Page:Popular Science Monthly Volume 7.djvu/746

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back three millions and forward one million of years. But we believe that most geologists agree with the statement of Sir Charles Lyell that "an attempt to assign chronological value to any except the latest geological epochs must in the present state of science be hopeless."

It may be stated, however, that during the period covered by the computations, three epochs of greatest eccentricity of the earth's orbit occurred, and in each of these it is found that the eccentricity is not uniform, but rises and falls. Hence the coming on and departure of each period may have been continuous, but by no means regular. Sometimes the changes were at the comparatively short intervals of 10,000 or 12,000 years, and the close relation these changes may have had to the life of the time, possibly to extinction of species, is plausibly suggested by Mr. Croll.

The period of great eccentricity to which the last glacial epoch is referred began about 240,000 years ago, and extended over a period of about 160,000 years. The conclusion, therefore, is that it closed about 80,000 years ago. It is not easy to understand, however, what value to attach to the words "close of the glacial epoch." Shall we say that it closed when the ice ceased to exist as a glacier along the shores of New England, or when it extended no farther southward than the Canadian highlands? The gigantic fields of ice which now cover both arctic and antarctic lands prove that, within areas more limited indeed than in former time, the glacial epoch still exists in its stern and sublime reality.

Whether we consider the facts of geology or those of astronomical computation, it seems evident that the growth and decline of the ice-sheet, and of the causes which produced it, have been in no sense cataclysmic or accidental, but secular; only after ages have passed are we enabled to realize, from the altered aspects of Nature, that a great change is in progress. At present the eccentricity of the earth's orbit is diminishing. In a little less than 24,000 years it will be "as nearly circular as it can ever be;" and if Mr. Croll's tables are to be relied on, no cycles of extreme heat or cold will occur for the next 150,000 years. We are entering a period of comparatively equable climate, arising from a more uniform distribution of solar heat over the surface of the globe.




THE "good, old" foundation upon which our fathers stood has been sadly shaken. Its complete overthrow has at times seemed inevitable. "Scientific men" have led the attacking army, and thus gradually brought themselves into disgrace with a portion of the