74 Kansas Academy of Science. 



and the summers twenty-two days shorter than at present, as 

 Croll's theory suggests (see above), the summer in the south- 

 ern hemisphere, since in said hemisphere the seasons are the 

 opposite of ours, would be twenty-two days longer than our 

 present northern summer, and, no doubt, on the whole much 

 hotter, and the winters of that epoch in said hemisphere would 

 be twenty-two days shorter and less cold than now. It is, 

 therefore, necessary to look for some other cause for the 

 frigid climate of the glacial epoch — a cause that will account 

 for the contemporaneous ice sheets, the one in the northern 

 hemisphere and the coexisting and equally extensive one in 

 the southern hemisphere. 



To find the real cause of the climate of said epoch, it is 

 necessary, it seems to me, to inquire, What was the glacial 

 epoch? What epoch, in comparison with the other epochs of 

 the earth's history, does it represent? To use the words of 

 Professor Le Conte : 'The Quaternary, of which the glacial 

 epoch was the first part, is a critical period." (Elements of 

 Geology, p. 619.) 



This definition leads to other and more complicated ques- 

 tions, some of which are : What are the critical periods of the 

 earth? and. Why do they occur? 



In answer to the first question, Le Conte says (Elements 

 of Geology, p. 619) that the critical periods of the earth's 

 history have been periods of oscillation of the earth's crust 

 between the great eras, periods of rest, and therefore of 

 changes of physical geography, marked by unconformity of 

 strata; and of changes of climate, marked by apparently 

 abrupt changes of species, /. e., periods of revolution and rapid 

 change. Again, Le Conte says, on the page opposite to the 

 one mentioned above, that three of the known critical periods 

 have been periods of cold in one or the other of the hemispheres 

 or in both, the latter being known to have occurred in the 

 glacial epoch. 



The second question, "Why do critical periods occur?" is 

 very hard to answer and involves a cosmical cause, a cause 

 which when once understood will explain not only the causes 

 of the glacial epoch, but most if not all of the geological 

 phenomena of our globe as well. As an answer to this question 

 the author will submit the following . 



