236 



DYNAMICAL GEOLOGY. 



209, 



Wherever the mass of the n^vi is sufficient to overcome the resistance to 

 motion, the true glacier begins. The ice is porous, because made of more or 

 less closely united grains. The grains, which are at first small, enlarge as 

 the stream descends, and in the Aletsch glacier some become two to three 

 inches in diameter. (Forel, 1880, 1890.) Moreover, the grains have a crys- 

 talline texture, as has been proved by examinations with polarized light. 



The porosity of glacier ice is made manifest by pouring on it aniline purple or indigo 

 sulphate ; the liquid penetrates it and gives it a marbled appearance. The specific gravity 

 of the iceberg ice off the west- Greenland coast has been found to be only 0-866, owing to 

 its abundant linear cells (Helland, 1877). 



2. Glacier regions. — In further illustration of the general characters of 

 glaciers, reference is first made to the Alps, the best known of glacial 

 regions. The Swiss Alps are divided into northern and southern ranges by 

 the east-and-west part of the valley of the Rhone, and the continuation of 

 the depression westward along the Trient and Chamouni. In the southern 

 range are two glacier regions, the western, of Mont Blanc, and the much 



larger eastern, of Monte Rosa, 

 besides some much smaller 

 areas. Mont Blanc has a 

 height of 15,784 feet, and 

 Monte Rosa of 15,163. In 

 the northern range, there is 

 the glacier region of the Ber- 

 nese Alps (so-named from the 

 Canton of Berne), in which 

 stand the Jungfrau, 13,671 

 feet high ; Eiger, 13,045 ; Fin- 

 steraarhorn, 14,026; and the 

 Aletschhorn, 13,800 feet. 



The map on page 235 

 represents the larger part of 

 the glacier region about Mont 

 Blanc, with 30 to 40 of its 50 

 glaciers. On the northwest 

 side is the valley of Cha- 

 mouni, or that of the river 

 Arve ; on the southwest, Allee 

 Blanche and Val Ferret, in 

 Savoy. The summit of Mont 

 Blanc is at B. As just stated, 

 each valley in the ice-covered 

 The largest extends from Mont Blanc, northeastward 

 to g, where it receives, and for the larger part is, the Glacier du Geant (G 

 being the Col du Geant). At m, where it is the Mer de Glace, it receives 



Union of the glaciers. Tyndall. 



area has its glacier. 



