390 University of California Publications in Geology [Vol. ll 



Rhythmic Bedding Due to Cyclic Changes of Climate 



The hypothesis of seasonal alternations appears quantitatively in- 

 adequate to explain the rhythmic bedding. It may be possible that 

 climatic variations extending over periods of many years might pro- 

 duce the bedding. 



STATEMENT OF HYPOTHESIS 



We might imagine as before, regions on the ocean bottom where 

 either radiolarian oozes were accumulating or where there was an 

 accumulation of precipitated silica. When the climate reaches the 

 most humid stage, mechanical sediment will reach a point where, 

 under ordinary conditions, silica is deposited. On this hypothesis 

 shale would be deposited only once in many years, during an un- 

 usually wet season. An extra dry year would produce no change in 

 the part of the basin where silica was being deposited. It is possible 

 that the entire thickness of a shale parting might be laid down during 

 one wet year, since terrigenous material would be more abundant than 

 silica. 



EVIDENCE IN FAVOE OF IDEA OF EXISTENCE OF CLIMATIC CYCLES 



There is a certain amount of evidence which indicates the exist- 

 ence of cyclic changes of climate. Many writers have pointed out such 

 evidence. 157 They have studied rainfall records, variations in glaciers, 

 and similar phenomena and have arrived at various periods of cli- 

 matic oscillation. For example, an eleven-year period and a thirty- 

 five-year period have been advocated, and it has been shown that there 

 is a rather regular alternation in sun-spot frequency corresponding 

 to these periods. 



Ellsworth Huntington, 158 from a study of ruins in various arid 

 regions of the world, has become convinced that the climate has 

 changed enough to render regions once habitable, now uninhabitable. 

 He has found evidence which leads him to believe that these changes 

 have taken place in a pulsatory manner. While his ideas with regard 

 to major periodic changes of climate have been disputed, on occa- 

 sion, the idea of shorter period climatic cycles finds support in the 



157 Ward, E. DeO, Changes of Climate, Pop. Sci. Mon., vol. 69, p. 458, 1906. 

 iss Geog. Jour., September and October, 1912. 



The Climatic Factor in Arid America, Carnegie Publication no. 192, Wash- 

 ington, 1914. 



