308 GEOLOGY. 



tricity may affect climate in two ways. " It may increase or dimin- 

 ish the difference between the summer and winter tempera- 

 ture." Take for example the second case. When the eccen- 

 tricity is greatest the distance of the earth from the sun, when 

 in aphelion, or farthest away, is 98,506,350 miles; and when 

 in perihelion, or nearest to the sun, it is only 84,293,650 miles. The 

 earth is, therefore, during such times, 14,212,700 miles further from 

 the sun in aphelion than when in perihelion. This is a greater dif- 

 ference by many millions of miles than now obtains. During such 

 a period of high eccentricity when winter occurs in aphelion, 

 it is also longer by 36 days than the summer. At the present 

 time the difference between the length of winter and summer 

 (from the 22d of September to the 20th of March, and from the 

 latter date to the 22d of September) is only seven or eight days. 

 Given, therefore, a winter 36 days longer than the present, with 

 the sun from 8,000,000 to 14,000,000 of miles further away from the 

 earth than now, and the mean temperature of the globe would not, 

 indeed, be greatly lowered, but the conditions of its reception would 

 be vastly different. One-fifth less heat would be received in winter 

 and one-fifth more in summer, which latter would be exceedingly 

 short. A long, cold winter and a short, hot summer would be the 

 result (Croll). What snows fell during autumn, winter and spring, 

 would not be dissipated by the short, hot summer. Snow would 

 accumulate and gradually form glaciers of immense thickness. An 

 indirect result affecting climate greatly would be a change of the 

 oceans' circulation. The warm currents that now lave northern 

 zones would be largely excluded from the glaciated hemisphere. 

 While, however, one hemisphere would be glaciated, the other 

 would have its winter in perihelion and its summer in aphelion. 

 This condition, according to Croll and Herschel, would " annihilate 

 the difference between winter and summer in temperate latitudes." 

 * Owing to the precession of the equinoxes, these conditions would 

 change from north to south of the equator every 10,500 years, or 

 thereabouts. Such periods of high eccentricity occurred three 

 times during the last 3,000,000 years — the last one commencing 

 240,000 years ago and ending 80,000 years ago, embracing a period 

 of 160,000 years. The cold and glaciation was most intense be- 

 tween 30,000 and 40,000 years after it commenced.' (Geike.) 



As the earth's orbit will continue to grow less eccentric for 

 23,900 years, during that time, at least, if this theory for the pro 



