290 WALKS AND TALKS. 



have an aggregate thickness, and the thickness of each Group, 

 System and Great System of strata. If we assume the whole 

 time represented in these formations to be eighty million years, 

 then the duration of the Eozoic jEon was eleven million 

 years ; the duration of the Coal Peroid, two and a half mill- 

 ions, the duration of the Mesozoic, eight millions ; of the 

 Caenezoic two and a half millions, and the time since the close 

 of the Tertiary, half a million; the Glacial period endured 

 352,000 years and the post-glacial interval has been 176,000 

 years. If we make Newcomb's calculations the basis of an 

 estimate, all the above intervals will be reduced to one-sixth. 

 The post-glacial period will be 30,000 years. These are, to a 

 large extent, random results; but they show, at least, that 

 the history of the world is embraced within a finite space 

 of time. 



Another method of gaining some conception of the length 

 of geological periods is based chiefly on changes in the eccen- 

 tricity of the earth's orbit. The earth's path about the sun, 

 as you remember, is an ellipse. But this ellipse is constantly 

 varying its elongation. For many centuries it increases its 

 elongation, and then for many centuries diminishes it. It was 

 shown by James Croll that when the eccentricity (elongation) 

 is greatest, the climates of the earth must be affected in such 

 a way as to lower the mean temperature of the northern 

 hemisphere. He contends that the advent of the Glacial age 

 was due to a state of maximum eccentricity. But the changes 

 in eccentricity are subject to a law by which the epochs of 

 maximum eccentricity have been calculated. The last max- 

 imum occurred 100,000 years ago, and the next preceding, 

 210,000 years ago. Mr. Croll thinks, therefore, that the 

 Glacial period began about 240,000 years ago, and continued 

 to 80,000 years ago, with a mild interglacial epoch 150,000 

 years ago. Now, if Mr. Croll is right in ascribing glacial 

 periods primarily to maximum eccentricity, it would appear 

 that the decline of continental glaciation took place about 

 80,000 years since. This high number is not confirmed by 

 any of the other methods of computation. There is nothing 



