172 Dr. Whewell on Glacier Theories. 



state of tension, produced by this distortion ; but the distortion 

 will be limited, and the downward projection of the curved 

 lines also limited, by the flexure which is possible for the tex- 

 ture. If in any part the texture gives way, we shall have fissures 

 nearly perpendicular to the transverse curves. On this sup- 

 position of a flexible glacier, we have (1) original transverse 

 straight lines drawn into curves o{ limitedjiextire \ (2) the sub- 

 stance in a state of distortion, from which hand specimens would 

 instantly recover; (3) a homogeneous texture of such speci- 

 mens. 



Now let this mass of caoutchouc become viscous or plastic. 

 What will now be the condition of the mass ? The sides and 

 bottom will still be held back by the friction ; the middle and 

 upper part will slide forwards, but not freely. This want of 

 freedom in the motion (arising from the viscosity) will produce 

 a drag towards the middle of the valley, where the motion is 

 freest; hence the direction in which filaments slide past each 

 other will be obliquely directed towards the middle. The 

 sliding will separate the mass according to such lines; and 

 though new attachments will take place, the mass may be 

 expected to retain the results of this separation in the traces 

 of parallel fissures in this direction. If the texture of the 

 substance yield to the tension which tends to separate it 

 directly, we shall have fissures in directions perpendicular to 

 the directions just described, and therefore convex upwards. 

 Original transverse straight lines will become curves, project- 

 ing downwards ; and the projection will increase without limit 

 as the sliding goes on. Hence, on this supposition of a vis- 

 cous or plastic glacier, we should have (1) original transverse 

 straight lines drawn into curves of tudimitcd flexuie; (2) the 

 texture of the substance in a state of equilibriun), so that hand 

 specimens have no distortion to recover from ; (3) a banded tex- 

 ture, the bands being traces of parallel fissures. 



It is well known that the latter set of ph£enomena agree with 

 the actual phaenomena of glaciers, and the former set differ 

 from them. To some persons it may seem harsh to call the 

 mass of a glacier plastic or viscous, when it seems so solid. 

 It may be said that we can hardly call a mass viscous, when 

 we have in it cliffs perhaps a hundred feet high. But let it 

 be considered, as I have said, that the difference of plastic and 

 very solid is only a difference of degree. Soft pitch will stand 

 in cliffs some inches high ; soft clay will stand in cliff's many 

 feet high ; clay may stand in cliff's hundreds of feet high, and 

 yet be plastic, if the mass be very large and the pressures dis- 

 tributed through it powerful enough to make one part move 

 past another part. We cannot doubt that clay might be hard 



