurated in the sectors beneath the ocean basins, 

 which are assumed to have shrunk radially, and to have crowded upon the ad- 

 jacent sub-continental sectors to the extent of causing a slight relative increase 

 in their elevations. 



It is also to be assumed that, more or less locally or regionally, the upper 

 parts of the oceanic and continental sectors, and perhaps subsectors, crowded 

 specially upon one another for considerable depths and in varying degrees, and 

 that these special deep-seated thrusts gave rise to the massive swellings that 

 constitute plateaus. In neither of these cases, however, was the lateral thrust 

 great, nor did it contribute much to superficial crumpling. It is assumed that the 

 interior shrinkage involved the descent, relative to the center, of the continental 

 sectors as well as the sub-oceanic, but to a slightly less degree. These continental 

 sectors have been spoken of as being crowded up, or caused to swell up, implying 

 elevation, in accordance with the common usage of terms in the literature of the 

 subject. These terms relate to the average level of the surface, our most available 



