68 



AQUEOUS AGENCIES. 



Fig. 



Fig. 59. 



row gorge out on an open plain. The lateral spreading of the glacier 

 causes it to crack longitudinally (Fig. 59). Fig. 60 is a longitudinal 

 vertical section of the same. 



Veined Structure. — Tyndall has shown conclusively that veins are 

 always at right angles to the line of greatest pressure, and that, there- 

 fore, they are produced by press- 

 ure. Thus fissures and. veins, 

 being produced by opposite 

 causes — one by tension and the 

 other by pressure — are formed 

 under opposite conditions. As 

 transverse fissures are produced 

 by the longitudinal stretching 

 of a glacier passing over a sa- 

 lient angle, so transverse veins 

 Fig. 60. are formed by the longitudinal 



compression of a glacier passing 

 over a re-entering angle. Fig. 60 is a section of the Ehone glacier 

 (Fig. 59), showing the crevasses (c c c) produced by the steep declivity, 

 and the veined structure (s s s) produced by the compression conse- 

 quent upon the change of angle on coming out on the plain. The 

 relation of crevasses and 

 vein-structure is still bet- 

 ter shown in the ideal sec- 

 tion (Fig. 61). 



Again, as marginal fis- 

 sures are produced by the 

 pulling of the central por- 

 tions upon the lagging 

 margins behind, so the 

 marginal veins are produced by the crowding or pushing of the swifter 

 central parts on the slower marginal parts in front (Fig. 62). The 

 marginal veins are, therefore, inclined to the margin about 45°, but 



Fig. 61. 



