THEORIES OF STRUCTURE. 



69 



pointing inward and downward, and, therefore, at right angles to the 

 crevasses. The relation of these to one another is shown in Fig. 63. 



Finally, as longitudinal fissures are produced by lateral spreading 

 (Fig. 59), so longitudinal veins are produced by lateral compression. 

 This is best seen where two tributaries meet at a high angle (Fig. 64) — 

 for instance, where the Glacier du Geant and the Glacier de Lechaud 



Fig. 62. 



Fig. 63. 



Fig. 64. 



form the Mer de Glace (Fig. 44). All these facts have been experi- 

 mentally illustrated by Tyndall. 



Physical Theory of Veins. — There is little doubt that veins are 

 formed by pressure at right angles to the direction of the veins; but 

 how pressure produces this structure is very imperfectly understood. 

 Probably at least a partial explanation is contained in the following 

 propositions : 1. White vesicular ice by powerful pressure is crushed, the 

 air escapes, and the ice is refrozen into solid blue transparent ice. 2. 

 Ice being a substance which expands in freezing, and, therefore, con- 

 tracts in melting, its freezing and melting point is lowered ~by pressure. 

 Therefore, ice at or near 32° Fahr. is melted by pressure. Now, the 

 glacier is under powerful pressure of its own weight, and the stress of 

 this pressure is ever changing from point to point by the changing 

 position of the particles produced by the motion. Thus the glacier in 

 places is ever melting under pressure, and again refreezing by relief of 

 pressure. The melting discharges the air-bubbles, and, in refreezing, 

 the ice is blue. 3. No substance is perfectly homogeneous, and of 

 equal strength in all parts ; therefore, this crushing and melting, and 

 consequent conversion of white into blue ice, take place irregularly in 

 spots. 4. As ice of a glacier acts like a viscous substance, the final 

 effect of pressure would be to flatten these spots, both white and blue, 

 in the direction of greatest pressure, and extend them in a direction at 

 right angles to the pressure, and thus create bands in this direction. 5. 

 Differential motion would also tend to bring the veins into the direction 

 indicated by Forbes. aJ 



