GEOLOGICAL CLIMATE IN HIGH LATITUDES. 361 



The fact that the earth's axis has a different obliquity from that 

 of the moon proves that a change occurred in the one or the other 

 after their separation ; and, since the moon remains so nearly in the 

 normal position, it must have been the earth that was changed. The 

 uniformity of biological conditions in all latitudes indicates that the 

 present obliquity had not been attained in Archaean time, nor in Palae- 

 ozoic, nor in Mesozoic, nor in the Eocene, nor in the Miocene, nor in 

 the earlier Pliocene ; then comes a blank during which the Glacial 

 epoch came and went, and, when again the record begins to be legible, 

 there are, for the first time in the world's history, indications of alter- 

 nating seasons.* 



In view of all these facts, it seems most probable that, in that 

 blank interval, the Glacial epoch, or, more largely, between the end 

 of the Miocene and the beginning of the Champlain, that movement 

 occurred which gave the earth seasons, unequal days and nights, and 

 greatly enlarged its limits of inhabitability. 



It requires no argument to show that an axis nearly perpendicular 

 would account for the otherwise inexplicable evenness of geological 

 climate. Although the Gulf Stream, or other currents, might bend the 

 isotherms, the temperature at any point would, with such an axis, have 

 remained constant. The conditions as to light and actinic force would 

 have been the same everywhere, save the variation due to greater or 

 less latitude. All this, however, is compatible with great cold ; hence it 

 remains to inquire why the polar climate was so warm. Many theories 

 have been advanced to solve this problem. I have neither space nor 

 time to discuss them now, and will only say that six or seven of the 

 earlier ones are ably treated by Searles V. Wood, Jr., in the " Geo- 

 logical Magazine" for September and October, 1876; also by Dr. 

 Croll, in his " Climate and Time." Dr. Croll's own theory I have dis- 

 cussed at large in " The Three Climates of Geology " (" Penn Month- 

 ly," June, July, and August, 1880), and have there pointed out what 

 seem to me insuperable objections to it.f 



Professor Whitney has lately put forth another theory, attributing 

 the early warmth to the sun itself being hotter in geological times 



* There probably were zones of climate in the latter half of the Tertiary, or at least 

 in the Pliocene, but these are quite compatible with the absence of seasons, since, with a 

 perpendicular axis, temperature, however it might differ on different parallels, would be 

 constant in each. 



f Since writing the above I have read Professor Woeikof's article in the "American 

 Journal of Science" for March, 1886, entitled " An Examination of Dr. Croll's Hypothe- 

 ses of Geological Climates." It is a careful testing of Dr. Croll's theories by applying 

 them to present facts, as to summer and winter variations of climate. He shows that no 

 such differences exist as Dr. Croll's theories demand. He sums up his conclusions (page 

 178) as follows : " The main points on which rests, so to say, the whole fabric in its expla- 

 nation of glaciation and geological climates generally the influence of winter in aphelion 

 and in perihelion during high eccentricity, and the calculation of temperatures in pro- 

 portion to the sun-heat received, are unfortunately fallacious." The article will well repay 

 the student of geological climate for its careful study. 



