900 PRINCIPLES OF STRATIGRAPHY 



may be the result of sudden readjustments, due to the redistribution 

 of material on the earth's surface by currents and by meteorological 

 agents. 



Accompanying the changes of position of the poles, is, of course, 

 a change in the position of the equator, the plane of which is always 

 at right angles to the axis of rotation of the earth. A further re- 

 sult of such changes is a variability of terrestrial latitudes gen- 

 erally. 



It is now commonly admitted that th.e movement of large bodies 

 of water, or of air, over the surface of the globe, and, more es- 

 pecially, an accumulation of vast masses of snow and ice in differ- 

 ent regions, may all be causes effecting slight displacements of the 

 earth's axis. More pronounced effects might follow from wide- 

 spread upheavals or depressions of the surface of the lithosphere. 



If the earth's center of gravity should be sensibly displaced, 

 momentous rearrangements, accompanied by pronounced wander- 

 ings of the pole, would result. It has long been known that the 

 center of gravity does not coincide with the center of figure of 

 the earth, but lies to the south of it. This is due to the greater 

 aggregation of dense material in the southern hemisphere, but 

 whether this is a result of original distribution of matter, or is 

 due to later readjustments, cannot be said. Simroth has even 

 suggested that the in- falling of a satellite, or second moon, into the 

 earth in the African region may have been a possible cause of the 

 displacement of the center of gravity, and that the pendulations of 

 the earth's axis were the result of a gradual readjustment. That 

 meteoric or extra-telluric bodies reaching the earth in great numbers 

 may sensibly affect the center of gravity, and hence the position 

 of the earth's axis, cannot be doubted. 



Earth Movements and Geosynclines. 



As already noted in Chapter XX, the term geosyncline was ap- 

 plied by Dana (in 1873) to the great earth troughs, whether simple 

 or compound, which are formed in regions of excessive deposition, 

 the Appalachian region being taken as the type of such troughs. 

 The idea of great downward bowings under heavy load was de- 

 veloped much earlier by James Hall, who held that this downward- 

 bending was in direct response to the load added by the sediment, 

 and thus, in effect, constituted an isostatic readjustment. Dana, 

 on the other hand, regarded the downward bending as primary, 

 and of the nature of a fold due to compressive strains, and the 



