306 DOMAIN XII. VOLCANIC. 



in this and other instances, the centre alone 

 is in the prismatic forms, which are some- 

 times found enclosed in amorphous lava, 

 identically the same with the columns, 

 sometimes in tufa, and sometimes even in 

 volcanic glass. But he seems never to 

 have seen or observed the remarkable arti- 

 culations, not only convex and concave, 

 but strengthened by projecting angles and 

 recipients, which were first noticed and 

 engraved by Da Costa, and afterwards by 

 Pictet, in their representations of the giants' 

 causey. This striking characteristic, which 

 seems unaccountably to have escaped most 

 writers, can scarcely be ascribed to mere 

 desiccation ; but seems rather to rival the 

 process by which nature produces regular 

 rock crystals, in the vast caverns of the 

 Alps, of enormous size, and weighing many 

 tons. 



coimnm of Other basaltic columns occur in Sicily 

 at Vizzini, where the columns are articu- 

 lated and a foot in diameter, but only a 

 few feet high, curiously arranged on a 

 curved basis ; and they gradually become 



