42 
Geology. 
mountains. In the former case^ a nearly horizontal sheet is 
formed which spreads outwards and gives rise to tongues when 
the expanding border encounters valleys leading down from the 
little plateau to the lower lands or the sea. Thus the tongues 
radiate from the parent mass. The motion is centrifugal. In 
the other case, the snows and ice gather from the amphitheatres 
and gulches into a common trunk, much as the water of 
a collecting area gathers into a river. The movement is concen- 
trative. The upper ice streams gather into a lower trunk stream. 
In some respects, therefore, the habit of the true Alpine glacier 
is the opposite of that of the local ice-cap. Notwithstanding this 
notable difference, the two forms grade toward each other, and 
may possibly grade into each other. Where several Alpine gla- 
ciers collect on the several sides.of a mountain, they jointly have 
a radiant movement. If their amphitheatres are conjoined, they 
approach the type of the ice-cap. If the summit of the mountain 
is broad and smooth, so as to permit a general snow covering, 
there may be a graduation of the one type into the other. 
The frequency of occurrence of local ice-caps on little plateaus 
between the inland ice and the sea is quite notable, and is not 
without importance as a factor in glacial study, for these 
local ice-caps define more sharply than the great inland sheet, the 
limitations of glacial accumulation. They enable us to observe 
quite accurately what elevations (combined with other condi- 
tions) are adequate to produce glaciation, and where the limit of 
adequacy is reached. So also, elevations being equated, they 
furnish the means for estimating the influence of meteoric condi- 
tions. In the case of the great ice-cap it is difficult to determine 
how far the protrusion of the ice upon the borderlands is due to 
local conditions, and how far to influences derived from the dis- 
tant interior, 
USE OF TERMS. The prevalence of ice-caps in Greenland 
has led to a special usage of glacial terms. The title glacier is 
usually applied simply to the ice tongues that protrude from the 
inland ice or from some local ice-cap. It is also applied to dis- 
tinct portions of the border of the ice-cap, particularly such as 
come down to the sea, even though they protrude but little, or 
