FREQUENT allusions have been made in the preceding pages to a period called the glacial, to which no reference is made in the Chronological Table of Formations given at p. 7. It comprises a long series of ages, chiefly of posttertiary date, during which the power of cold, whether exerted by glaciers on the land, or by floating ice on the sea, was greater in the northern hemisphere, and extended to more southern latitudes than now.

It often happens that when in any given region we have pushed back our geological investigations as far as we can, in search of evidence of the first appearance of man in Europe, we are stopped by arriving at what is called the ‘boulder clay’ or ‘northern drift.’ This formation is usually quite destitute of organic remains, so that the thread of our inquiry into the history of the animate creation, as well as of man, is abruptly cut short. The interruption, however, is by no means encountered at the same point of time in every district. In the case of the Danish peat, for example, we get no farther back than the recent period of our Chronological Table (p. 7), and then meet with the boulder clay; and it is the same in the valley of the Clyde, where the marine strata contain the ancient canoes before described (p. 47), and where nothing intervenes between that recent formation and the glacial drift.