116 Prnrerdings of the Royal Irish Acwlemti. 



only weather back like the general surface of the igneous sheet. Some of the 

 spheroids, however, show a distinct tendency to flaking and onion-structure, 

 even under their lichen-covered surface. When cut across by the rock- 

 slicing machine, a zone of soft decomposed rock a millimetre or so thick is 

 seen, and shows that alteration is still operating from the surface. 



The fact that original joints determine such onion-like weathering is 

 clearly seen near Carrick-a-rede, in the county of Antrim, where the fresh 

 basalt can be broken up into balls of various sizes, some of which are only 

 two centimetres or so in diameter. The structure, in fact, is a coarse repre- 

 sentative of the delicate perlitic structure of glassy igneous rocks. Even in 

 glassy rocks, asG. P. Scrope showed long ago in the Ponza Islands,* globular 

 structure may appear on a coarse scale. Scrope describes the pitchstone of 

 the Chiaja di Luna as having " a tendency to the columnar division, the 

 columns separating into large globes or ellipsoids, placed one above the other. 

 These balls, when they have been exposed a short time to the weather, 

 desquamate at a touch into numerous concentric coats, like those of a bulbous 

 root, inclosing a compact nucleus, of wliich tlie lamime have not been 

 sufficiently loosened by decomposition ; though the application of a ruder 

 blow will produce a still further exfoliation. The globes vary from a few 

 inches to three feet in diameter. . . . These varieties of natural division are 

 certainly not produced by decomposition, which has evidently only assisted 

 in disclosing an original configuration." 



Robert Mallet' jwinted out a similar tendency to di\ide into globes in a 

 " trap-rock " near Galway. Here no decomposition had reached the mass, 

 and the structure was unsuspected until the rock was blasted by gunpowder 

 during quanying operations. 



Weathering action from the e.xposed surface, aided by the battering of 

 rain, has doubtless produced the hollows that surround the spheroids of the 

 Picture-Rock. Had the destructive agent been subterranean, as in the case 

 of the rotting of rock-masses to produce kaolin, the coies of the spheroids 

 would have become entirely detached. 



The walls that stand up round each compartment of the rock are evidently 

 due to some strengthening of the groundmass by material intillered from llic 

 joint-surfaces. The resisting ridges are sometimes worn away to a knife- 

 edge, but arc sometimes 3 cm. or more in width. Each is marked near the 

 centre by a plane of weakness, along which it divides when struck with the 



'"Notice on the Geology o( the Poiii« I«le»." Trwu. Geol. Sue, London, st^r. 2, vol. ii. 

 (1827). p. 205. 



= " On an unobarrv.tl »:ructiitr in Trmp Bocka of coiint> Gal»«y." 'Jnins. K. lii.-li .\cs<l., 

 Tol. xviii. (1837). p. 7J : and I'roc. R. Iiisb .*..-..d., toI. i. il836-40), p. 56. 



