upon its period of north-polar glaciation. Its last glacial epoch in the operation of this cause occurred in the Northern Hemisphere in its acme of intensity at a period 11,300 years ago.
The effect of these irregularities in the motions of the earth on the climate has been ably discussed by Mr. James Croll, of the Geological Survey of Scotland ("Transactions of the Geological Society of Glasgow," vol. ii., part iii., p. 177), and detailed calculations on the periodicity of these variations have been made by Mr. Stockwell, of Cleveland, Ohio.
Let us endeavor to picture the recurrence of one of these coincidences, and to rehearse some of the phenomena of an actual period of continental ice.
The precipitation of the winter season is all preserved on the ground in the form of snow and ice. It constitutes what has been denominated névé. The advent of the summer season is not powerful enough to melt the accumulations of the long winter. The névé is simply converted into granulated ice. Another winter adds to the thickness left by the preceding. Another summer changes it to ice. Some water may be the result, but it is congealed in the streams, or perhaps escapes to the ocean. This succession is continued, with a slow increment of cold, through thousands of years. The ice-mantle becomes continental in its extent. Its thickness reaches hundreds of feet. Toward the pole this may be increased to thousands or tens of thousands. It has a great weight. It presses upon itself. Its lower portions yield to the inequalities of the rocky surface. The mass seeks the valleys. It slides down the mountain-sides, carrying the débris which it detaches in its descent. It covers the broad plains. The accumulations toward the north, ever increasing, press out toward the south, the foot of the ice-sheet. A general movement is developed by reason of the gravity of the mass, the fracturing and regelation of its parts, and the molecular forces that allow it to yield under pressure. Each recurring summer develops more or less water. This water perhaps enters the openings and crevasses, and washes out some of the obstructions, facilitating the general progress. The main water-sheds separating valleys serve also as ice-sheds. The valleys are more rapidly dug out by the rasping and ploughing movement of the glacier than are the highlands. In the valleys the ice flows most rapidly. In the valleys, also, the ice is prolonged much farther into warmer latitudes than on the highlands. Southward, prolongations of the ice-sheet follow the north-south outcropping edges of argillaceous formations. Lake Michigan lies in one of these troughs. Lake Huron lies in another. Lakes Erie and Ontario are only shallow basins dug out of soft rocks by ice that passed southwestwardly. The shale-bed that gave rise to Lake Ontario also determined the location of Georgian Bay and of Green Bay. The basin of Lake Erie is much shallower toward the west end than toward the east, and it finally runs out alto-
