Ch. XXIX.] 



STRUCTURE OF VOLCANIC ROCKS. 



485 



The columnar structure is by no means 

 peculiar to the trap rocks in which augite 

 abounds ; it is also observed in clinkstone, 

 trachite, and other felspathic rocks of the 

 igneous class, although in these it is rarely 

 exhibited in such regular polygonal forms. 



It has been already stated that basaltic 

 columns are often divided by cross joints. 

 Sometimes each segment, instead of an 

 angular, assumes a spheroidal form, so that 

 a pillar is made up of a pile of balls, usually 

 flattened, as in the Cheese-grotto at Beit- 

 rich-Baden, in the Eifel, near the Moselle 

 (fig. 637). The basalt there is part of a 

 small stream of lava, from 30 to 40 feet thick, which has proceeded from 

 one of several volcanic craters, still extant, on the neighboring heights. 



Columnar basalt in tho Viccntin. 

 (F-M-tis.) 



Fi?. 637. 



Basaltic pillars of the Kasegrotte, Bertrich-Baden, halfway between Treves and Coblentz. 

 Height of grotto, from 7 to 8 feet. 



The position of the lava bordering the river in this valley might be repre- 

 sented by a section like that already given at fig. 635, if we merely sup- 

 posed inclined strata of slate and the argillaceous sandstone called grey- 

 wacke to be substituted for gneiss. 



In some masses of decomposing greenstone, basalt, and other trap rocks, 

 the globular structure is so conspicuous that the rock has the appearance 

 of a heap of large cannon balls. According to the theory of M. Delesse, 

 the centre of each spheroid has been a centre of crystallization, around 

 which the different minerals of the rock arranged themselves symmetri- 

 cally during the process of cooling. But it was also, he says, a centre of 

 contraction, produced by the same cooling. The globular form, therefore, 

 of such spheroids is the combined result of crystallization and contraction.* 



* Delesse, sur les Roches Globuleuses, Mem. de la Soc. Geol, de France, 2 ser, 

 torn. iv. 



