72 H. D. MACGINITIE 



One of the distinguishing features of the Glacial ages seems to 

 have been the cyclic or periodic character of the climate — the 

 alternation of Glacial and non-Glacial stages. A new approach to 

 the problem of "fossil" temperatures has been developed by Urey 

 and his students (Urey, 1948; Emiliani, 1955, pp. 538-546). The 

 relative proportion of the oxygen isotope of mass 18 in carbonates, 

 for example, is inversely proportional to the temperature at which 

 the carbonate was formed. The ratio of the two isotopes (mass 18 

 and 16) can be determined accurately by spectrographic means. 

 Although many difficulties are still to be worked out, this method 

 of investigating past temperatures holds much promise and some 

 interesting results have already been achieved by Urey and by 

 Emiliani. Investigation of sea- bottom cores from the equatorial 

 and subtropical Atlantic Ocean has shown that there were no 

 periodic or cyclic temperature fluctuations during the Miocene or 

 Oligocene and that the "cause responsible for Pleistocene climatic 

 variations was not effective during Tertiary times" (Emiliani, 

 1956, pp. 285-287). 



The distribution of living plants and animals in the present 

 temperate regions can be explained only on the basis of the violent 

 climatic fluctuations of the Pleistocene. The extent of these climatic 

 fluctuations can hardly be overemphasized. For instance, the climate 

 on the Michigan Peninsula varied from that of the frigid continental 

 icecap to climates warmer than at present. Along the front of the 

 glaciers there were changes from tundra through steppe to broad - 

 leafed forests, and back again. Studies have indicated that the snow- 

 line in all the high mountains of the world descended during times 

 of glaciation. It has been estimated (Leopold, 1951; Antevs, 1954; 

 Flint, 1957, p. 304) that the snowline on the mountains of New 

 Mexico, for example, descended approximately 4,000 feet below that 

 of the present. There is evidence from deep-sea cores that the 

 surface of the ocean also underwent refrigeration, even in the tropics. 

 Through the study of fossil pollen from scattered bogs, it has been 

 indicated that a cool-temperate climate extended from western 

 Florida to near Austin, Texas (Brown, 1938; Davis, 1946, pp. 

 193-196; Potzger and Tharp, 1947, 1954). The lower levels of these 

 bogs contain pollen of spruce and fir species now characteristic of 

 forests around the Great Lakes. The distribution of the bogs indi- 

 cates that these cool-temperate forests could hardly have been small 



