O SURFACE GEOLOGY. 



nearly circular ellipse, so that at one point of its orbit it is somewhat 

 farther from the sun than at the opposite point. This eccentricity of the 

 earth's orbit is not constant, but increases and diminishes through long 

 periods. During the past fifty thousand years it has been comparatively 

 small, and will continue so for the same time to come. The last period 

 of great eccentricity began about 240,000 years ago, and lasted 160,000 

 years. During this time the winters which occurred farthest from the 

 sun, or in aphelion, would be longer and colder than now. The sum- 

 mer's heat would be increased in the same proportion, but it is argued 

 that its length would not suffice to melt the annual accumulation of snow. 

 This would gain slowly in depth, and become solidified, till a large part of 

 this hemisphere would be enveloped in ice. At the same time the oppo- 

 site side of the globe would have a short, mild winter, and a long, cool 

 summer. Owing to other astronomical causes, known as the precession 

 of the equinoxes and motion of the line of apsides, these different climates 

 would not be permanent for each hemisphere during the whole of this 

 long period, but they would be several times changed, prevailing by turns 

 on each side of the equator. In 21,000 years the hemisphere which at 

 first had its winter at aphelion would have passed through a cycle, in 

 which its place in winter would have traversed the entire orbit, — falling 

 after half this time at perihelion, and finally arriving at its first position. 

 This theory accordingly supposes that an ice-sheet was produced several 

 times about each pole, alternating with long intervals of genial tempera- 

 ture, in which it disappeared. Stratified deposits of sand or clay contain- 

 ing organic remains have been found in Europe, underlaid and overlaid 

 by till, proving the existence of mild inter-glacial epochs. Equally certain 

 proofs of these are rarely found in America. Thick beds of modified 

 drift in the midst of till occur in New Hampshire, but they do not appear 

 to prove a disappearance and return of the ice-sheet. 



If glacial epochs are produced by a great eccentricity of the earth's 

 orbit, we should also expect indications of ice-action in the older rocks, 

 and probably many coarse conglomerates have been formed in this way. 

 The remote date to which this theory assigns the last glacial period is not 

 improbable, as the amount of erosion effected by Niagara river since the 

 ice age, and other facts bearing on this question, indicate a similar lapse 

 of time. This, however, seems but as yesterday when it is compared 



