PART I. CHAPTER VIII. 



113 



Columnar and Globular Structure. 



although in these it is rarely exhibited in such regular polygonal 

 forms. 



It has been already stated that basaltic columns are often di- 

 vided 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 Ber- 

 trich-Baden, in the Eifel, near the Moselle. (Fig. 102.) The 



Fig. 102. 



Basaltic pillars of the Kiisegrotte, Bertrich- Baden, halfway between Trevet and 

 Coblentz. Height of grotto from 7 to 8 feet. 



basalt, there, is part of a small stream of lava, from 30 to 40 

 feet thick, which has proceeded from one of several volcanic cra- 

 ters, still extant, on the neighbouring heights. The position of 

 the lava bordering the river in this valley, might be represented 

 by a section like that already given (Fig. 100. p. 112.), if we 

 merely suppose inclined strata of slate and the argillaceous sand- 

 stone called greywacke 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. 



A striking example of this structure occurs in a resinous tra- 

 chyte or pitchstone-porphyry in one of the Ponza islands, which 

 rise from the Mediterranean, off the coast of Terracina and 

 Gaieta. The globes vary from a few inches to three feet in di- 

 ameter, and are of an ellipsoidal form. (See Fig. 103.) The 

 whole rock is in a state of decomposition, " and when the balls," 

 says Mr. Scrope, " have been exposed a short time to the wea- 

 ther, they scale off at a touch into numerous concentric coats, 

 like those of a bulbous root, inclosing a compact nucleus. The 



