From the Alps gigantic glaciers descended to the lowlands in all directions. Then the Rhine glacier moved out far beyond the mountains and joined with the glaciers of Savoy and Dauphiny on the plains of France, while from the Southern Alps glaciers invaded the fertile plains of Italy. Glaciers of similar size and extent descended into the valleys of the Rhine and Danube. The Pyrenees, some of the higher mountains of the Spanish plateau, the higher mountains of France, the Apennines, the Carpathians, the Balkans, the Urals, all had their ice-sheets. Iceland and the Faroe Islands were buried under ice, and even Corsica had snowfields and glaciers, some of which were not small. Nearly one half of North America was buried in ice. Strangely enough, it was not the whole northern half, but the north-eastern half that was specially ice-invaded, and, more strangely still, not so much the mountainous portions, though these were affected, as the plains. Alaska was largely free from ice except on or about the mountains: and there was less ice on the western plains than in the valley of the Mississippi. Much the greater part of the four million square miles of ice-field lay on the plains of Canada and in the upper Mississippi valley. The Missouri and Ohio rivers like two great arms embraced the borders of the ice-fields to which they owe their origin. We do not propose to examine the several theories which have been proposed to account for this extraordinary cold, for none is completely acceptable or accepted, but we may just mention them. Dr. Croll a century ago suggested that the cold may have been due to the alterations in the shape of the earth's orbit, alterations which astronomers tell us take place regularly, though very slowly and at intervals of millions of years. If so, this glacial period was only the last of many glacial periods; the traces of the earlier ones having, however, been for the most part obliterated and destroyed.