518 GEOLOGY. 



the probabilities seem to be much against it. The declining series of 

 oscillations already noted seems to have reached its last term. If 

 carbon dioxide is an influential factor, its artificial production, which 

 is rapidly increasing, appears likely to more than offset the consump- 

 tion by natural processes, and hence to tend toward amelioration of 

 climate; but the factors that cooperate to produce glaciation are too 

 complex, in our view, to warrant more than a comfortable presumption 

 of future immunity from ice-invasions until another great deformation 

 shall have taken place in the distant future. 



The end of the deformation period. — So, also, it is not wholly clear 

 that the deformative period which started in the late Tertiary, and 

 extended through the Pleistocene, is yet completed. We are accus- 

 tomed to regard it as essentially passed, notwithstanding some move- 

 ments still in progress; and, in the main, this seems to be justified 

 by the probabilities of the case. It is uncertain whether the existing 

 and very recent movements are to be regarded as portions of the main 

 deformative movement, or as secondary adjustments following the 

 main movement, or as but instances of the class of gentle movements 

 that are ever in progress, even if we do not raise the question, as some 

 geologists would, whether there is any real periodicity of movement 

 at all. 



The region of the lower St. Lawrence has been elevated relatively 

 since the retirement of the ice, as is well attested by fossiliferous marine 

 beds, and by shore-lines 600 feet above the present sea-level. This 

 movement seems to have affected the North Atlantic coast from New 

 England northward, but quite unequally at different points. It has 

 been suggested by several writers that this relative rise might be chiefly 

 a resilience from the depression due to the weighting and cooling which 

 the region had suffered during the glacial stages. 



A recent movement in the region of the Great Plains seems also 

 to be suggested by certain physiographic features. Extensive tracts 

 in central Kansas and Nebraska bear an aspect of pronounced topo- 

 graphic youth, suggesting that they have been lying, until recently, 

 near the neutral horizon between erosion and deposition, and have 

 lately been raised on the western side. In the Dakotas, there are 

 broad gradation plains of abandoned river-courses which cross the 

 present valley of the Missouri River. Their present gradients, and 

 their elevation above the present river-bottoms of the region, also 



