144 



T. G. BONNET ON COLUMNAR. FISSILE, 



to the annexed diagram (fig. 2), together with the following extract 

 from my note-book may make this plain : — 



" The columns are frequently rather rude and quadrangular 

 in shape ; in part, however, they are tolerably regular and hex- 

 agonal. They are bent in a most singular manner, so that those 

 toward the outside curve round and roughly overarch an interior nearly 

 vertical sheaf or boss. These columns are cut by numerous cross joints 

 which run parallel to great curving divisional planes, and traverse 

 their axes at a high angle. Thus the columns appear to be built 

 up of slabs. The cross joints are commonly not at right angles to 

 the axis, the inclination sometimes not exceeding 50°. In parts 

 this platy structure becomes so conspicuous as almost to obliterate 

 the prismatic ; and a very singular effect is produced by this alterna- 

 tion of bands of ' frilled ' and ' crimped ' rock. The whole mass, 

 whether distinctly columnar or not, has more or less of a fissile 

 structure, which is of course most distinct where the columns are 

 least so, and is (so far as I could see, and as one would expect) 

 parallel to the great curving divisional planes. This description 

 applies especially to the south-western part of the hill ; when a 

 view is obtained of the whole of the western face, as from the slopes 

 on the opposite sides of the valley, the same complicated double 

 structure will be seen continued throughout." 



Turning now to the lloche Tuilliere, we find that its eastern face 

 is divided into a number of large columns ; these are very distinctly 

 marked, and are cut by a great number of cross joints, which are 

 not at right angles to the axis of the columns, but dip outwards. 

 These joints also, when viewed from a distance, are seen not to 

 traverse the face of the cliff horizontally, but to arch upward from 

 its two extremities so as to become horizontal in the centre, 



Fig. 3. — Diagram of Columns and Cross 

 joints {lloche Tuilhere). 



Fig. 4. — Fissile structure. 

 (Roche Tuillihre). 



^ 



A B. Vertical joints. C C. Direction of cross 

 joints. D. Direction of southern end of cliff. 



following very nearly the curve of its sky-line, which has no doubt 

 been determined by the direction of these divisional planes. On 

 climbing the steep wooded talus to the base of the columns their 

 structure becomes more evident. In shape they are rather less 



