672 DYNAMICAL GEOLOGY. 



2500 feet ; in some parts of the Arctic, 3500 feet ; on the south side of 

 the Himalayas, 2000 feet, and on the north, 2600 ; while in the equable 

 climate at the equator in the Andes, the snow-line is 1000 feet below 

 the year-line of 32°. In Norway, the glacier-limit is 4000 feet below 

 the line of 32°. 



The lower limit of a glacier sometimes varies several miles in the 

 course of a series of years. A succession of moist years increases 

 the thickness of the glacier, and thereby its tendency downward ; 

 while dry years have the reverse effect. If the moist years have 

 also long, hot summers, the descent and lengthening of the glacier 

 will be further promoted, — since glaciers move most rapidly in 

 summer. But hot, dry years would shorten it, by diminishing the 

 ice, and especially at the lower end. 



Lowering the mean temperature of a place by cooling the summers would 

 lower the glacier-limit. Great Britain and Fuegia are in nearly the same lati- 

 tude; and yet in Fuegia the snow-line is only 3000 feet above the sea. If, by 

 any means, the climate of Great Britain could be reduced to that of Fuegia, it 

 would cover the Welsh and Irish mountains with glaciers that would reach 

 the sea, the snow-line being but 1000 to 2000 feet • above it; and the same 

 cause would place the snow-line in the Alps at 5000 to 6000 feet above the sea, 

 instead of 9000. This change of temperature involves a removal of tropical 

 sources of heat, or an increase of arctic sources of cold. The diversion of the 

 Gulf Stream by the submergence of Darien has been thought of as a means for 

 the former; but it unfortunately leaves the winds blowing in their old direction, 

 and these cannot be so easily managed. An increase of arctic lands by such 

 elevations as have taken place in former times would accomplish the latter. 



6. The law, rate, and method of flow. — The law of flow is essentially 

 that of rivers. There is friction along the bottom and sides of the 

 glacier, and cohesion in the ice adjoining. The flow is, consequently, 

 most rapid at the surface ; and the axis of greatest velocity varies 

 from the medial line to one side or the other of it, according to the 

 bends in the course of the valley. The motion is so slow that there 

 is no atmospheric friction to retard the surface-movement. The 

 greater rapidity of the middle portion is shown by the fact that the 

 transverse ridges made at an ice-cascade, like that of the Rhone, 

 and the lines of earth and sand in the chasms, become afterwards 

 arched in front, as shown in fig. 951, in which the crevasses c are at 

 first transverse, but curve below the cascade. The arch is sometimes 

 very much elongated, almost to a triangular form, as in the Geant 

 portion of the Mer de Glace. This is well illustrated in figs. 949, 

 950, from Tyndall : the right-hand half of the figure corresponding 

 to the Geant Glacier (the cascade in which is alluded to on p. 670) 

 has the transverse bands (carrying dirt and stones) elongated 

 into triangles, while in the other half of the Mer de Glace there 



