68 GEOLOGY OF OHIO. 



drawn from the geological facts observed in the Old World. We are, there- 

 fore, apparently driven to seek a solution of the problem in some extra- 

 terrestrial or cosmical cause. One has been suggested, in the variation 

 in the eccentricity of the earth's orbit, which places the subject in an 

 altogether new light, and promises, at least, to lend important aid in re- 

 moving the obscurity which has hitherto hung over it. This suggestion 

 was first made by Sir John Herschell, but it has been recently advocated 

 with so much originality and force by Prof. James Croll, of Glasgow, that we 

 may almost consider him as its author. At the instance of Prof. Croll, Mr. 

 Stone, of the Greenwich Observatory, made careful determinations of the 

 eccentricity through several millions of years, running forward and back- 

 ward from the present time. It was thus ascertained that it passed, at 

 remote intervals, through maxima and minima of considerable magni- 

 tudes. During the period of the greatest eccentricity, the earth, in aphel- 

 ion, would be about 100,000,000 miles from the sun, or over 8,000,000 

 further than now, while in its perihelion it would be proportionally 

 nearer. As the amount of heat received by the earth from the sun 

 would be the same in its maxima and minima of eccentricity, it might 

 be supposed that the climate would not be affected by this cause ; but 

 when the precession of the equinoxes is taken into account, it can be 

 shown that the winter in the northern hemisphere was sometimes thirty- 

 six days longer than the summer; the heat received being, during the 

 winter, one-fifth less than now. Hence, though the summer was one-fifth 

 hotter, it was not sufficiently long to melt the snow and ice of winter; 

 and thus the effects of the cold winter might be cumulative* in each 

 hemisphere through what may be called the winter half of the great year 

 (of 21,000 years) produced by the precession of the equinoxes. Prof. 

 Croll estimates that the influence of extreme eccentricity, acting in the 

 manner described, might be sufficient to depress the average annual tem- 

 perature of London 40° Fahrenheit, and thus produce an arctic climate. 

 We have space only for the results and not for the processes of Prof. 

 Croll's theory, but the subject will be found discussed in great detail in 

 his papers published in the London and Edinburgh Philosophical Magazine 

 1867 to 1871. It. is but fair to state that Prof. Croll's conclusions have 



* This effect would be the result of the difficulty with which ice is melted when 

 once formed. It requires eight tons of rain, at 58° Fahrenheit, to melt one ton of ice ; 

 and large bodies of ice, in melting, surround themselves with vapors which intercept 

 the sun's rays and retard the melting process. Such vapors rising to the height of a 

 few hundred or, at most, a few thousand feet, are congealed to snow, to be remelted, 

 or to pass into neve, and thence into ice again. Thus they become both active and 

 passive agents in preventing the melting of ice fields. 



