308 GEOLOGY. 



the outset, it is likely to occur later as the result of the melting and 

 wave-cutting which disturb their equihbrium. The great majority 

 of bergs do not travel far before losing all trace of stony and earthy 

 debris, but the finding of glacial material in dredgings far south of all 

 glaciers shows that they occasionally carry stones far from land. 



Fig. 284.— End of Muir glacier, Alaska. (Reid.) 



THE INTIMATE STRUCTURE AND THE MOVEMENT OF 



GLACIERS. 



With the preceding account of glaciers in mind, we may return to 

 a closer study of their origin, their intimate structure, and their mode 

 of motion. The key to this study is the thesis that a glacier is a mass 

 of crystalline rock — ^the purest and simplest type of crystaUine rock 

 known — since it is made up of a single mineral of simple composition 

 and rare purity, which never appears in a soHd state except in the crys- 

 talline form. 



The growth and constitution of a glacier. — The origin and history 

 of a glacier is little more than the origin and aggregate history of the 

 crystals that compose it. The fundamental conception of a glacier is 

 therefore best obtained by tracing the growth of its constituent crystals. 

 A basal fact ever to be kept in mind is that water in the solid form is 

 always controlled by crystalline forces. When it solidifies from the 

 vapor of the atmosphere it takes the form of separate crystals (Figs. 286- 

 291). Perfect forms are developed only when the flakes fall quietly 

 through a saturated atmosphere which allows them to grow as they 

 descend. Under other conditions, the crystals are imperfect in growth 

 and are mutilated by impact. But however modified, they are always 



