COLUMNAR AND GLOBULAR STRUCTURE. 
511 
ainous district in the South of France, where, in the midst of 
a region of gneiss, a geologist encounters unexpectedly sev¬ 
eral volcanic cones of loose sand and scoriae. From the crater 
of one of these cones, called La Coupe d’Ayzac, a stream of 
lava has descended and occupied the bottom of a narrow val¬ 
ley, except at those points where the river Volant, or the tor¬ 
rents which join it, have cut away portions of the solid lava. 
The accompanying sketch (Fig. 588) represents the remnant 
of the lava at one of these points. It is clear that-the lava 
once filled the whole valley up to the dotted line d a; but 
the river has gradually swept aw^ay 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 com¬ 
posed, as usual in this country, of three parts: the uppermost, 
at a, being scoriaceous; the second, 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 and are nearly horizontal at/*, their position hav¬ 
ing been everywhere determined, according to the law before 
mentioned, by the form of the original valley. 
In the annexed figure (589), 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 Vivarais, last mentioned, the 
basalt of this country was evident¬ 
ly 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 augite abounds; it is also 
observed in trachyte, and other 
feldspathic rocks of the igneous 
class, although in these it is rarely Columnar basalt in the Vicentin. 
exhibited in such regular polygo- (Fortis.) 
nal 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. 590). The basalt there is part of a small 
* Fortis, Mem. sur ITIist. Nat. de ITtalie, tom. i., p. 233, plate 7. 
Fig. 589. 
