Prior to the enunciation of this theory by Croll, the famous English geologist, Sir Charles Lyell, from measurements of the strata, had calculated that the last period of glaciation occurred about as Croll stated, and that a period of cold and ice far more intense and extensive occurred some four or five hundred thousand years earlier. Mr. Laing has shown that, in order to make such conditions as must have existed at this time, not only is a low temperature necessary, but a certain amount of land must have an elevation sufficient to give the required initial fall to the ice river, so that it may move over the obstacles in its way, and that the higher such elevations in the Arctic zones, and the greater the humidity of the air when it strikes such elevated polar plateaux, the more augmented will be the probability of glacial activity. The rapidity of the glacier's movement can have no bearing upon the duration of the glacial period, inasmuch as a certain length of time may have been required for the ice-cap to form and push forward to a certain place, and it may have remained there for an indeterminate period, governed only by the amount of snow deposited upon the original source, and the rapidity of melting at the moraine. In Eastern England, no less than four distinct boulder clays have been found separated by the débris deposited from the moraines of each ice sheet, and a few hundred miles away in France, the record is so certain that we know that the Arctic fauna and flora gave away twice for that of the warmer parts of the Temperate zones. We are certain that both that portion of Scandinavia and Canada, which were the centers of the great European and American ice-caps, had an elevation greatly in excess of what it is to-day, at the time of the glacial epoch. During the first glaciation, Eastern Canada, or that part south of Hudson's Bay, was certainly twenty-five hundred feet higher than it is now, and the area covered by ocean formations or marine beds to the southward, show that at the same time these sections were very much lower than they are at the present day. On the other side of the Atlantic Ocean, the elevation in Norway was at least a couple of thousand feet more than at present; while both England and Ireland have risen a considerable amount since this period.