io6 The American Geologist. August, isoo 
tion of the problem, and each must add its store of observed 
facts to clear away the mysteries which have obscured the 
truth. 
Chapter IV. 
Epitome of Past Climatic Variations. 
The record left by the last of the series of climates through 
which the earth has passed is the evidence upon which geolo- 
gists have established the existence of the Ice age. 
To speak of the Ice age as a past epoch in the earth's his- 
tory is not quite correct. Over the greater part of polar lati- 
tudes this age is yet in existence at all altitudes; in temperate 
latitudes, the area over which its present existence extends are 
more restricted and are confined to elevations between 5,000 
and 15,000 feet according to latitudes; in the tropical zone a 
still greater restriction as to area and altitude exists, glaciers 
being found only at bights above sea level approximating 16,- 
000 to 20,000 feet. It is therefore possible to speak of it as a 
past age only in a somewhat restricted sense; and the evidence 
of progressive retreat found at the base of existing glaciers 
justifies the opinion that it is a passing age. 
From the strata of each of the geological ages, and almost 
from those of each period, evidence has been found of ice ac- 
tion.* This evidence is so widely distributed and has been 
identified with such a degree of certainty in some instances 
that it must be accepted as proof that ice has been an agent 
from the earliest geological ages. Whatever may be the doubt 
or obscurity attaching to some of these evidences of ice action, 
it must be admitted that during each age from the Palaeozoic 
to the present, glacier ice has formed and has recorded and is 
recording its existence in a legible and characteristic manner. 
We must therefore conclude that the glacier was an early 
agent in geology, and that elevated areas were as liable to be 
glaciated in pre-Quaternary time as they are now. W^e nuisl 
admit and weigh all the evidence regarding these glacier 
marks, and solve the problems of climatic evolution without 
omitting or ignoring this evidence. At the same time, it will 
not do to build up an ice age on insufficient data. Glacier ice 
*01dham, Geol. Mag., Vol. XXIII. 1886. White. Am. Geologist, 
Vol. Ill, 1889, pp. 222-230. 
