THEORIES OF STRUCTURE. 



67 



tudinal vertical section (Fig. 56, c) shows the manner in which they 

 dip. Fig. 57 is an ideal glacier cut in several directions, and combining 



Fig. 56. — Sections of a Glacier. 



Fig. 57. — Ideal Diagram, showing 

 Structure of Glaciers (after Forbes). 



in one view the three sections given above. It is generally impossible 

 to trace the veins around from side to side. Sometimes they are most 

 distinct on the margins, and then are called marginal veins ; some- 

 times at the point of the loop — transverse veins ; sometimes tributaries 

 running together, as in the figure (Fig. 57) — the interior branches of 

 the two loops coalesce, and are flattened against one another, and form 

 longitudinal veins. 



Fissures. — These are also marginal, transverse, and longitudinal. 

 The marginal fissures are shown in Fig. 43 ; they are always at right 

 angles to the marginal veins. 



Theories of Structure. 



Fissures. — There can be no doubt that the great fissures or crevasses 

 are produced by tension or stretching, and that their direction is always 

 at right angles to the line of greatest tension. Thus the transverse 

 fissures are produced by the stretching of the glacier in passing over a 

 salient angle. The marginal fissures are produced by the dragging or 

 pulling of the swifter central portions upon the slower marginal por- 

 tions. It has been proved by Hopkins, the English physicist and geolo- 

 gist, that the line of greatest tension from this cause would be inclined 

 45°, with the course of the glacier as shown by the arrows (Fig. 58). 

 The fissures should be at right angles to these lines, and, therefore, also 

 inclined 45° with the margin, and running upward and inward. The 

 longitudinal fissures are best seen where a glacier runs through a nar- 



