. Elements of geology. Geology. PART 1. CHAPTER VII. 97 Scorise Pumice Lava. of trap. It derives its name from the Greek word amygdala, an almond. The origin of this structure cannot be doubted, for we may trace the process of its formation in modern lavas. Small pores or cells are caused by bubbles of steam and gas confined in the melted matter. After or during consolidation these empty spaces are gradually filled up by matter separating from the mass, or infiltered by water permeating the rock. As these bubbles have been sometimes lengthened by the flow of the lava before it finally cooled,
. Elements of geology. Geology. PART 1. CHAPTER VII. 97 Scorise Pumice Lava. of trap. It derives its name from the Greek word amygdala, an almond. The origin of this structure cannot be doubted, for we may trace the process of its formation in modern lavas. Small pores or cells are caused by bubbles of steam and gas confined in the melted matter. After or during consolidation these empty spaces are gradually filled up by matter separating from the mass, or infiltered by water permeating the rock. As these bubbles have been sometimes lengthened by the flow of the lava before it finally cooled, the contents of such cavities have the form of almonds. In some of the amygdaloidal traps of Scotland, where the nodules have decomposed, the empty cells are seen to have a glazed or vitreous coating, and in this respect exactly resemble scoriaceous lavas, or the slags of furnaces. Yi<y. 88. The annexed figure re- presents a fragment of stone taken from the upper part of a sheet of basaltic lava in Auvergne. One half is scoriaceous, the pores being altogether empty, the other part is amygdaloidal, the pores or cells being mostly filled up with carbonate of lime, forming white kernels. ScoricB and Pumice may next be mentioned as po- rous rocks, produced by Scoriaceous lava in part converU^^ an amyg- actlon of gaseS On ma- <Mo2«?.—Montagne de la Veille, Department of i i , Puy de Dome, France. terials melted by volcanic heat. Scorics are usually of a reddish brown and black colour, and are the cinders and slags of basaltic or augitic lavas. P/imice is a light, spongy, fibrous substance, produced by the action of gases on trachytic and other lavas; the relation, however, of its origin to the composition of lava is not yet well understood. Von Buch says it does not occur where only Labrador-felspar is present. Lava.—This term has a somewhat vague signification, hav- ing been applied to all melted matter observed to flow in streams from volcanic vents. V^^hen this matter
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