148
T. G. eonney on columnab, fissile,
intervening between these and the curved boundary of the ball, which, as a rule, nowhere touches them.
These curves generally point in one direction, sometimes upward, sometimes downward ; occasionally, however, cases may be noticed where they are differently directed in adjoining columns*. So far as I can remember, they are most frequent where the rock exhibits the spheroidal structure which I am about to describe ; but before doing this I should wish to call attention to a curious form of cross- jointing which 1 observed in some of the basalt columns at the upper part of the hill of Bonnevie, Murat (fig. 8).
Here, in the upper part of the excavation opened for quarrying, a curious interruption to the regularity of the columns may be observed in the form of a sort of projection, like a part of a capital from which springs the base (curving till the old direction is
Fig. 8. — Crots joints at Bonnevie.
Pig. 0. — /Structure of Columns in the Lava at lioyat.
A. Plane of parallel joints.
resumed) of another column. A closer examination shows that this structure is connected with a series of cross joints, which cut the columns (here at angles of about 60°), and to whose plane the axes of the imperfect prisms forming the brackets are parallel. These joints are from 4-6' apart; and the ordinary cross-jointing in the other parts of the hill seems rather regular.
A singular case of columnar and joint-structure occurs in the valley of the Tirtaine, near the grotto of the springs between Old and New lloyat (Auvergne). Here the lava-stream, which is generally irregularly jointed, and sometimes (according to Mr. Scrope) spheroidal, is rudely columnar at its base (fig. 9). The columns are irregularly hexagonal, and about 18" in diameter. At first sight they seem cross-jointed at intervals of about 12" or 15";
- As is the case (pointed out, by Mr. Scrope, Geol. Mag. Dec. 11, vol. ii. p. 412)
in the columns in possession of our Society.
