248 



ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



the sea level. The climate then was most favorable and has since 

 declined. 



In the diagrammatic presentation of the alternation of glacial and 

 interglacial stages, it is not to be supposed that this proceeds in just 

 the same way in different regions. Their varying size and altitude 

 make it possible that their maxima and minima of glaciation or 

 upheaval and sinking will not absolutely coincide, and the glaciation 

 of different regions not be of equal duration. Further, their geo- 

 graphical situation can in many ways alter the influence of cold or 

 hot sea currents, winds, and other agencies. Notwithstanding, the 

 changes of climate must on the whole correspond in the different 

 regions all over the globe, regulated as they are by the general chill- 

 ing and heating of the oceans and by the general sinking and rising 

 of the sea level. 



Fio. 1. — A schematic sketch illustrating the relations of glaciation, crustal movement, 

 and oscillation of sea level 



The theory here proposed contains a general thesis: That the al- 

 ternating pliothermic and miothermic conditions depend upon 

 changes between pediocratic and orocratic relief, and a special thesis 

 that the succession of glacial and interglacial ages during a miother- 

 mic period depends upon isostatic and eustatic oscillations of level. 

 The theory thus sees the causes of the different climates during past 

 eras in different geographical circumstances. Attempts to solve the 

 puzzling question from this point of vicAv were made almost a hun- 

 dred years ago and have never been abandoned. One by one the 

 facts and ideas on which the above theory is founded have been put 

 forward and with the increase of our geological knowledge they have 

 grown ever clearer and more convincing. I believe, therefore, that 

 the relief theory will be aj)proved as the true solution of the clima- 

 tologic problem in geology, though it may be only after discussion 

 and improvement. 



