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Dr. Croll's Excentricity Theory

Sir,—In your last number Mr. Searles V. Wood advances what he considers to be "the conclusive objection" to Dr. Croll's theory of excentricity as a cause of the glacial epoch, viz. that North America was glaciated further south than Europe, in proportion to its present difference of winter climate, while Dr. Croll admits his theory "to be baseless unless there was a complete diversion of the warm ocean currents from the hemisphere glaciated."

I do not myself remember that Dr. Croll ever made such an admission, and it is certainly not necessary for the application of his theory. But whether there was a partial or a complete diversion of the Gulf-stream from the coasts of Europe, the result anticipated by Mr. S. V. Wood—a complete similarity in the extension of ice over the two continents—was not to be expected, because they are subject to very different conditions, independently of the action of ocean currents.

Europe is interpenetrated by seas having a southward opening, while the mass of land in Western Europe is trifling compared to that of North America. Transfer the Mediterranean to America and you have a sea entering south of Cape Hatteras, and extending quite across the continent to the Sierra Nevada of California, with northward branches reaching to Lake Huron! The influence of such a sea receiving the waters of one of the largest tropical rivers (the Nile), together with the broken form of the western coast of Europe and the narrowness of the land, must be alone sufficient to give Western Europe an insular climate as compared with Eastern America. But at the same time we have on the American side conditions tending in the very reverse direction. The enormous ice-bearing masses of Greenland and Grinnell's Land immediately to the north and north-east, and the Highlands of Labrador in the latitude of the Germanic plain, combined with the great cul-de-sac of Hudson's Bay, to receive icebergs from the north, and pile them up in its southern inlet, almost in the latitude of London, must have tended to lower the climate of North America during the Glacial epoch as much as the Mediterranean and the Bay of Biscay must have ameliorated that of Europe.

These causes of difference of climate depend on broad geographical facts, which we have every reason to believe existed during the Glacial epoch as they do now, and they appear to me amply sufficient to account for the 10° or 12° further southward extension of the ice in America than in Europe, even if the Gulf-stream were "completely diverted." But I do not believe it was completely, but only partially diverted and also diminished in intensity, and it therefore still exerted some differential action on the climates of the opposite coasts of the Atlantic. I would also point out that the difference between the latitudes of points with the same winter isothermals in West Europe and East America averages about 20°, which is much greater than the difference of the

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limit of glaciation in the countries, and this would show that some equalizing effect was produced by the diminished and partial diverted Gulf-stream, as Dr. Croll's theory requires.

Having recently been subjecting the whole of the evidence on the subject of "geological climates" to a careful examination, I may state, that I have arrived at an important modification of Dr. Croll's theory, which will, I believe, obviate the chief objections that have hitherto been made to it. The subject will be fully discussed in a volume I am now engaged in printing.