26 Proceedings of the Ohio State Acadeiiix of Scieiice 



crustal quiet, f^eneplain production requires that land should 

 remain for a long period in ahout the same positi<)n in regard to 

 base-level. If the land surface is being reduced one foot in live 

 thousand years, and if the average height of a continent after up- 

 lift is two thousand feet, the production of a peneplain would 

 take ten million years. This value is suggestive only. Towards 

 old age the rate of downward erosion becomes much less, and, on 

 the other hand, in but few cases are peneplains developed over a 

 whole continent. Yet the value is true in the sense that tlie times 

 of quiet must be measured in terms of millions of years. While 

 separating these periods of relatively stable crust have been the 

 periods of crustal movement of rock folding, and faulting, an:l 

 of up-arching of broad areas. 



The dissected upland plains which we have been describing 

 were recognized in the mountainous parts of Great Br'«^ain, and 

 were explained by Ramsey and later English geologists as plains 

 of marine denudation, which had been later raised and dissected. 

 Evidently we have here a problem in psychology. To the Eng- 

 lishman on his wave-girt island, with its relatively small river 

 systems, sea action was more noticeable than stream w^ork. The 

 American geologists of the western surveys had the erosive work 

 of rivers before them on a stupendous scale. There is of course 

 today abundant reason for believing that peneolains are of sub- 

 aerial and not of submarine origin ; and this explanation is now 

 accepted by English geologists for their "plains of marine denuda- 

 tion," but it is interesting that each country had a theory sug- 

 gested by its own geological environment. 



CLACIAL GEOLOGY. 



The progress in glacial geology in the last twenty-hve years 

 has been very great, both in itself and in its bearing on general 

 questions. I shall speak of three main lines of work: 



1. The subdivision of the Pleistocene. 



2. Former glacial periods. 



3. Theories of the glacial oeriod. 



The Subdirision of the Pleistocene. 

 Twenty-five year ago the subdivision of the Pleistocene 

 glacial period was not generally recognized. Le Conte (Elements 



