CLIMATE SINCE THE LATE CRETACEOUS 77 



Kansas, and the prairies will move eastward and northeastward. 

 Is this warming and drying trend due to the enormous amounts of 

 carbon dioxide being returned to the atmosphere through the 

 burning of fossil fuels, as Plass believes, or is it due to some secular 

 change in solar radiation? In this connection, accumulating evidence 

 indicates that short-wave and corpuscular radiations from the sun 

 appear to fluctuate rather markedly, and that the amounts of such 

 radiation absorbed by the outer atmosphere may produce rather 

 large effects in modifying atmospheric circulation patterns (Craig 

 and Willett, 1951 ; Willett, 1953, pp. 62-69). 



In his study of past climates the writer has become convinced 

 that the fundamental cause of world climatic changes has its origin 

 in small fluctuations in the amount and kind of solar radiation 

 (see also Willett, 1953, pp. 57, 61; Flint, 1957, pp. 481-509). Opik 

 (1958) has developed what is perhaps the first plausible mechanism 

 for long-term changes in solar radiation. These solar changes are 

 considerably modified by the resulting changes in ocean tempera- 

 tures, concentrations of H2O and CO2, and, on a more local scale, 

 by topographic changes. The problem is a complex one, but the 

 writer, on the basis of present evidence, always returns to the 

 conclusion that the primary cause lies outside the earth itself. 

 Brooks' solar-topographic hypothesis (Brooks, 1951, pp. 1016-1017) 

 offers an adequate and satisfying explanation of the observed 

 facts. 



REFERENCES 



Antevs, E. 1954. Climate of New Mexico during the last Glacio-pluvial 

 /. Geol, 62: 182-191. 



Axelrod, D. I. 1956. Mio-Pliocene floras from west central Nevada. 

 Univ. Calif. Pubis. Geol. Sciences, 33: 1-322. 



— . 1957. Late Tertiary floras and the Sierra Nevada uplift. 



Bull. Geol. Sac. Am., 68: 19-46. 



Bogolepov, K. V. 1955. Stages in the development of the Tertiary vegeta- 

 tion in the Angara region of the Enisei ridge. Doklady Akad. Naiik 

 SSSR, 100: 985-988. 



Brooks, C. E. P. 1949. Climate through the Ages. McGraw-Hill, New York. 



. 1951. Geological and historical aspects of climatic change. 



Compendium Meteorol., pp. 1004-1023. 



Brown, C. A. 1938. The flora of Pleistocene deposits in the western 

 Florida parishes; West Feliciana Parish, East Baton Rouge Parish, 

 Louisiana. Louisiana Geol. Survey, Geol. Bull., 12: 59-96. 



