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. 



