618 



STEUCTURE OF VOLCANIC ROCKS. 



[Oh. XXIX. 



Fig. GS9. 



pies the bottom of a narrow valley, except at those points where the 

 river Volant, or the torrents which join it, have cut away portions of 

 the solid lava. The foregoing sketch (fig. 688) represents the rem- 

 nant of the lava at one of the points where a lateral torrent joins the 

 main valley of the Yolant. It is clear that the lava once filled the 

 whole valley up to the dotted lino 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, in 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 columns, which are 

 vertical on the banks of the Yolant, where they rest on a horizontal 

 base of gneiss, but which are inclined at an angle of 45° at </, and 

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

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

 original valley. 



In the annexed figure (689) 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.* Unlike those 

 of the Yivarais, last mentioned, the ba- 

 salt of this country was evidently sub- 

 marine, and the present valleys have 

 since been hollowed out by denudation. 

 The columnar structure is by no means 

 peculiar to the trap rocks in which augite 

 abounds ; it is also observed in clink- 

 stone, trachyte, and other felspathic 

 rocks of the igneous class, although 

 in. these it is rarely exhibited in such 

 regular polygonal forms. 

 It has been already stated that basaltic columns are often divided 

 by cross joints. Sometimes each segment, instead of an angular, 

 assumes a spheroidal form, so that a pillar is made up of a pile of 

 balls, usually flattened, as in the Cheese-grotto at Bertrich-Baden, in 

 the Eifel, near the Moselle (fig. 690). The basalt there is part of a 

 _ ill stream of lava, from 30 to 40 feet thick, which has proceeded 

 from one of several volcanic craters, still extant, on the neighboring 

 heights. The position of the lava bordering the river in this valley 

 might be represented by a section' like that already given at fig. 635, 

 if we merely supposed inclined strata of slate and the argillaceous- 

 sandstone called graywacke to be substituted for gneiss. 



In some masses of decomposing greenstone, basalt, and other trap 



Columnar basalt in the Vicentin. 

 (Fortis.) 



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



