10)1 



GLACIERS. 



GLACIERS. 



1)34 



and on which a new moraine is collected. This recedes like the first, 

 and so on ; so that were it not that the moraines of the opposite 

 sides sometimes become confounded into one, and because the motion 

 of the ice on the irregular slopes of the valley disturbs the order and 

 parallelism of the banks, they might serve to determine the age of 

 the glaciers. 



In winter, as well as in summer, there is continually a quantity of 

 water flowing out from the lower parts of the glaciers, though much 

 less abundantly in the former than in the latter season. This water 

 proceeds from the thawing of the under surface of the glacier, occa- 

 sioned by subterraneous heat. In the winter it oozes from under the 

 ice in small streamlets ; but in the spring and summer mouths, when 

 it is greatly increased in quantity, it bursts away the ice from before 

 it, and gushes out in plenteous streams from the caverns it has exca- 

 vated; some of these grottoes are 100 feet high, and from 50 to 80 

 feet wide, presenting very various and sometimes highly picturesque 

 appearances. The torrents of the glaciers are remarkable for the 

 whitish-blue colour of their waters, which they maintain for a 

 distance of some leagues. 



There is a phenomenon, which the Germans call ' gletschergeblase," 

 which results from the sudden escape of the air imprisoned beneath 

 the glaciers. On a change of temperature this escapes through the 

 crevices in strong currents of insupportably-cold wind, driving like 

 snow-dust the fine icy particles with which it is loaded. 



All the Alpine valleys being inclined planes, it is natural to suppose 

 that the glaciers must slip down by their own weight, whenever any 

 circumstance destroys their adhesion to the sides and bottoms of. the 

 valleys. This adhesion is constantly diminished, even in the depth of 

 winter, by the natural warmth of the earth, which thaws the under 

 surface of the glacier ; but as this takes place only in those parts 

 where the great thickness of the ice protects the soil from the effects 

 of external cold, the mass by this action is but partially disengaged, 

 and therefore still maintains its position. But when the warmth of 

 summer heats the Boil all around, and thaws the ice at its surface and 

 edges, the liberation of the glacier goes on with rapidity, aided as it is 

 moreover by the erosion of the uuderflowiug currents, and the abra- 

 sion of the lumps of ice and the stones which they bear along. Then 

 it is that the whole mass, obeying the impulse of gravity, slips down 

 and invades the fertile valleys below, presenting the singular spectacle 

 of an ice-field terminating on flowery meadows and contiguous to rich 

 harvests. The limits which the descending glaciers attain are subject 

 to variation. 



Great attention has been lately given to the causes of the descent 

 of glaciers. The hypotheses by which the descent of the vast masses 

 of frozen snow down the valleys of the Alps and other mountainous 

 regions has been explained, or attempted to be explained, are essentially 

 two. First, the original notion of Saussure, put in its most original 

 form, that the glacier-masses descended the valleys by the force of 

 gravity ; and secondly, the later notion of Charpentier, that the icy 

 masses were pushed down the valleys by an internal expansion caused 

 by congelation of water in their internal cavities. Each of these 

 original notions takes at least two forms. Saussure's hypothesis, 

 indeed, appears in three modifications of importance. 



1 A. The view of Mr. R. Mallet, communicated to the Geological 

 Society of Dublin, recognises the descent of the glacier by gravity, 

 but adds the hydrostatic pressure of water below the glacier upward 

 and forward ; and it is impossible to deny to this speculation the 

 merit of removing some considerable difficulty in the reception of 

 the general hypothesis of Saussure. 



1 B. The view of Professor James Forbes, who, besides noticing 

 p'-culiar structures (the blue bands) in the glacier ice, and measuring 

 the velocity of glacier movements in different parts of the valley, 

 and in the central and lateral parts of the ' ice-current' (if we may so 

 apeak), has proved the bending of the ice from a straight transverse 

 line during its movements ; and hajs, by means of artificial prepara- 

 tions, imitated some of the peculiar glacial structures. His view is, 

 that glaciers descend the valleys iu consequence of so much mutual 

 yielding and adjustment (plasticity) of their parts as to entitle them 

 to be regarded as viscous or semifluid masses, flowing slowly under 

 the influence of gravity. 



1 C'. Mr. Hopkins, recurring to the original notion of Saussure, 

 maintains the mechanical probability of the glaciers sliding down 

 tlfuir containing valleys^ as solid bodies; that is to say, sliding in 

 consequence of the general slope of the valley, and not in consequence 

 of the internal displacements and readjustments of the icy mass. 

 Displacements and readjustments happen in this view no less than iu 

 Mr. Forbes'g hypothesis. The icy mass is subject to extension, to 

 flexure, and fracture under tension, and a variety of accidents, which 

 Mr. Hopkins has employed to explain the unequal rates of move- 

 ment, the varying forms of fissures, Ac. in glaciers. Ice may be 

 regarded as in a certain degree plastic, since it is flexible ; but none of 

 these things interfere with the essential condition of this hypothesis, 

 namely, the sliding of the whole ice-mass on its bed. This sliding is 

 in a great measure due to the perpetual slow fusion of the lower 

 surface of the ice, which leaves it in a constant state of disintegration, 

 Ac. Mr. Hopkins has made experiments on the sliding of ice ; from 

 which it results, that even at angles of inclination from the horizon 

 much below 1 degree, the movement of the ice masses on a rough 



flagstone, while the lower surface was subject to slow fusion, was 

 very discernible, and found to follow a simple law proportioned to 

 the sine of the angle of inclination. 



2 A. The second branch of the hypothesis alluded to, originating 

 with Charpentier, has further ramified into two forms. Charpentier 

 thought the congelation of water in many fissures of the glacier 

 must necessarily urge portions of it forward in the direction of least 

 resistance. But as the glacier is sliding by day and by night, iu 

 summer and in winter, sliding unequally in different parts of its 

 descent, and not with such an order of inequality as fits to the 

 hypothesis; and as there is no assignable reason (in this hypothesis) 

 for the formation of new fissures, and the continuation of the 

 process, this whole speculation (the dilation hypothesis) has beeu 

 abandoned. 



2 B. Another form has been given to it by M. Agassiz. This 

 eminent explorer of the Alpine glaciers ascribes their onward move- 

 ment to congelation of water, not in the cavities of Charpentier, 

 but in minute capillary fi.-jsures and spaces among the granular 

 constituent masses of the glacier. With such a power of expansion, 

 altogether independent of gravity, M. Agassiz esteemed it possible 

 to allow of the movement of glaciers, even across level countries, 

 and thus to account for many geological phenomena difficult to be 

 otherwise explained, as erratic blocks and other diluvial phenomena. 

 Mr. Hopkins has, however, exclusively shown that by the means 

 supposed no such power of onward movement can be exercised. Mr. 

 Forbes has proved the actual phenomena of glaciers to be incon- 

 sistent with the assumption of such congelation as a cause and with 

 the effects ascribed to it by Agassiz. It is clear from all experiments 

 and mechanical reasonings, that gravity causes the descent of glaciers ; 

 but there remain some further researches to be made into the true 

 internal structure of ice, and the true state of the lower surface of 

 glaciers, before we can regard the inquiry as complete. 



The most recent writers on the phenomena of the glaciers are the 

 Messrs. Schlageutweit, who have published a work on the ' Physical 

 Geography of the Alps.' 



The investigations of Agassiz and others into the history of. the 

 glaciers of the Alps, &c., and their former greater extension, have 

 rendered it very probable that this enormous ice-power has been 

 actively at work in early geological periods in situations where now 

 no fields of ica ever occur. Observations of effects such as glaciers 

 are known to produce on surfaces which they traverse ; smoothed 

 and rounded rocks grooved surfaces striatiou parallel to the grooves 

 appear to require the existence and movement of glaciers down 

 some of the valleys which intersect the Snowdoniau, the Cumbrian, 

 and Irish mountains. One of the effects attendant on glaciers is the 

 transport of detritus on the surface of the ice till it melts, or iu 

 the case of the glacier meeting the sea, breaks off in an iceberg to 

 be drifted by oceanic currents. It is believed by Agassiz that the 

 immense single blocks of stone which lie on the Jura, opposite the 

 valleys of the Alps, have been deposited there by glaciers, not 

 drifted by water-currents, and this speculation he has applied to far 

 greater areas and more difficult cases, such as the accumulation of 

 erratic blocks, gravel mounds, and diluvial heaps in level regions like 

 those which margin the ancient tertiary Bay of Dublin, or abound in 

 the central plains of England. From so great an extension of this 

 speculation, founded as it is on au erroneous hypothesis of glacial 

 movement, Mr. Hopkins's demonstrations warn us to dissent ; but if, 

 as may be easily believed, the ancient glacier streams of Cumberland 

 delivered the detrital blocks of Shap and Carrock into the sea by the 

 breaking off of icebergs, these may have been drifted by currents to 

 Staffordshire, to the mouth of the Tyue, and the valleys of York and 

 Holderuess. 



It may be objected that for such an icy covering to Snowdon and 

 Skiddaw, a great reduction of climate is necessary, aud that this is 

 not consonant with the general tenor of geological inferences, which 

 point to more elevated temperatures in early periods. But the reply 

 is easy. The evidence of warm climates in northern zoues relied on 

 by geologists applies to far earlier periods ; and in respect of this 

 comparatively late period, a reduction of temperature to the extent 

 required for the productiou of glaciers on the mountains of Wales 

 aud Cumberland is perfectly possible by a mere change of the dispo- 

 sition of laud and water since in fact the temperature of the 

 British Islands is now iu excess above the mean of their latitude by 

 5, 10, 15, and more, and this excess is merely due to oceanic 

 currents and other conditions which vary with the distribution of 

 land and water. 



Notwithstanding the immense accession of snow and ice which tho 

 glaciers receive every winter, and which is much greater than what 

 could possibly be thawed by the mere effect of a short summer in tho 

 higher Alpine regions, it is found that they have not sensibly increased. 

 If for one or a few years in succession some of the glaciers are observed 

 to descend lower than usual, they are found iii the following years to 

 recede proportiouably ; thus they are confined within certain limits 

 by a compensating process of nature. The evaporation from ice, and 

 particularly from snow, is considerable even iu winter, aud goes on 

 with great rapidity iu a dry aud rarefied air ; and subterranean heat, 

 as we have already observed, produces throughout the year a certain 

 diminution of the glaciers at their under surface. In the summer the 



