CLOUDS AND RIVERS, ICE AND GLACIERS. 185 



the glaciers tliat we have visited the little superficial 

 furrows here described ; and you have, moreover, 

 noticed that in the furrows mainly is lodged the finer 

 dirt which is scattered over the glacier. They sug- 

 gest the passage of a rake over the ice. And when- 

 ever these furrows were interrupted by a crevasse, the 

 veined structure invariably revealed itself upon the 

 walls of the fissure. The surface grooving is indeed 

 an infallible indication of the interior lamination of 

 the ice. 



470. "We have tracked the structure through the 

 various parts of the glaciers at which its appearance 

 was most distinct ; and we have paid particular atten- 

 tion to the condition of the ice at these places. The 

 very fact of its cutting the crevasses at right angles is 

 significant. We know the mechanical origin of the 

 crevasses ; that they are cracks formed at right angles 

 to lines of tension. But since the crevasses are also 

 perpendicular to the planes of structure, these planes 

 must be parallel to the lines of tension. 



471. On the glaciers, however, tension rarely occurs 

 alone. At the sides of the glacier, for example, where 

 marginal crevasses are formed, the tension is always 

 accompanied by pressure ; the one force acting at right 

 angles to the other. Here, therefore, the veined 

 structure, which is parallel to the lines of tension, is 

 perpendicular to the lines of pressure. 



472. That this is so will be evident to you in a 



