894 



PRINCIPLES OF STRATIGRAPHY 



meridian which passes through the rotation and oscillation poles 

 forms the culmination circle {Kulminationskreis), and it divides 

 the earth into a Pacific and an Atlantic-Indian hemisphere. (Fig. 

 244.) Each hemisphere is again divided hy the equator into a north- 

 ern and a southern quadrant. The meridian of 10° E. longitude 

 from Greenwich, which bisects each hemisphere, is called the 

 oscillation circle, because it is upon it that the poles swing back 

 and forth. The culmination circle is so called because each point 

 reaches its closest approximation to the poles when it crosses this 

 circle. 



Fig. 244. Map of the earth, divided by the oscillation circle (near the 

 meridians of 10° E. and 170° W. of Greenwich) into two hemi- 

 spheres, according to the theory of P. Reibisch. The oscillation 

 /'o/f.f form tlie centers of the hemispheres. The vertical meridian 

 is the culiiiiiiatioii circle (80° on the left, 100° on the right). 

 The concentric rings around the oscillation axis mark the paths 

 along which the points cut by them move during the pendulation. 

 (After Simroth.) 



The form of the geoid, i. e., the difference in length of the axis 

 of rotation and the equatorial diameter, amoimting to more than 40 

 kilometers (43 km., or nearly 2y miles; see ante, Chapter I), is re- 

 sponsible for the transgressions and regressions of the sea, and the 

 elevation and subsidence of the land. The sea, the mobile element, 

 assumes the flattened geoid form with every position of the axis 

 of rotation, but the land, owing to its greater rigidity, does not so 

 readily assume this form. Hence, as every point approaches the 

 pole during the pendulation, the waters recede from it, while at 

 the same time they rise over the points approaching the equator. 

 The difterences between rise and fall of the water are greatest on 

 the oscillation circle and decrease progressively to the oscillation 



