EMERGENCES 503 



by rigidity. This gives new and very strong emphasis to the idea that the 

 earth is a failing structure, not a competent structure" (34). 



"The indications are, therefore, that when an elevated area under which 

 there is complete isostatic compensation is unloaded by erosion the underlying 

 material to a depth of 71 miles increases in volume mainly because of chemical 

 changes induced by the decrease in pressure, and partly also because of 

 changes in the gases from solution to the free state. This increase in volume 

 raises the surface. It also increases the pressure at each level above the 71- 

 mile depth, and tends to bring it back toward the value which it had at that 

 level before the unloading. 



"This expansion process alone is not sufficient, however, to maintain an iso- 

 static adjustment indefinitely. 



"As the process progresses — a continuous expansion in the underlying mate- 

 rial keeping pace approximately with continuous unloading by erosion at the 

 surface — the pressure near the bottom of the expanding column will become 

 considerably less than it is at the same level in other areas at which no un- 

 loading by erosion is taking place. So too. near the top of the expanding 

 column the pressures will tend to be somewhat greater than at the same level 

 in other areas. The result of these differences in pressure at any given hori- 

 zontal surfaces will be to set up, sooner or later, a great slow undertow from 

 the ocean areas toward the continents, and a tendency to outward creeping at 

 the surface from the continents toward the oceans" (38). 



"The undertow should be most powerful a short distance inside the conti- 

 nental borders, and hence the mountain building should be most active there. 

 . . . Such mountain ranges should be unsymmetrical. thereby indicating 

 that the pressure came from the ocean side" (39). 



Emergences. — The emergences will next be considered. Of these there 

 were at least fourteen of Paleozoic and Mesozoic time, nine of which are 

 thought to have appeared rapidly. For eight the period of emergence 

 was relatively short, while one — the Cayugan emergence — was of long 

 duration. The other five appear to represent periods of slow emergence, 

 as follows : The Chemung, Keokuk, and Kaskaskia emergences belong to 

 one long interval of negative strand-line movement, beginning in the late 

 Hamilton and persisting to the end of Paleozoic time, emphasized by the 

 three above named sharp but small superimposed emergences of short 

 duration. It may therefore be concluded that the emergences referred to 

 were in reality also rapid, so that eleven of the fourteen represent, relative 

 to the transgressions, short beats in chronology. The other three, being 

 slow in their making, were of long duration and of great significance. 

 These are: (1) The Taconic revolution, beginning early in the Trenton 

 submergence and persisting well into the Siluric ; (2) the Appalachian 

 revolution, beginning in early Pennsylvanic time and continuing well into 

 the Mesozoic, varying according to the region, and ( 3 ) the Laramide revo- 

 lution, beginning in the Xiobrara, and to all appearances enduring to the 

 present lofty continent. 



XLV — Bull. Geol. Soc. Am., Vol. 20, 1008 



