850 T HE WONDERS OF GEOLOGY. Lect. VIII. 



of the tertiary epoch have already been noticed (ante, 

 p. 271). This columnar structure has been proved by some 

 highly interesting experiments, to have originated in the 

 manner in which refrigeration took place. Mr. Gregory 

 Watt * melted seven hundred weight of basalt from Rowley 

 Regis (ante, p. 774), and kept it in the furnace several 

 days after the fire was reduced. It fused into a dark- 

 coloured vitreous mass, with less heat than was necessary to 

 melt pig-iron ; as the mass cooled, it changed into a stony 

 substance, and globules appeared ; these enlarged till they 



2 

 Lign. 190. — Basaltic Columns, from the Giants' Causeway. 



Fig. 1 . A block partially decomposed, exhibiting the primitive spheroidal figure of 

 the prism. 2. Portions of columns, consisting of several joints. 3. The 

 concave surface of a joint. 



pressed laterally against each other, and became converted 

 into polygonal prisms. 



The articulated structure and regular forms of basaltic 

 columns have, unquestionably, resulted from the crystalline 

 arrangement of the particles in cooling ; and the concavities, 

 or sockets, have been formed by one set of prisms pressing 

 upon others, and occasioning the upper spheres to sink into 

 those beneath; thus the different layers of spheres have 

 * Philosophical Transactions for 1804. 



