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DYNAMICAL GEOLOGY. 



liest of the ages, the Archaean, mountains were raised to a height of 

 many thousands of feet and left standing on sure foundations. Great 

 subsidences had previously taken place ; accumulations of Archaean 

 sedimentary beds, a score or more of thousands of feet in thickness, 

 had been laid down ; and, finally, the mountains were formed out of 

 the sedimentary beds, and made to stand high and firm. The Adiron- 

 dacks, for example, were certainly 5,000 feet (plus what has been lost 

 during subsequent ages through denudation) above the level of the 

 sea-beaches of the Potsdam or Primordial era, the first period of the 

 Lower Silurian, for the Potsdam sandstone was in part a beach de- 

 posit ; and they continued to stand aloft, with small changes of level, 

 during all the long Paleozoic periods, when the crust of the Appala- 

 chian region, to the south, was slowly subsiding. And they have stood 

 there ever since (besides other Archaean heights to the south and 

 southwest, and those of North Carolina of even greater altitude) be- 

 cause the crust was able to support them ; which capability was owing 

 either to some virtue in the arching of the crust in that part, or to the 

 nature of the crust, or the degree of viscidity below it. The average 

 specific gravity of the Adirondack rocks is probably a tenth greater 

 than that of the Paleozoic sediments of the Appalachian area. Hence 

 the crust could hold up mountains in the earlier as well as later ages. 



2. Expansive Force of Vapors proceeding from the viscid material 

 beneath the Crust. — Such vapors would act upward, and the result 

 would be simply a bulging of the surface, with the opposite effect, 

 possibly, on their condensation. But if a real cause, these vapors 

 should have been most abundant and energetic early in the earth's 

 history. The fact that the great mountains of the globe received the 

 larger part of their altitude after the Cretaceous period, and hence 

 near the end of geological time, and that volcanoes reached their 

 maximum at the same time, is quite strong evidence that the interior 

 liquid of the globe had lost nearly all that was vaporizable, under the 

 existing conditions of heat and pressure, before the earth was firmly 

 crusted over. 



3. Lateral Pressure within the Crust from Contraction of the cool- 

 ing viscid layer beneath. — This contraction of the cooling underlying 

 layer would result either in one or all of the following results : (1) in 

 breaking the continuity of the layer itself, and producing rents opening 

 downward ; (2) in making open spaces between it and the crust ; (3) in 

 forcing the hard crust above, if adhering to it, to become accommo- 

 dated to its own decreasing size. Owing to the second of the results 

 of contraction here mentioned, gravity, or the weight of the crust, 

 would produce in it lateral or tangential pressure, with its conse- 

 quences ; and the third would add more or less to this pressure. 



