io8 The American Geologist. ' August, i899 
the level of the sea in the Equadorian Andes, or upon Mt. 
Kenia in equatorial Africa, among the dwarfed remnants of 
actual glaciers doing their work under the same laws as those 
of Greenland, and from these hights trace the evidences of 
their work as they grow fainter and older, until they fade into 
the transformed drift of the tropical plains below. At all tem- 
perate latitudes where glaciers exist the same gradations of 
distinctness can be traced in descending to sea level; and as at 
each latitude and at all altitudes the existing glaciers are so far 
as known yet retreating, this retreat must be accounted for by 
known laws now active.* 
Below the altitudes at which glaciers are now found in 
temperate and tropical regions exist evidences of greater ex- 
tension. The distinctness of these evidences varies inversely 
as the mean annual temperature of the localities in which they 
occur; that is, the farther either in latitude or altitude the evi- 
dence of past glacial action is from existing glaciers the fainter 
and more modified is the evidence. This marks a progressive 
retreat from tropical to polar regions and from sea level up- 
ward. 
As this retreat is yet in progress, as has been observed 
at all known glaciers, it is apparent that at the present time 
the Ice age is yet passing away, and that it yet remains upon 
a considerable portion of the earth; it is strictly a past age 
only for the great temperate and tropical portions of the globe 
and even in these it is^ in existence at considerable altitudes 
above sea level ; the ultimate limit of the effect of solar energy 
in modifying the conditions of the Ice age has therefore not yet 
been reached, for the progress of this effect is not only noted 
by the geological and physical tracings of the vast glacial re- 
treat but by the observed progressiveness of this retreat. 
The evidence relied upon to establish the existence of gla- 
cial periods consists in: 
1. Forms of life suitable only to arctic or boreal temper- 
atures, and particularly marine life of this character. 
2. Types of topography shaped only by the action of ice; 
and built up of irregular, semi-stratified clays, sands, gravels, 
and sub-angular and rounded boulders. 
3. Moraines marking the final or temporary limits of gla- 
*See authorities just quoted. 
