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The state of impending; also, that which impends. "Impendence of volcanic cloud." Ruskin.

Situated between hills; -- applied especially to valleys lying between volcanic cones.

is sometimes used as equivalent to plutonic rocks. It is then contrasted with effusive or volcanic rocks. -- In*tru"sive*ly, adv. -- In*tru"sive*ness, n.

Volcanic ashes, consisting of small, angular, stony fragments or particles.

lucent trapezohedral crystals. It is a silicate of alumina and potash. It is found in the volcanic rocks of Italy, especially at Vesuvius.

A spherulitic cavity often with concentric chambers, observed in some volcanic rocks, as in rhyolitic lavas. It is supposed to be produced by expanding gas, whence the name.

he solid earth, the portion undergoing change through the gradual transfer of material by volcanic eruption, the circulation of underground water, and the process of erosion and deposition. It is, therefore, regarded as a third mobile envelope comparable with the hydrosphere and atmosphere.

Native sulphate of ammonia, found in volcanic districts; -- so named from Mascagni, who discovered it.

A glassy volcanic rock of a grayish color and pearly luster, often having a spherulitic concretionary structure due to the curved cracks produced by contraction in cooling. See Illust. under Perlitic.

A volcanic rock, formed by the cementing together of sand, scoria, cinders, etc.

e Plutonists; igneous; as, the Plutonic theory. Plutonic action (Geol.), the influence of volcanic heat and other subterranean forces under pressure. -- Plutonic rocks (Geol.), granite, porphyry, and some other igneous rocks, supposed to have consolidated from a melted state at a great depth from the surface. Cf. Intr…

Volcanic ashes from Pozzuoli, in Italy, used in the manufacture of a kind of mortar which hardens under water.

A very light porous volcanic scoria, usually of a gray color, the pores of which are capillary and parallel, giving it a fibrous structure. It is supposed to be produced by the disengagement of watery vapor without liquid or plastic lava. It is much used, esp. in the form of powder, for smoothing and polishing. Called…

Cellular slaggy lava; volcanic cinders.

A volcanic area or vent which yields only sulphur vapors, steam, and the like. It represents the stages of the volcanic activity.

minute spherical crystalline body having a radiated structure, observed in some vitreous volcanic rocks, as obsidian and pearlstone.

either combined as in the sulphides (as pyrites) and sulphates (as gypsum), or native in volcanic regions, in vast beds mixed with gypsum and various earthy materials, from which it is melted out. Symbol S. Atomic weight 32. The specific gravity of ordinary octohedral sulphur is 2.05; of prismatic sulphur, 1.96.…

A white to gray volcanic tufa, formed of decomposed trachytic cinders; -- sometimes used as a cement. Hence, a coarse sort of plaster or mortar, durable in water, and used to line cisterns and other reservoirs of water. [Formerly written also tarras, tarrace, terras.]

A friable volcanic rock or conglomerate, formed of consolidated cinders, or scoria.

or passage for air or any fluid to escape; as, the vent of a cask; the vent of a mold; a volcanic vent. Look, how thy wounds do bleed at many vents. Shak. Long't was doubtful, both so closely pent, Which first should issue from the narrow vent. Pope.