Our next topic for consideration is one of the most vexed questions among geologists, the Glacial period which immediately preceded the Advent of Man. In treating of this it will be safest first to sketch the actual appearances which present themselves, and then to draw such pictures as we can of the conditions which they represent. The most recent and superficial covering of the earth's crust is usually composed of rock material more or less ground up and weathered. This may, with reference to its geological character and origin, be considered as of three kinds. It may be merely the rock weathered and decomposed to a certain extent _in situ_; or it may be alluvial matter carried or deposited by existing streams or tides, or by the rains; or, lastly, it may be material evidencing the operation of causes not now in action. This last constitutes what has been called drift or diluvial detritus, and is that with which we have now to do. Such drift, then, is very widely distributed on our continents in the higher latitudes. In the Northern Hemisphere it extends from the Arctic regions to about 50° of north latitude in Europe, and as low as 40° in North America; and it occurs south of similar parallels in the Southern Hemisphere. Farther towards the equator then the latitudes indicated, we do not find the proper drift deposits, but merely weathered rocks or alluvia, or old sea bottoms raised up. This limitation of the drift, at the very outset gives it the character of a deposit in some way connected with the Polar cold. Besides this, the general transport of stones and other material in the northern regions has been to the south; hence in the Northern Hemisphere this deposit may be called the _Northern_ Drift. If now we take a typical locality of this formation, such, for instance, as we may find in Scotland, or Scandinavia, or Canada, we shall find it to consist of three members, as follows:-- 3. Superficial Sands or Gravels. 2. Stratified Clays. 1. Till or Boulder Clay. This arrangement may locally be more complicated, or it may be deficient in one of its members. The boulder clay may, for example, be underlaid by stratified sand or gravel, or even by peaty deposits; it may be intermixed with layers of clay or sand; the stratified clay or the boulder clay may be absent, or may be uncovered by any upper member. Still we may take the typical series as above stated, and inquire as to its characters and teaching.