iF may be assumed that the circumstances essential to 
the formation and movement of glaciers are these: 
1. Abundance of moisture in the atmosphere. 
2. ÅA low temperature, due either to great elevations in 
low latitudes or to high latitudes with or without such eleva- 
tions of land. I 
These conditions insure such accumulations of show above 
the line of perpetual frost as will sooner or later descend below 
the line of perpetual snow and be changed to ice and water. 
The water forms glacial rivers, and the ice will move 
as a plastic mass to a line determined by the amount of 
snow on the one hand and the climate on the other. 
The advancing movement of the glacier is accompanied 
by erosion and scratching of the rocks below and by the 
formation of different kinds of moraimes, as till or blue 
boulder-clay, and yellow unstratified masses, — terminal, 
lateral and superficial moraines. Simultaneous with these 
phenomena, we have the action of the glacial rivers, consisting 
in a partial denudation of the moraines, and the formation 
of stratified gravel, sand and clay. 
The glacial phenomena of the glacial period were as 
follows: 
1. The sinking of the temperature, accompanied by the 
formation and increase of glaciers. 
2. The motion of the ice to its extreme limit. 
3. The formation of moraines, of which a part were 
moved forward and constituted terminal moraines, while 
another portion was passed over by the ice and then became 
bottom- or ground-moraines. 
4. "The removal by this advancing glacier of the glacial 
river-deposits, or their being covered by the glacier itself 
and its ground-moraine. 
A geological section at the edge of the ice would then 
present either 
