Page:Quarterly Journal of the Geological Society of London, vol. 32.djvu/216

144 144

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