242 EARTH-CRUST MOVEMENTS AND THEIR CAUSES. 



ing and all elevation to removal of weight. Probably this is a true 

 cause, but not the main cause of such movements. Doubtless the prop- 

 osition is true, but its converse is even much more so. It is certain 

 that thick sediments may cause subsidence, but it is much more certain 

 that subsidence, however determined, will cause continuous sedimenta- 

 tion by ever renewing the conditions of sedimentation. It is true that 

 removal of weight by erosion will cause elevation, but it is more cer- 

 tain that elevation 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 Cre- 

 taceous times this region was subsiding, until at the end of the Creta- 

 ceous 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 Cretaceous, 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 natural enough, however, if elevation was the cause and ero- 

 sion the effect, for the effect ought to lag behind the cause. It is evi- 

 dent, then, that we must look elsewhere — that is, in the interior of the 

 earth — for the fundamental 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 

 elevation in this case is associated with, and in spite of, loading. The 

 elevation, as we all know, commenced in late Tertiary and culminated 

 in early Glacial. This elevation was, at least, one cause, probably tiie 

 main cause, of the cold and the ice accumulation, but the elevation con- 

 tinued in spite of the accumulating load of ice. Finally, however, the 

 accumulating load prevailed over the elevating force and the previously 

 rising area began to sink, but only because the interior elevatory forces 

 had commenced to die out. Then with the sinking commenced a mod- 

 eration of the climate, melting of the ice, removal of load, and conse- 

 quent rising of the crust to the present condition, but far below the 

 previous elevated condition, because the elevating forces, whatever 

 these were, had in the meantime exhausted themselves. If it had not 

 been for the interference of the ice load, I suppose that instead of the 

 double oscillation which actually occurred there would have been a 



