As the earth became divided, so that there were a number of continents and oceans, its climate became diversified. This was in part accomplished by the changes in the course of the ocean currents, such as our Gulf Stream; in part it may have been by slight variation in the sun's heat. However brought about, from very ancient days to the present time large portions of the earth's surface have occasionally had climatal conditions which cause the rainfall to descend in the form of snow, the snow falling in such quantities that it did not melt away in the summer season. This condition now exists about either pole, and to a certain extent on the high mountains, even those of tropical lands.
From time to time, owing to the variable adjustments of climate, these periods of excessive snow have endured for ages, in which the glacial sheet has extended in either hemisphere far towards the equator. In our present day the earth is just escaping from the last of these wonderful ice epochs. At a time so recent that it may be called a geological yesterday the greater part of Europe and of North America was buried beneath accumulations of snow, or rather of ice formed from it, the sheets having in places the depth of a mile or more, and, according to their strange nature, moving slowly over the surface, crushing and grinding the rocks as they went, until the ice either reached the sea, where it would float off as icebergs, or a place on the land where it was far enough south to be melted away.
THE ICE SHEET WAS DEEP ENOUGH TO FLOW OVER THE TOP OF MOUNT WASHINGTON.
On the surface of North America the ice sheet, the remnant of which still covers Greenland, expelled all life from the region of Canada and the United States from a line a little to the east of the Rocky Mountains, and in general north of the Ohio and the Mississippi rivers to the sea-coast. It was deep enough to flow over the top of Mount Washington in New Hampshire, and a primitive man (for there were such in those days) might possibly have journeyed over all the realm without discerning the least trace of the earth's rock surface, for even the higher mountains were buried.
We do not yet know how many of these glacial periods there have been, or whether they occur at the same time in both the northern and southern hemispheres, but it is clear that they have been of frequent occurrence. In the intervals between the ice epochs warm conditions appear to have prevailed even up to the pole of the hemisphere, which was shortly afterwards to experience the dreadful winter of an ice-time. Thus, at a period which in its geological sense was not long before the last glacial epoch, the Greenland district bore a forest much like that which now exists in parts of the Southern States of this country. It seems probable from the history of the past that the next revolution in our northern hemisphere will dissipate the ice about the arctic pole, and make a wide realm now uninhabitable to man fit for his use.
The foregoing little sketch of a few of the great events of the earth's history does not take into account the greatest of them all, the coming of man. But the conditions which surround the appearance of this flower of the earth are as yet so imperfectly known that they cannot well be considered.