240 ANNUAL. REPORT SMITHSONIAN INSTITUTION, 1924 



upheaval to levels higher above the sea than those at which the 

 summits of the mountains stood before the erosion began. 



There is thus good evidence that the relief of the continents 

 gradually grew higher and more uneven in the course of the Pliocene 

 and Pleistocene ages, and that the elevation of the mountains over 

 the valleys, of the highlands over the lowlands, and of the con- 

 tinents over the sea, reached its maximum in the beginning of the 

 Quaternary period, whereafter it has rather diminished. By analogy, 

 we have the right to assume that the continents also had their 

 highest and most mountainous reliefs at the beginning of the elder, 

 i. e. of the Caledonion, Hercynian, and Alpinian cycles. Like- 

 wise, the relief must, in higher or lower degree, have been elevated 

 and mountainous at the orogenic phases within the cycles, though 

 not so high as after the three chief ages of diastrophism. 



During the long anorogenic phases of the cycles, the mountains 

 and hills were broken down and the continents finally peneplained. 



The changes of relief show themselves also in the nature of the 

 sediments. Conglomerates, sandstones, grits, and other coai^se 

 clastic rocks prevail in the systems deposited during and after the 

 orogenic phases. They testify to the activity of running water 

 with strong power of erosion and transport ; that is, that the ground 

 was high and sloped considerably. In the systems from the end of 

 the anorogenic phases, again, muddy deposits and limestones are 

 predominant. The rivers did not bring coarse material down to the 

 sea, in many cases not even finest mud, since the land was low and 

 plain. 



The above suggestions concerning relief in the past and present 

 may be briefly summed up. 



Geological events have not proceeded uniformly, but cyclically. 

 There were not at all ages lofty mountains and highlands in some 

 regions, with plain lowlands, or even lower mountain districts, in 

 others, as it is the case nowadays. For the formation and destruc- 

 tion of mountains has not during all time outbalanced each other. 

 In some periods the former, in others the latter, process was 

 prevailing. Only after the orogenic phases was the relief comparable 

 to that of the present day, or still higher and more broken. For 

 such a condition I will use the term oroci'atic. During anorogenic 

 periods, again, the continents became more or less peneplained. I 

 call such a condition fediocratic. Of course, there are gradual 

 transitions between these two kinds of relief. 



The changes of climate can also be fitted into the frame of the 

 geological cycles. 



In some ages in the history of the earth ice caps have covered 

 vast areas, just as in recent times. I have named such periods 



