268 Mr. J. Ball on the Veined Structure of Glaciers. 



some force must have been at work in the space between E and 

 F to effect this change in the direction of the planes of the 

 veined structure. Nothing of the kind, however, need take 

 place. The compression at E, and the resulting transverse 

 structure, are merely superficial as compared to the entire thick- 

 ness of the glacier. In the neutral zone, which has undergone 

 neither compression nor tension during the passage of the ice- 

 catai-act, the structure originally developed at A may well sur- 

 vive, and when superficial waste has cleared away the portion 

 affected by transverse compression, the old longitudinal structure 

 comes again to the surface. 



Why we never see indications of a cross-bar structure in cases 

 where pressure is applied in a new direction to ice already pos- 

 sessing the veined structure, is a question that seems to me to 

 deserve further examination. Experiments such as I have already 

 suggested, which should include this inquiry, might probably 

 give interesting results. 



Before closing these remarks, I beg to call the attention of 

 observers to a point of glacier structure which has obtained but 

 little notice. At the lower extremity of glaciers, where a stream 

 issues from a cavern in the ice, there is almost always apparent 

 a tendency to split in curved surfaces, which, in part at least, 

 cut perpendicularly the planes of the veined structure. It is to 

 this tendency that the caverns whence the glacier torrents issue 

 owe their arched form. It is clear that this is no merely acci- 

 dental hollowing out of the cavity above the stream ; for above 

 the cavern successive fissures may be seen, sometimes but an 

 inch or two in width, all strictly parallel to the curve of the 

 interior arch. Does this indicate a structure extending through- 

 out the entire of the glacier ? I have sometimes thought so, 

 and imagined it to be analogous to the jointing of crystalline 

 rocks ; but I do not venture to speak with confidence on the 

 subject. I believe partial subsidences near to the banks, and 

 even in the central region of great glaciers, to be more common 

 than is usually supposed, and to be accompanied by fissures 

 which tend to cut at right angles through the surfaces of the 

 veined structure. 



In conclusion, I beg to express the conviction that, with the 

 key to a true explanation of the most important phajnomena of 

 glacier structure, for which we are indebted to Professor Tyndall, 

 a systematic inquiry into the application of his theoretical views 

 to all the facts presented in the ice-world, and into those residual 

 phenomena to which he has not turned his attention, would 

 amply reward the labours of a competent observer, and is re- 

 quired, in spite of all that has been hitherto done, to complete 

 our knowledge of the glaciers. 



