2i8 The American Geologist. October, 1902. 
pleasure and profit, "The Founders of Geology," by Archi- 
bald Geikie, (1897), "Illustrations of the Huttonian Theory 
of the Earth," by Playfair (1802), and "Theory of the Earth," 
by James Hutton (1795). 
Classification. 
For convenience in considering this subject it may be di- 
vided into two general classes : glaciers of the present, and 
glaciers of the past. The former class may be said to include 
the existing glaciers of the mountainous portion of the globe, 
together with the polar ice sheets, while the latter class con- 
siders particularly the great continental glaciers. 
We may further distinguish broadly bf^tween the two 
prominent types of present glaciers, which have been termed 
the Alpine or mountain glaciers, and the polar ice-fields of 
the Greenland type.* First, the area of the polar glaciers is 
enormous, compared with that of the Alpine; secondly, the 
maximum rate of motion is much greater in polar than in 
Alpme glaciers ; and thirdly, the economic results produced 
by the one are vastly different from those produced by the 
other. 
A glacier in general may be defined as a gigantic mass 
of ice formed from snow falling in altitudes above the 
snow line, and subsequently assuming a gram^lar form, which, 
as it moves by gravitation to lower levels, beco^mes compacted 
into a mass known as neve, which gradually takes on- a charac- 
teristic blue, crystalline structure as the air is squeezed out by 
motion, and compression, and becomes glacier ite. 
The Great Aletsch and Gonier glaciers of the Swiss Alps, 
are typical examples of mountain glaciers, where, as 
is often the case, the main ice stream is produced by tlie com- 
bination of many tributary streams. The St. Elias Alps in 
North America, however, furnish the grandest examples of 
this type (which is known as Alpine because it was first 
systematically studied in the Swiss Alps) from the fact 
that Mt. St. Elias and Mt. Logan, its giant neighbor (19.500 
*NoTK. — Some authors include in their classification, glaciers of the 
Scandinavian' type, as .separate from those here given. Such a clas- 
sification may be preferable in some i-espects; the reason for not dis- 
tinctively including it her<3 is, that a glacier of the Scandinavian type 
is defined, according to Johnson's Encyclopeda, as "A broad sheet of 
ice accumulating on .a mountainous plateau." et cetera. Therefore this 
type appears to find a place under "glaciers of the moi'intainous portions 
of the globe," as already, stated. 
