|CAUSES, STAGES, AND TIME OF THE ICE AGE.|
IF we could see the entire earth at once, by some grand extension of our range of vision, as we might walk around a geographic globe a hundred feet in diameter, and examine it fully, with comparison of all portions of its area, probably no other features of the great terrestrial panorama would be so impressive as the wonderful diversity of climatic conditions. At the same time with perpetual summer on the equator and throughout nearly all of the intertropical zone, a wintry covering of snow and ice would be seen on all lands in high latitudes about one or the other pole. While every bounty of luxuriant plant and animal life is present to attract the traveler and furnish him sustenance in the central zone, the rigorous climate which is gradually encountered in approaching the poles, and the general decrease and limitation of both flora and , have opposed insuperable obstacles to the most eager and courageous explorers. About four hundred and fifty miles at the north, and about eight hundred and fifty miles at the south, lie beyond the farthest limits of exploration; and more than double these distances must be crossed, respectively, if one would pass, according to Nansen's hope and plan, from one side to the other of the hitherto untraversed circumpolar areas.
During the Ice age, or Glacial period of geology, very extensive and thick sheets of land ice, like those now enveloping the Antarctic continent and the interior of Greenland, overspread the northern half of North America (excepting the greater part of Alaska) and northern Europe, with nearly the whole of the British Isles. The southern boundary of the North American ice sheet crossed Nantucket and Martha's Vineyard, Block Island, Long Island, and Staten Island. On the mainland it extended through northern New Jersey and northeastern and northwestern Pennsylvania, being indented by a great angle, whose apex was at Salamanca in southwestern New York. Thence it reached southwest and west across southern Ohio, Indiana, and Illinois, and through central Missouri, into northeastern Kansas; and beyond, it curved far northward, crossing eastern Nebraska and South and North Dakota. From near Bismarck it again trended westward through Montana, Idaho, and Washington, to the Pacific Ocean not far south of Puget Sound. North of this line an area of about four million square miles, stretching to the Arctic archipelago, was covered with ice hundreds and thousands of feet deep.
The comparatively small present ice sheet of Greenland covers