Page:Popular Science Monthly Volume 10.djvu/608

590 already at hand, collected by European observers, that the glacial epoch itself was not continuous, but intermitted by a warmer time during which the ice retreated to reoccupy that portion of its former territory from which it has now finally retired. Prof. Dana has contributed some evidence of a similar action on North American territory. But for our present purpose a general view of the Ice period is all that we need. Evidence is at hand that the glacier, at the time it traversed our territory, was accompanied by plants and animals different from those now inhabiting the Atlantic States.

Remains of the reindeer have been discovered by Prof. Dana in clay-beds thrown together by the action of ice. This animal is now, as we know, confined to arctic regions, but then ranged the valley of the Connecticut. And there has been a sort of natural trap set for the animals and plants of that time, which caged a part of them, so that we may examine some of their live descendants.

It has been found that the condition of the tops of high mountains, such as Mount Washington in New Hampshire, and that of high northern regions, are very similar. It is calculated that a change of one degree Fahrenheit takes place in the temperature for every three hundred feet of vertical height. On a level the same change occurs for every sixty miles as we journey northward. We should have to travel, for instance, from Boston to Hudson's Bay, as Agassiz has shown, before passing over the same range of climatic changes as we do in one day in the Alps, thus causing a narrow strip of Alpine flora to correspond to a broad zone of northern vegetation. The mountains are thus compressed models of the physical conditions of the latitudes of the surface.

In the tropics we have mountains crowned with ice, whose summits reproduce the condition of the north-pole; and, as we descend their sides, we pass through belts of climate ever increasing in warmth, to the plain beneath, where we meet with the condition of the torrid zone. Now, during the glacial epoch, when the surface of our Middle States was covered with a coat of ice, the plants and animals had been swept southward of the White Mountains. They bloomed and lived in the spring-tides that softened the edge of the glacier, and enjoyed the short summer that there ensued at the source of streams fed from the melting ice. But, when the glacier retired, the summers over this region becoming longer, and the winters shorter, plants and animals followed the ice and their congenial climate northward to the valleys of New Hampshire. Out of these valleys the glacier finally departed also, but not without leaving some of its retinue behind. After the main glacier had left the valley, Mount Washington and Mount Adams still remained largely covered with ice, and a system of local glaciers filled the clefts and gorges of the hills. Allured by these, some of the plants and insects were retained and did not follow the bulk of their companions who were on their long march to the