1845.] MODELS ILLUSTRATING THE VEINED STRUCTURE. 79 



front. Plate V.* shows a more accurate drawing taken from 

 such a model. 



The lines of sliding separation occur most distinctly marked 

 near the sides, where the friction is greatest, and the central 

 parts are forced past the lateral parts, on account of the less 

 embarrassed and consequently swifter motion of the centre ; 

 and they incline to the centre although the breadth of the 

 channel be perfectly uniform. But the forces which tear asunder 

 the parts (when such exist) act perpendicularly to the former, 



Fig. 19. 



and produce dislocations and fissures, which perfectly correspond 

 to the direction and appearance of the crevasses of a glacier, 

 that is, they are convex upwards or towards the origin of the 

 glacier. It is the former of these lines of separation, or diffe- 

 rential motion, which constitute and trace out with an exact 

 parallelism the veined structure which I have described as form- 

 ing the normal structure of all true glaciers. Plate VI.* is a 

 representation of a very beautiful plaster model of more con- 

 sistence than the other, in which the swelling of the surface and 

 the direction of the open cracks produced by direct thrusts are 



* [Plates V. and VI. of the Philosophical Transactions for 1846, which exhibit 

 more elaborately the markings shown in the diagram of Plate I., fig. 2, of this 

 volume, are not reproduced on account of their elaborate nature, but fig. 19 of this 

 page gives a somewhat rude idea of one of them. J 



