20 THIRD LETTER ON GLACIERS. [1842. 



unfrequently parallel and uniform like a ribboned calcedony or 

 jasper. 



I will, for brevity, merely state the modifications which this 

 fundamental type undergoes, bringing together glaciers of all 

 classes, but reserving the detail of examples and proofs, of which 

 my experience has already furnished me with a great number, 

 to another occasion. If a glacier be long and narrow, as the 

 Lower Aar, or the Mer de Glace of Chamouni, the frontal dip 

 is the least conspicuous part of the phenomenon ; and if it ter- 

 minate in an icy cascade, as in the second case, it might escape 

 observation altogether. The vertical planes parallel to the 

 length, or nearly so, usurp nearly all the breadth of the glacier, 

 and^ only in the centre is a narrow space, where not unfrequently 

 the structure appears quite undefined. I have satisfactorily 

 made out, however, in every glacier which I have had the 

 means of examining^with that view, that the conoidal structure, 

 however obscured, exists in all parts of the true glacier, modified, 

 according to its length and breadth, in the manner which figs. 

 3 and 4 indicate. I need not add, that these rude sketches are 



Fig. 3. Fig. 4. 



ife 



not intended to be considered as rigorously exact, but only to 

 explain generally my meaning. 



There is yet another modification, but only a modification, 

 of the above, namely, in the case of extremely steep glaciers, but 

 which are coherent, and not crevassed into pyramids. There 

 are numberless examples of these in all the higher valleys of 

 the Alps, which do not descend into the hollows, but festoon 



