lava

[Ital.=a torrent of rain overflowing the streets; from Ital. lavare; Lat. lavo to wash; Sp. lava; Fr. lave. (Mahn.)] Petrol . & Geol .: A term used vaguely of all molten matter existing within or flowing in streams from volcanic vents, but more specifically confined to the latter, the former being called trap. The lower part of it which has consolidated slowly and under pre ure, tends to be of stony consistency, while the upper is scoriaceous If, however, the lava flow came first and became scoriaceous, it remains so, even if stony material be piled above it. The more compact lavas are often porphyritic, and contain crystals. So do the scoriaceous kinds, the crystals having been derived from older rocks and remained infusible when the original matrix in which they had been imbedded was melted. Lavas vary greatly in composition. Some are trachytic, as in the Peak of Teneriffe; many are basaltic, as in Vesuvius and Auvergne; others andesitic, as in the volcanoes of Chili. Some of the most modern in Vesuvius consist of green augite, and many of those of Etna of augite and Labrador felspar. [LEUCITE, SANIDINE.] lava-flow, s. A sheet of lava whien has flowed forth from some more or le adjacent volcanic vent. If it is subsequently covered by sedimentary strata, alteration by igneous action is seen only on the str a um below, whereas, in the case of intrusive sheets, the rocks both above and below the sheet are altered.