1857.] Observations on Glaciers. 323 



The veined or laminar structure of glacier ice was next consi- 

 dered, which Professor J. D. Forbes in his earlier writings compared 

 to slaty cleavage. His theory of the structure is, perhaps, the only 

 one which has made any profound impression, and it may be briefly 

 stated as follows : — Owing to the quicker flow of the centre of a 

 glacier, a sliding of the particles of ice past each other takes place ; 

 in consequence of this sliding, fissures are produced, which, when 

 filled with water, and frozen in winter, produce the blue veins of 

 the glacier. To account for the obliquity of the veins to the sides 

 of the glacier, a drag towards the centre is supposed to take place, 

 producing a differential motion in this direction, which results in 

 the formation of fissures. But at the centre of the glacier this drag 

 towards it cannot be supposed to exist ; and to account for the 

 veins, or laminated structure of the centre, which under normal 

 conditions is transverse to the axis of the glacier, it is supposed 

 that the thrust from behind, meeting an enormous resistance in 

 front produces a differential motion of the particles in a direction 

 approximating to the vertical; and that in consequence of this 

 motion fissures are produced, which, when filled and frozen, produce, 

 as in the other cases, the blue veins. Now, the only fact here is 

 that of differential motion parallel to the length of the glacier. It is 

 not established that the colds of winter reach to a depth suflScient to 

 produce the blue veins, which it is affirmed form a part of " the in- 

 most structure " of a glacier. Again, the lamination in some cases 

 presents itself in the form of transparent lenticular masses imbedded 

 in the general white ice ; and the differential motion referred to 

 would be mechanically inadequate to produce detached cavities 

 corresponding to these masses, which vary greatly in size, and in 

 some cases accurately resemble the greenish spots in slate rock, 

 when a section perpendicular to the cleavage of the rock is exposed. 

 Further, as the motion of the glacier takes place both in summer 

 and winter, it is to be inferred that the fissures are formed at both 

 seasons of the year. If formed in winter, they cannot be filled 

 and frozen that season for want of water ; and if formed in summer 

 they cannot, while summer continues, be frozen, for want of cold. 

 Hence, at the end of ?ach summer, if the above theory be correct, 



by their expansion rupture the ice, and produce the crepitation referred to. 

 The observation is an interesting one, whatever difficulty we may find in 

 accepting the explanation. An experiment made on the 31st of January, 

 appears to me to account for the observation in a satisfactory manner. Snow 

 having fallen, I was early at work compressing it ; and on removing a plate of 

 the compacted mass from the press, I noticed, as the ice melted, a sparkling 

 motion of the surface. To imitate the action of the sun, an iron spatula was 

 heated, and on bringing it near to the compressed snow, the jumping of the sur- 

 face caused by the issue of the air through the film of water which covered it, 

 was greatly augmented. On removing the spatula, the motion subsided. To a 

 similar action on the part of the sun, which melts the surface of the glacier, 

 and thus liberates the air bubbles with which it is filled, the observation of M. 

 Agassiz is in all probability to be referred. 



