of the Ice of Glaciers. 247 



time to time made in it as it passes through that point, and if 

 we conceive this cut to be constantly tilled up with water which 

 freezes to a plate of transparent blue ice iDserted into the white 

 opaque ice of the glacier, and, entering into the economy of its 

 motion, is slowly drawn out and attenuated by the extension of 

 the transverse plate into a conoidal surface of greater area from 

 year to year, the extent of which attenuation may be estimated 

 by a comparison of the lengths of the successive curved lines 1 1, 

 2 2, 3 3, 4 4 } &c. in the diagram (fig. 2) with the straight line A B 

 forming the base of the diagram, then we shall, I think, have 

 conceived the formation of a structure identical with the veined 

 structure. I do not think this filling of a cut with water the means 

 by which a plate of blue ice is actually inserted transversely into 

 the mass of the glacier. It may be effected by an extraordinary 

 thrust to which the ice is somewhere subjected, by which the white 

 opaque ice is melted across a transverse section, and then, when re- 

 lieved of the thrust, frozen again into transparent ice, on Sir W. 

 Thomson's principle ; or we may conceive a transverse fissure to 

 be filled with ice-debris or snow, and this ice-debris or snow so in- 

 troduced into the substance of the white opaque ice, by dint of the 

 tearing asunder, thinning out, extension, and kneading which it 

 thus undergoes, to be converted into transparent ice*, whilst the 

 mass in which it is imbedded retains its character of white ice. 



Professor Tyndall has shown that a process not unlike this 

 takes place at an ice-fall. The ice pushed over the edge of the 

 fall opens, as the leaves of a thick book open when it is placed 

 on its back. These openings of the ice get filled with ice-debris, 

 snow, and sometimes mingled dirt, as wool or any other compres- 

 sible substance might be inserted between the leaves of the book. 

 As the ice reaches the bottom of the fall, the force, whatever it is, 

 which presses it forwards closes these openings, shuts up the 

 book, and the inserted ice and snow, compressed into the sub- 

 stance of the glacier, submits to its law of the differential motion, 

 and thins itself out and stretches and prolongs itself into the 

 conoidal laminse of the veined structure. 



Such an ice-fall is represented in fig. 5, which by the permis- 

 sion of Professor Tyndall I have copied from his paper " On the 

 Veined Structure," in part i. of the Philosophical Transactions 

 for 1859, p. 286. 



Professor Tyndall thus describes it, as seen by him on the ice- 

 fall of one of the branches of the lower glacier of Grindelwald : — 



" The glacier, descending from its neve, reaches the summit of 



* Principal Forbes speaks (Phil. Mag. March 1859) of the "glassy struc- 

 ture of ice as attainable by the cohesion under pressure (especially if aided 

 by motion with friction, or kneading) of the semiopaque and porous mate- 

 rial of the glacier." 



