much discussed and controverted topic. Seven 

 theories have been presented from time to time, 

 accounting for these important epochs, chief of 

 which is the very interesting 



ASTRONOMICAL THEORY. 



This theory, first presented by Mr. Croll, and en- 

 dorsed by Professor Geikie and many other English 

 geologists, " attributes the Glacial Age to the com- 

 bined influence of precession of equinoxes and 

 secular changes in the eccentricity of the earth's 

 orbit," whereby the seasons, summer and winter, 

 would have a disparity of nearly five weeks instead 

 of one week, as at present: this disparity, it is 

 claimed, would produce Glacial and Thermal 

 Ages alternately, every 21,000 years. 



However, Professor Le Conte, America's most 

 renowned geologist, controverts this theory, in 

 part, quoting from many authorities, showing that 

 but one Glacial Age can be proven. Referring to 

 the researches of Professor Wallace and others he 

 asserts that the phenomenon is the result of several 

 agencies astronomical, geological, and geograph- 

 ical producing a severe Glacial Age of great 

 length, with two cumulative periods of greatest 

 severity and a Sub-Thermal period between; the 

 Age commencing about 240,000 years ago, continu- 

 ing 160,000 years, and ending 80,600 years ago. 



GREAT DESTRUCTION BY COLD. 



During this Glacial Age the plants were driven 

 slowly, generation after generation, a few feet at a 

 time, down across the North Temperate Zone, by 



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