z 9 2 THE POPULAR SCIENCE MONTHLY. 



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 Glas- 

 gow," vol. ii., part iii., p. 1V7), and detailed calculations on the perio- 

 dicity of these variations have been made by Mr. Stockwell, of Cleve- 

 land, Ohio. 



Let us endeavor to picture the recurrence of one of these coinci- 

 dences, 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 de- 

 nominated neve. The advent of the summer season is not powerful 

 enough to melt the accumulations of the long winter. The neve 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 debris 

 which it detaches in its descent. It covers the broad plains. The ac- 

 cumulations 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 per- 

 haps 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 fol- 

 low the north-south outcropping edges of argillaceous formations. 

 Lake Michigan lies in one of these troughs. Lake Huron lies in an- 

 other. 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- 



