244 
NATURE 
[Fuly 15, 1886 
in definite ways it receives specific names. When dis- 
tributed along the margin of an ice-stream it forms /afeva/ 
moraines. Two glaciers uniting, the right lateral moraine 
of one combines with the left lateral moraine of the other 
to form a medial moraine at the line of contact, the ice- 
streams flowing on side by side as a single compound- 
glacier. The debris carried to the extremity of a glacier 
and deposited about its foot is known as a /ermzna/ or 
Jrontal moraine. 
In flowing through a valley ice is subjected to stress, 
which causes it to fracture and form open fissures termed 
crevasses. When a glacier passes over a steep ascent it 
becomes broken by a great number of fissures, and not | 
infrequently falls to the base of an escarpment in detached 
blocks, forming an ice cascade, but heals its scars and 
flows on as a solid mass below. The fissures formed 
when a glacier passes over an inequality in its bed are 
commonly transverse to the direction of flow, but may 
take other courses, depending on the nature of the 
obstruction, change of slope, &c. Marginal crevasses, 
resulting from the friction of the ice-stream against its 
banks and the consequent more rapid flow of the central 
portion, usually leave the shore at a moderate angle and 
tend up-stream. 
Glacier ice has been found to exhibit a definite struc- 
ture, known as lamination, or as ribboned or banded 
structure, produced by the alternation of thin plates or 
strata of compact bluish ice with others more porous. 
As shown by Tyndall’s experiments, this arrange- 
ment is the result of pressure, and is analogous to slaty 
cleavage. 
Owing to unequal melting, the surface of a glacier is 
usually extremely irregular, the parts protected by 
moraines standing in higher relief than the clearer por- 
tions. Still further diversity is formed by boulders 
perched on columns of ice, which they have protected 
from melting as the general surface wasted away. These 
are termed glacéa/ fables. At other times the ice bristles 
with a multitude of acicular pyramids, or is melted into 
holes and ice-wells, each having a stone or mass of dirt 
at the bottom. 
The melting of the surface of a glacier gives rise to 
many rivulets and brooks, which course over it in channels 
of ice, frequently plunging into yawning crevasses, and 
finally joining the sub-glacial stream that issues from 
beneath every glacier. These glacier-born streams are 
always heavy with comminuted rock, ground fine by the 
moving ice. 
Such in brief are the principal characteristics of alpine 
glaciers. 
At the present time continental glaciers are confined 
to the arctic and antarctic regions, and have been less 
thoroughly explored than the alpine forms common in 
more temperate latitudes. Glaciers of this class are 
characterised by their broad extent and by not being 
confined by definite walls; their #vcs are large, frequently 
covering nearly the entire glacier, and their surfaces are 
free from boulders and debris, for the reason that they are 
regions of accumulation, and also because mountains 
seldom rise above them. Owing to inequalities in the 
country over which these great fields pass, they are not 
infrequently broken by crevasses; and, as on smaller 
glaciers, the melting of the surface gives origin to numerous 
streams, frequently of large size, which become ponded 
and form lakes in basins of ice, or plunge into open 
fissures and disappear in the body of the glacier. Exist- 
ing continental glaciers are believed in all cases to flow 
from the interior towards the coast, and hence may be 
considered as acquiring motion in all directions from a 
centre of accumulation. When alpine glaciers increase 
sufficiently to cover an entire mountain-range and form a 
confluent ice-sheet, they approach and may pass into the 
continental type. It is not impossible that a mountains 
range of very modest dimensions might give origin to a 
| we call a névé. 
quaquaversal glacier of vast proportions. It is perhaps 
not out of place to suggest in this connection that the 
glaciers which formerly covered the New England State- 
and Canada were of this character. 
In framing a definition of a glacier it is evident that we 
must include both alpine and continental types, and also 
embrace the secondary phenomena that are commonly 
present. A glacier is an ice-body originating from the 
consolidation of snow in regions where the secular accu- 
mulation exceeds the dissipation by melting and evapora- 
tion, z.e. above the snow-line, and flowing to regions 
where loss exceeds supply, z.e. below the snow-line. 
Accompanying these primary conditions many secondary 
phenomena, dependent upon environment, as crevasses, 
moraines, lamination, dirt-bands, glacier-tables, ice- 
pyramids, &c., may or may not be present. Thus, glaciers 
even of large size may exist without moraines ; in such 
an instance glacier-tables, ice-pyramids, ice-wells, &c., 
would be absent. We may conceive of a glacier flowing 
through a channel so even that it would not be broken 
by crevasses, but such instances must be extremely rare. 
The most common of the numerous secondary features 
seems to be the laminated structure of glacial ice, but 
even this is not always distinguishable in ice-bodies that 
are unquestionably true glaciers. 
Although the definition we have presented may assist 
in understanding the nature of a glacier, yet it is mani- 
festly open to objections. If we consider the snow-line 
as defining the limit between the évé and the glacier 
proper, it is evident that there must be numerous excep- 
tions to the rule. As before remarked, during certain 
years, and in many instances for a term of years, the 
snow-line is much lower than at other times, and may 
completely conceal the glacier beneath. Again, an ice- 
stream may terminate in the sea and be broken up and 
form icebergs before the differentiation into #évé and 
glacier proper has been reached. 
From all that has been determined concerning the 
nature of glaciers it is evident that they form one of the 
transition stages in the history of the snow that falls in 
certain regions, and like genera and species in the organic 
kingdom cannot be limited by hard-and-fast lines, but 
may be classified by comparison with typical examples. 
From the snow, hail, and frozen mists of a mountain-top 
are formed the accumulations of granular ice-snow which 
By pressure and alternate melting and 
freezing, the zévé passes into compact ice, which acquires 
motion and is termed a glacier; but the plane of separa- 
tion is indefinite, and one merges into the other by 
insensible gradations. 
The morainal material carried by glaciers is deposited 
when melting takes place, and frequently forms character- 
istic accumulations that still retain the name of moraines. 
The debris along the border of an ice-stream is frequently 
left as ridges or irregular terraces on the sides of a valley, 
marking the former height of the ice-flood. At various 
stages in the retreat of the ice the lateral moraines are 
united by terminal moraines which cross the former bed 
of the glacier in irregular but usually crescent-shaped 
piles, between which the valley bottom is usually deeply 
filled with unassorted debris, and frequently occupied by 
lakelets. When a glacier is prolonged from the mouth of 
a valley on a plain, it builds out its lateral moraines 
perhaps for many miles, and when it retreats these are 
left as parallel embankments, not infrequently hundreds 
of feet high and sometimes miles in extent. __ 
The movement of glaciers causes friction, which results, 
as the study of living glaciers has shown, in the smoothing 
and scratching of the rocks over which the ice passes. 
The boulders, pebbles, and sand held in the bottom and 
sides of the glacier produce smooth and polished surfaces, 
crossed by scratches and grooves having an exceedingly 
characteristic appearance, which, when once understood, 
it is difficult to mistake for the results produced by other 
