672 JOSEPH BAKRELL 



rock upon the land. These relations give a depth of 54 km. to 

 S and 88 km. to T. 



It should be repeated, however, in closing this topic, that the 

 solutions here given are approximate only and assume that iso- 

 static compensation results in lateral stress-differences which show 

 the same distribution of forces as a diagram of hydrostatic pressures, 

 differing only in magnitude. The writer is inclined to think that 

 the actual facts of nature call in most cases for some depression in 

 depth of the critical points beyond those here shown. Especially 

 is there likely to be under the margins of a continent in isostatic 

 equilibrium some permanent lateral stress-difference within the 

 asthenosphere, due to the compensation above and tending toward 

 a landward undertow. Upon the unbalancing due to erosion and 

 sedimentation this would cause the lateral stress-differences within 

 the asthenosphere to rise more quickly to the low elastic limit and 

 permit more readily than would otherwise be the case a regional 

 readjustment toward isostasy. 



RELATIONS OF UNDERTOW TO THE ZONE OF COMPENSATION 



Present status of the problem. — The causes of vertical movements 

 Button' made twofold. He clearly distinguished on the one hand 

 between those internal forces leading to expansion or contraction, 

 which tend, by producing changes in density, to create isostatically 

 a new surface relief, and, on the other hand, those isostatic re- 

 adjustments following erosion and sedimentation, readjustments 

 which tend not to make a new, but to restore the older, relief. 

 Folding he regarded as unrelated to the former, as a result of the 

 latter. He had shown earlier (in fact, he had the honor of being 

 the first to show) that the time-sanctioned hypothesis of cooling 

 as a cause of crustal shrinkage and consequent mountain-making 

 was inadequate to account for either the distribution or amount of 

 folding.^ From this he was led to regard folding as due, not to any 

 js'nd of contraction, but as a compressive movement of one section 



' "On Some of the Greater Problems of Physical Geology," Bull. Phil.Soc. Wash., 

 XI (1P89), 51-64. 



* C E. Button, "A Criticism upon the Contractional Hypothesis," Am. Jour. 

 Sci., VIII (1874), 113-23. 



