ISOSTASY AND THE EARTH'S INTERIOR FORCES. 12 



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in the Appalachian region in preparation for the Appalachian's birth 

 unless there were continuous pari passu subsidence ever renewing the 

 conditions of sedimentation. 



Now there can be no doubt as to the value of this principle, but there 

 is much doubt as to the extent of its application. The operation of ex- 

 terior causes, such as transfer of load by erosion and sedimentation, are 

 so comparatively simple and their effects so easily understood that we 

 are tempted to push them beyond their legitimate domain, which in 

 this case is to supplement and modify the more fundamental movements 

 derived from interior causes. We are thus tempted to generalize too 

 hastily and to conclude that all subsidence is due to weighting and all 

 elevation to removal of weight. Probably this is a true cause, but not 

 the main cause of such movements. Doubtless the proposition is true, 

 but its converse is even much more so. It is certain that thick sedi- 

 ments may cause subsidence, but it is much more certain that subsi- 

 dence, however determined, will cause continuous sedimentation by ever 

 renewing the conditions of sedimentation. It is true that removal of 

 weight by erosion will cause elevation, but it is more certain that eleva- 

 tion is the cause of removal of matter by erosion. 



Take again the Plateau region as an example. We have seen that 

 during the whole Carboniferous, Permian, Triassic, Jurassic, and Creta- 

 ceous times this region was subsiding, until at the end of the Cretaceous 

 the earth's crust here had bent downward 12,000 or 15,000 feet. Shall 

 we say it went down under the increasing load of sediments ? Why, 

 then, did it, from a previous land condition, ever commence to subside ? 

 And why, when the load was greatest, namely, at the end of the Creta- 

 ceous, did it begin to rise ? Again, from that time to this it has risen 

 20,000 feet. Of this about 12,000 feet have been removed by erosion, 

 leaving still 8,000 feet of elevation remaining. Now if this elevation be 

 the result of removal of weight by erosion, how is it that a removal of 

 12,000 feet has caused an elevation of 20,000 feet? This result is natu- 

 ral enough, however, if elevation was the cause and erosion the effect, 

 for the effect ought to lag behind the cause. It is evident, then, that we 

 must look elsewhere — that is, in the interior of the earth — for the funda- 

 mental cause, although, indeed, the effects of this interior cause may be 

 increased and continued by the addition and removal of weight. 



But perhaps the best illustration of the distinctness of the two kinds 

 of causes of these movements is found in the oscillations of the Quater- 

 nary period. I say best because in this case the effects of the two may 

 be disentangled and viewed separately, and this in its turn is possible 

 because the loading in this case is not by mere transfer from one place to 

 another, and therefore is not correlated with unloading. In fact, the 



