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II. VOLCANIC ROCKS , GASES, AND LAVA-STREAMS.
1. Volcanic Rocks. — Lava is a general term for anyrock of a lava-stream. It may be very compact. — that is,without a cellule ; or it may contain air-cavities or vesiclesthat were made by expanding vapor while the lava was stillliquid, and so be a vesicular lava. But vesicular lava may'be very openly vesicular, like a furnace slag, and then itis scoriciceous lava ; or it may be made chiefly of vesicles,and then it is a scoria , the scum or froth of the liquidlava.
Lava , on cooling rapidly, sometimes becomes glass, or hasa thin glassy crust; but on cooling slowly' the same materialmay become stone, by the conversion of glass into stony par-ticles, which are the constituent minerals of the lava. Thelighter scoria of a volcano is mostly glass. Generally, butnot always, minute portions of the glass remain in the mostsolid lavas. By extreme slowness of cooling, such as mayoccur in Nature in the interior of great igneous masses orunder a deep covering of rocks, the texture of lava may be-come coarsely crystalline, like granite or syenyde ; so that,although granite has little resemblance to ordinary lava, itmay be as truly an igneous rock as the lavas of Kilauea orVesuvius , and, as Judd, Hague, Iddings, and others haveshown, be part of the same eruption with scoriaceous lavas. 1
The more common kinds of lava consist as follows : —
I. Of feldspar, as the chief ingredient.
II. Of a feldspar, with one of the two iron-bearing min-erals,-augite, hornblende.
I. Lavas consisting chiefly of a Feldspar. — Theselavas are generally light in color and light in weight. Thespecific gravity is 2.5 to 2.8. The feldspar is ordinarilyorthoclase or potash-feldspar.
1 See page 4.