218 Mr. R. Mallet on the Origin and Mechanism of 



p. 406) says, "also in those curved columns of basalt where .... 

 no joints are apparent we may suppose that some tendency of 

 an original set of spheroids to develop themselves more in one 

 direction than another, from some local cause, has been so con- 

 tinued as to produce the general curve observed ;" while Scrope 

 says (' Volcanos/ 2nd. edit. pp.* 99-100), " sometimes the co- 

 lumns have taken a gradual and graceful curve, probably in 

 consequence of some slow movement impressed upon the lava 

 matter during the process of consolidation." The last merely 

 repeats the notion expressed by Bakewell (Introduction to Geo- 

 logy, 1st edit. 1813, p. 112), who says, " In some situations 

 basaltic columns are bent, apparently by the force of incumbent 

 pressure when in a soft and yielding state." 



The first of these notions is devoid of any value^ resting, as it 

 does, upon the false hypothesis originated by Gregory Watt, 

 that prismatic basalt has been produced by the squeezing toge- 

 ther of spheroidal lumps ; while the two latter are untenable on 

 the ground already proved, that the splitting into prisms can- 

 not commence until after the mass has beeu cooled enough to 

 become rigid. Curved prisms are frequently jointed ; and had 

 they been formed originally straight, and subsequently bent by 

 pressure, however slowly applied, after the mass had become 

 rigid, it is scarcely conceivable that transverse fractures of the 

 prisms should not be found, and that the transverse joints, if 

 produced in the way explained in the preceding pages, should 

 not have been opened upon the convex side of the curve ; indeed 

 the most natural supposition would be that the bending-force 

 would dislocate the straight and rigid prisms at the joints and 

 form bundles of straight prisms meeting each other at very ob- 

 tuse angles. It is not to be denied, however, that severe pres- 

 sure has in some instances been transmitted through masses of 

 prismatic basalt sufficient to generate a distinct slaty cleavage 

 in the prisms after their production from previously homoge- 

 neous material. Thus, in the neighbourhood of Mont d'Or les 

 Bains is a huge isolated mass of prismatic basalt, all the sepa- 

 rate joints of which have a slaty cleavage, so that they may be 

 split into a coarse sort of roofing-slabs, whence the mass 

 takes its local name of " ]a tuillerie." The pressure has varied 

 in direction in different parts of the mass, the prisms at one side 

 of which possess a cleavage approaching to a plane transverse to 

 their axes, while at the remote side of the mass the plane of 

 cleavage is inclined at a small angle to the axes. But there are 

 no evidences that the severe pressure sufficient to produce this 

 slaty cleavage has bent or distorted the prisms themselves — thus 

 presenting an additional proof that the basaltic mass was already 

 rigid when the splitting into prisms took place, and that the 



