484 



STRUCTUKE OF VOLCANIC ROCKS. 



[Ch. XXIX. 



the sea. In fig. 634, a small portion of this dike is represented 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 sur- 

 faces. If these surfaces, therefore, instead of being either perpendicular 

 or horizontal, are curved, 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 Vivarais, a mountainous district 

 in the South of France, where, in the midst of a region of gneiss, a 

 geologist encounters unexpectedly several volcanic cones of loose sand 

 and scoriae. From the crater of one of these cones, called La Coupe 

 d'Ayzac, a stream of lava descends and occupies the bottom of a nar- 

 row valley, except at those points where the river Volant, or the torrents 

 which join it, have cut away portions of the solid lava. The accom- 

 panying sketch (fig. 635) represents the remnant of the lava at one of 



Fig. 635. 



Lava of La Coupe d'Ayzac, new Autraigue, iu the province of Ardeche. 



the points where a lateral torrent joins the main valley of the Volant. 

 It is clear that the lava once filled the whole valley up to the dotted 

 line d a ; but the river has gradually swept away all below that line, 

 while the tributary torrent has laid open a transverse section ; by which 

 we perceive, iu the first place, that the lava is composed, as usual in this 

 country, of three parts : the uppermost, at a, being scoriaceous ; the 

 second, b, presenting irregular prisms ; and the third, c, with regular col- 

 umns, which are vertical on the banks of the Volant, where they rest on a 

 horizontal base of gneiss, but which are inclined at an angle of 45° at g, 

 and are horizontal at/, their position having been everywhere determined, 

 according to the law before mentioned, by the concave form of the origi- 

 nal valley. 



In the annexed figure (636) a view is given of some of the inclined and 

 curved columns which present themselves on the sides of the valleys in 

 the hilly region north of Vicenza, in Italy, and at the foot of the higher 

 Alps.f Unlike those of the Vivarais, last mentioned, the basalt of this 

 country was evidently submarine, and the present valleys have since been 

 hollowed out bv denudation. 



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



f Fortis. Mem. sur l'Hist. Nat. de l'ltalie, torn. i. p. 233, plate 1. 



