20 Barrett — Movements of the Strand Line 



last stages of upwarping over the centers of glacial load. The 

 actual evidence at hand does not decide between these hypo- 

 theses. The association with the close of glaciation appears to 

 favor a genetic connection with deglaciation, but on the other 

 hand it remains to be demonstrated why the extra-marginal 

 zone should rise together with the region directly glaciated, or 

 that the cycle was restricted to such an extra-marginal zone. 



Possible effects of radial shrinkage. 



The fourth line of intersection with the problem of recent 

 movements was the result of a query raised by Professor 

 Schuchert in 1909 as to the quantitative effect of earth shrink- 

 age in producing an increased speed of diurnal rotation, and a 

 greater oblateness of the earth's figure ; tending to raise the 

 level of ocean waters toward the equator, and to depress the 

 ocean level near the poles. Calculations made by the writer 

 for Schuchert in regard to the possible magnitude of this effect 

 suggested that it might be of very real influence.* A radial 

 shrinkage of a mile, it was estimated, would increase the differ- 

 ence between equatorial and polar radius by about 90 to 100 

 feet. If this difference were taken up wholly by adjustment 

 of ocean level and not by internal earth adjustment, a supposi- 

 tion which can be true to only a limited degree, then a shrink- 

 age of a mile in radius would raise the water level at the 

 equator about 35 feet, while at the poles it would sink about 

 60 feet. In latitude 35 degrees the water level would suffer 

 no change. 



The rigidity of the earth in an east-west direction is also 

 found to be different from that in a north-south direction and 

 may have some influence on mode of crustal yielding. 



But to be of influence in a result, a factor must not only be 

 real ; it must, furthermore, be of quantitative importance. Such 

 great folding and thrusting movements as have occurred in 

 the later Tertiary suggest that earth shrinkage is an important 

 factor. Schuchert has called attention to the greater persist- 

 ence of equatorial epeiric seas and the readvance of waters 

 more frequently from lower toward higher latitudes. 



The southward bends of certain rivers such as the Delaware 

 and Susquehanna along the strike of soft formations has 

 received various explanations. In so far as land emergence is 

 produced by sea-bottom subsidence or is coincident with move- 

 ments of crustal shortening, the retreating sea should also 

 slightly change its slope. It is possible that such a slight 

 change in slope of sea level at the time the lower river courses 

 were established on a newly emerged coastal plain may be at 



* Charles Schuchert, Paleogeography of North America, Bull. Geol. Soc. 

 Am., vol. xx, pp. 505-508, 1910. 



