AND SPHEROIDAL STRUCTURE. 



151 



lenticular space. In this, two internal spheroids were exposed by 

 the exfoliation of the face, which were not connected with any cross 

 joints. In the adjoining mass four large, rude and very irregular 

 spheroids were shown (the bounding surfaces being incomplete or 

 with more than one centre). Similar instances of spheroids discon- 

 nected from the main joints may be seen in a quarry called Turner's 

 Pit in the Rowley-Eegis basalt, where also, in some cases, the nuclei 

 of the spheroids are further subdivided, so that imperfect spheroids 

 are enclosed by a spheroidal shell, like the twin kernels of a nut (fig. 

 13). It is therefore clear that, though the spheroids often do corre- 



Fig. 12. — Spheroids in an 2in- 

 jointed column near Le Puy. 



Fig. 13. — Complicated spheroidal, 

 structure {Rowley-Regis bascdt). 



( }— f , , -1 



spond with the spaces between cross joints in the column, there is no 

 necessary connexion ; both maybe due to a somewhat similar cause; 

 but the spheroid can exist without the cross joint, and vice versa. 



Is it, then, possible to find any one cause which will explain these 

 various divisional structures from the fissile to the spheroidal ? I 

 think that which has already been suggested (I may say proved) 

 for some of them, viz. the contraction of a cooling mass, is capable 

 of explaining all. Mr. Mallet has shown that if a mass of molten 

 rock be cooling uniformly from a surface, it will, in consequence of 

 the mathematical principle of least action, break into hexagonal 

 prisms at right angles to the surface of cooling. By the same 

 principle, if a cube were contracting in consequence of a uniform 

 loss of heat from each of its sides, it would be more likely to rupture 

 internally (supposing that the solidification of the exterior prevented 

 diminution of volume) in spheroidal shells ; for the more rapid loss 

 of heat from the angles would tend to bring the isothermal surfaces 

 within into a rudely spherical form ; and then, when the strain 



