54S GEOLOGY, 



and plateaus are probably the incidental results of the great abysmal 

 and continental readjustments. 



The great movements are probably to be attributed to stresses that 

 gradually accumulated until they overcame the rigidity of the thick 

 massive segments involved, and forced a readjustment, Tn accumula- 

 ting these stresses, some local yielding on weak lines and at special 

 points was an inevitable incident in distributing more equably the 

 accumulating stresses. So, also, the first great readjustments probably 

 left many local strains and unequal stresses which gradually eased 

 themselves by warpings, minor faultings, etc., so that some minor move- 

 ments w^ere a natural sequence of the great movements. But there 

 were doubtless many local and superficial causes, such as irregular gains 

 and losses of heat, regional loading and unloading, solution, hydration, 

 etc., that have caused local or regional movement, and which have little 

 to do with the great deformations of the earth's body. As implied 

 above, the gentle, nearly constant movements probably fall mainly 

 into a different category from the great periodic movements. Both will 

 be considered further. 



The differential extent of the movements.— Between the highest 

 elevation of the land and the lowest depth of the ocean, there is a ver- 

 tical range of nearly twelve miles. There may have been higher eleva- 

 tions, relatively, in past times, but probably not deeper depressions; 

 and so, if we assume that the surface was once perfectly spheroidal, 

 this may be taken as a maximum expression of differential movement, 

 not absolute vertical movement. From the Thibetan plateau, where a 

 considerable area exceeds three miles in height, to the Tuscarora deep, 

 where a notable tract exceeds five miles in depth, the range is eight 

 miles, which may fairly represent the vertical range of rather massive 

 differential movement. From the average height of the continents 

 to the average abysmal bottoms of the oceans the range is nearly 

 three miles, which may be taken as the differential movement of the 

 great segments. Under certain hypotheses of the origin and early 

 history of the earth, to be sketched later, the surface is not assumed 

 to have been perfectly spheroidal originally, and hence the present 

 irregularities do not necessarily imply so great differential movement. 



If the protruding portions of the lithosphere were graded down 

 and the basins graded up to a common level, this level would lie about 

 9000 feet below the ocean-surface. This equated level is the best basis 



