112 



LYELL'S ELEMENTS OF GEOLOGY. 



Columnar Structure of Volcanic Rocks. 



Fig. 100. 



Lava of La Coupe d'rfyzac, near dntraigue, in the Province of Jirdechc. 



the main valley of the Volant. It is clear that the lava once 

 filled the whole valley up to the dotted line da,' but the river 

 has gradually swept away all below that line, while the tributary 

 torrent has laid open a transverse section ; by which we per- 

 ceive, in the first place, that the lava is composed, as usual in 

 this country, of three parts ; the uppermost at a, being scoriace- 

 ous ; the second, 6, presenting irregular prisms ; and the third, 

 c, with regular columns, 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 then horizontal at f, 

 their position having been every where determined, according to 

 the law before mentioned, by the concave form of the original 

 valley. 



In the annexed figure (101.) a 

 view is given of some of the in- 

 clined 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.* Unlike those 

 of the Vivarais, last mentioned, the 

 basalt of this country was evidently 

 submarine, and the present val- 

 leys have since been hollowed out 

 by denudation. 



The columnar structure is by no 

 means peculiar to the trap rocks in 

 which hornblende or augite predo- 

 minate ; it is also observed in clink- 

 stone, trachyte, and other felspathic rocks of the igneous class, 



Fig. 101. 



Columnar basalt in the Vicentin. 

 (Fortis.) 



* Fortia, Mem. sur 1'Hist Nat. de 1'Italie, torn. i. p. 233. plate 7. 



