Ch. XXIX.] 



STRUCTURE OF VOLCANIC ROCKS. 



617 



tical ; in a vertical dike they are horizontal. Among other examples 

 of the last-mentioned phenomenon is the mass of basalt, called the 

 Chimney, in St. Helena (see fig. 686), a pile of hexagonal prisms, 64 



Fiff. GS6. 



Fig. 6S7. 



laiWBiiip 



Small portion of the dike 



Volcanic dike composed of hori- 

 zontal prisms. St. Helena. 



feet hig-h, evidently the remainder of a narrow dike, the walls of rock 

 which the dike originally traversed having been removed down to the 

 level of the sea. In fig. 687, a small portion of this dike is repre- 

 sented on a less reduced scale.* 



It being assumed that columnar trap has consolidated from a fluid 

 state, the prisms are said to be always at right angles to the cooling 

 surfaces. If these surfaces, therefore, instead of being either per- 

 pendicular or horizontal, are cuived, the columns ought to be inclined 

 at every angle to the horizon ; and there is a beautiful exemplification 

 of this phenomenon in one of the valleys of the Yivarais, a moun- 

 tainous district in the south of France, where, in the midst of a 

 region of gneiss, a geologist encounters unexpectedly several volcanic 



Fig. 688. 



Lava of La Coupe d'Ayzac, near Antraignes, in the Department of Ardeche. 



cones of loose sand and scoria3. From the crater of one of these 

 cones, called La Coupe d'Ayzac, a stream of lava descends and occu- 



Seale's Geognosy of St. Helena, plate 9. 



