North America in the Ice Period - The causes of the glacial period

A Few years ago the scientific world was startled by the assertion that at no very remote period, geologically speaking, the climate of the northern hemisphere had been very much colder than at present; and that the arctic conditions which now prevail in Greenland—with perpetual snow-sheets, and glaciers reaching the sea—extended as far south as the middle of the present temperate zone.
At first, seriously questioned by most, strenuously denied by some, this theory was found to be sustained by such abundant and indisputable evidence—the inscriptions left by the glaciers themselves—that it was not long before it had secured a general acceptance from geologists. Since then there has been a vast amount of theorizing and investigation, to determine if possible the causes of these remarkable changes of climate…
Although the glaciated area on our continent has been as yet but partially explored, abundant proof has been gained, as it seems to me, of the truth of the following propositions:  
1. That glaciers once covered most of the elevated portions of the mountain-belts in the West as far south as the thirty-sixth parallel, and all the eastern half of the continent to the fortieth parallel of latitude.
2. That the ancient glaciers which occupied the area described were not produced by local causes, but were the exponents of a general climatic condition.
3. That they could not have been the effect of a warm climate and an abundant precipitation of moisture, but were the results of a general depression of temperature, and therefore afford proof of the truth of what is called the glacial theory…