10 IOWA STUDIES IN NATURAL HISTORY 



high lands exist demonstrates that there are forces which 

 give lands high altitudes and that these forces, on the 

 average, at least balance the processes of degradation. 

 These renewing forces are diastrophic. 



There is not perfect agreement among geologists con- 

 cerning some of the phases of diastrophism, but the prin- 

 ciples involved in land formation are fairly well agreed up- 

 on. Lands are due to lithospheric contraction. As the 

 lithosphere shrinks, the ocean basins settle more than the 

 continental platforms, the capacity of the ocean basins in- 

 creases, the water withdraws from the continental plat- 

 forms, and lands are increased in area or height or both. 

 If lands have been reduced to low levels and the lithosphere 

 shrinks, these lands are left higher by the withdrawal of 

 the sea and a new cycle of erosion is inaugurated. 



If diastrophism were a continuous process, land would 

 be reduced slowly if general degradation exceeded uplift, 

 it would remain at a generally constant level if degradation 

 and uplift were equal, and it would become slowly higher if 

 uplift took place more rapidly than degradation. The 

 height of land would depend upon a balance betv/een dias- 

 trophic uplift and degradation by all agents of which run- 

 ning water is chief. 



Pronounced diastrophism manifests itself periodically 

 rather than continuously. Degradation goes on uninter- 

 ruptedly between periods of diastrophism, but sooner or 

 later the uplift comes, degradation is renewed and new 

 cycles of erosion are inaugurated. 



The relative duration of erosion cycles and diastrophic 

 periods now becomes important. If the diastrophic period 

 is longer than the erosional cycle, land is totally destroyed 

 and then formed again. If the periodic uplifts come so 

 frequently and the land is uplifted each time so high that 

 the land added by each uplift is not entirely destroyed be- 

 fore the next uplift, the history is one of land increase, 

 partial degradation, further increase, partial degradation, 

 and so on. Neither the cycle of erosion -nor the diastrophic 

 period is of determinate duration, and therefore there can 

 be no invariable rule in their relative values, but a study 



