Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/463

1869.] MURPHY—GLACIAL CLIMATE. 353 of descent. This might not have much effect on the climate of Central Europe, but it would have a very great effect in those high latitudes where the glaciers would reach the sea and give origin to icebergs; for we know that icebergs have great influence as transporters of cold.

In particular cases the effect of a comparatively slight fall of summer temperature would be very great. I quote from Forbes's 'Norway and its Glaciers,' p. 215:—

"Though the surface actually covered by perpetual snow in Norway be small, yet the mountainous districts and tablelands everywhere approach it so nearly that the snow-plane may be said to hover over the peninsula, and any cause which should lower it even a little would plunge a great part of the country under a mantle of frost."

And again, p. 243:—

"It is exceedingly probable that a diminution of the temperature of the summer months by 4° only would at once place one-fourth of the surface of Norway within the snow-line; and so vast a mass of snow would refrigerate the climate, especially the summer temperature, to such a degree as would unquestionably pour glaciers into the head of every fiord in western Norway. . . . The lowering of the snow-line over so large a surface would deteriorate the climate and lower the mean temperature, which would lower the snow-line still further."

The change in the eccentricity of the earth's orbit is in all probability amply sufficient to account for this or a much greater change in summer temperature.

I take the following data from Mr. Croll's paper. The recently ascertained error in the old determinations of the sun's distance affects both distances alike, and consequently does not affect their ratio. Along with the maximum distances of the sun at present and at greatest excentricity, I state the proportionate quantities of heat the earth will receive under those two different conditions:—

Sun's maximum distance.

Ratio of heat received.

At present                 96,473,205 miles 100 At greatest excentricity. 102,256,873 "     90

So that in the one case the earth receives about one-tenth less heat than in the other.

The sun's maximum distance occurs at present a little after the midsummer of the northern hemisphere. When it occurred at the same time of the year during the period of greatest excentricity, the earth at our midsummer was receiving only nine-tenths of the quantity of heat which it now receives at that time of the year. I cannot calculate the effect on climate; but it must have been very great, not only directly, by depressing the snow-line, but, as Forbes remarks in the place cited above, indirectly by chilling the air—and, I will add, by filling the North Sea with the icebergs which must have broken off from the glaciers that filled the Norwegian fiords, as