The Evolutioji of Climates. — Ma?ison. 103 
The most distinguished scientists who have investigated 
this subject, have no considerable number of followers; and 
equally distinguished co-workers accept in part only, or reject 
entirely, the conclusions reached by their fellows. 
The most conservative and deeply read scholars express 
them.selves in the doubtful and cautious terms just quoted, 
or admit that they "must be content to work and wait." 
The disagreements as to cause, extend also to actual facts 
and phenomena, and we find geologists of high repute dis- 
agreeing as to whether certain phenomena, existing in warm 
temperate and tropical latitudes be glacial or not. Nor is this 
disagreement that which usually appertains to the nebulous and 
debatable fields of all branches of science, but it is fundamental. 
It relates to the very principles upon which the sciences of 
geology and physical geography rest. 
Confusion is also introduced through lack of uniformity in 
the terms used; the period of maximum glacial extension is 
spoken of as the Ice age, the great Ice age, the Glacial period, 
the last Glacial epoch, etc. 
The use of the adjective great suggests that there have been 
other ice ages; and the term last glacial epoch, suggests others 
of a series. 
It is not possible in the present stage of the question to 
frame a definition which would give general satisfaction — that 
is, a definition which would reasonably fix the time limits of 
the Ice age. It is probable that ice has had a periodical and 
variable existence upon restricted areas throughout the greater 
portion of geological time. In a broad sense, therefore, this 
age embraces that period of the world's history during which 
ice has been a geological agent; it would embrace the earlier 
geological ages, during which ice formed on elevated peaks; 
the Carboniferous and Permian eras during which ice. sheets 
became quite extensive; the Quaternary period during which 
glacial conditions reached a maximum; and the present era 
during which polar regions and mountains above the snow line 
are yet subjected to ice action. 
As ordinarily understood and used by geologists the terms 
Ice age, Glacial period, and Glacial epoch are synonymous and 
embrace an indefinite period of time between the early Quater- 
nary and the present era, during which glacial conditions were 
