. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 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
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 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
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Keywords: ., bookcentury1800, bookdecade1860, booksubjectgeology, bookyear1868
