150 



T. G. BONNEY ON COLTJMNAE, FISSILF. 



roidal structure, though on a minute scale ; and here examination 

 of the rocks, both macroscopic and microscopic, proves that the 

 structure is in no way connected with previously formed cubes, or 

 capable of being explained by simple exfoliation during decompo- 

 sition. But the following instances of its occurrence will, I think, 

 show that no theory of mere decomposition is adequate to explain 

 the facts. 



The annexed diagram (fig. 11) is taken from a large spheroid ex- 



Fig. 10. — Spheroidal structure in 

 Agglomerate (Burntisland). 



Fig. 11. — Sjpheroid in Basalt 

 near Murat. 



hibited in a mass of basalt by the roadside on the way from Murat 

 to the bridge over the Alagnon (Auvergue). A glance at it will, 

 I think, show that the curious prolongation of the outer layer is 

 impossible to explain on any principle of weathering. I may add 

 that this occurrence of an isolated spheroid or small group of sphe- 

 roids in a mass which generally does not show the structure is by 

 no means rare. 



But I can produce yet stronger cases. A few kilometres from Le 

 Puy on the lloute de Brioude, close to where a road turns off to 

 Polignac, is a mass of columnar basalt rather decomposed, part of 

 which exhibits veiy well the spheroidal structure. Here spheroids 

 may be seen, one above another, enclosed three or four at a time in 

 a columnar shell without any dividing cross joints, so that they are 

 just like Dutch cheeses packed in hexagonal cases (the interstices 

 being filled up). The lid of the box has more or less fallen away, 

 and exposed the contained spheroids (See fig. 12). 



Again, a mass of trachyte, intrusive in trachytic ash, by the road- 

 side on the flank of the Puy Gros (near the Roche Sanadoire men- 

 tioned above) showed structures bearing on this as well as other 

 points of interest. The surface in contact with underlying tuff 

 exhibited a. platy structure for about three inches; then came a 

 layer rudely spheroidal ; above that were some irregularly vertical 

 joints ; two of these curved in opposite directions so as to enclose a 



