212 - GEOGNOSY AND GEOLOGY. 
sometimes giving rise to lava arches such as are met with in Iceland (pi. 
52, fig. 3). When the lava currents pour into lakes, they sometimes dry these 
up; an example of this is furnished by the voleano Krabla in Iceland, 
whose inner crater is represented in pl. 50, fig. 6. It first attracted atten- 
tion by its mighty eruption, May 17,1724. Among the phenomena of this 
eruption, which continued for six years, was the advance of a single lava 
stream to Lake My-vatn, a distance of nearly six miles, and drying it up 
almost entirely. 
The cooled lava has generally a very rough surface, caused by superficial 
bubbles and scoria. The interior is either amorphous, or separated in tabular, 
columnar, or spheroidal form. The spheroidal state is frequently exhibited 
by obsidian, which occurs in great extent in Iceland. The bubbles or vesicu- 
lar cavities which are seen in the outer portions of the current, and which are 
not rarely heaped up so as to form large masses of pumice, are ellipsoids of 
greater or less perfection, and with the longer axis parallel to the direction of 
the stream. 
Lava presents itself in general under three forms; as glassy, as stony, and 
as crystalline. The glassy has arisen from a very rapid cooling, and is 
furthest removed from the crystalline. It has a highly conchoidal and sharp- 
edged fracture, is brittle, more or less transparent, and of a vitreous lustre. 
Stony lava has a strong resemblance to stone ware; it is of earthy dull frac- 
ture, generally entirely opake, and intermediate between glassy and crystalline 
lava. 
Lava streams may be referred to three classes: trachytic, basaltic, and 
leucitophyric. ‘Trachyte lavas are characterized by feldspar or its minerals, 
which, however, is only recognisable when the aggregation state of the rock 
is decidedly crystalline. Crystalline trachytic lava forms a true trachyte, 
which generally is of a light color, not seldom modified by the presence of 
hornblende, specular iron, micaceous iron, and true brown mica. It 
frequently passes into porphyritic, so as sometimes to constitute a trachyte 
porphyry. The stony trachytic lava is often granulated, and contains 
vesicles and particles of glassy feldspar. The glassy is formed by pumices 
and obsidian ; it is either pure, or contains porphyritically separated particles 
of feldspar. 
Augite predominates in basaltic lava, accompanied by labradorite and 
various ferruginous minerals, for which reason these rocks are generally of 
a dark color. They are very similar to the true volcanic basalt, often so 
much so as to present no difference in petrographical character, this 
difference being only deducible from oreographical peculiarities. A true 
obsidian and pumice do not occur among the basaltic lavas, and a few 
glassy varieties of some similarity to these, are distinguishable by the simple 
blowpipe test that the former yield white, the latter dark globules on fusion. 
The crystalline granular basaltic lava exhibits a true dolerite, and for this 
reason is sometimes called dolerite lava: it characterizes Etna in particular. 
The stony lava is of a greyish black and brown color, with a compact 
vesicular, or slaggy interior. Here belong the Muhlstein of Niedermennig, 
not far from the Laacher Lake, as well as some basaltic lavas of Auvergne. 
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