CHARACTERISTICS OF VOLCANOES.

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A common kind, breaking with a rough surface of fractureand little lustre, is called Trachyte ; some black hornblendecrystals are often present, and tables of feldspar called sani-din, and rarely small crystals of black or brown mica. If itcontains quartz disseminated through it, visibly or not, it isnamed llhyolyte (from the Greek for to flow ) or Quartz-trachyte ; when in the form of glass, it is Obsidian ; whenpartly glassy, and therefore pitch-like or resin-like in lustre,it is Pitchstone; when a fine light scoria, Pumice.

A rock having the composition of trachyte or quartz-trachyte, hut smooth, flint-like, in surface of fracture, is calledFelsyte ; and when coarsely crystalline, the feldspar beingin crystalline grains, and the quartz in visible grains, it iscalled Granulyte ; and the latter, when mica is present, isthe common crystalline rock called Granite . A felsytespotted with whitish feldspar crystals is a variety of Por-phyry, or a por-phyritic Felsyte.

Leucite, a potash-hearing mineral related in composition toorthoclase, is prominent in some volcanic rocks. Its crystalsa re twenty-four-sided garnet-like forms. It is called leucite,from its whitish color. A leucite-rock occurs in Wyoming ,an d another kind is common at Vesuvius .

IP Lavas consisting of a Feldspar and other Ingre-dients. — In the other common igneous rocks the feldspar1X1 a y be (1) orthoclase; (2) oligoclase, which contains thealkalies, soda and lime, with more soda than lime, — or therelated andesine; or (3) labradorite, which also is a soda-linie feldspar, but with more lime than soda.

I • The Orthoclase and Hornblende or Augite Rocks. — Theserocks are Syenyte, Quartz-syenyte, Augite-syenyte, andSo me others which are not among the rocks of ordinary

lavas.

Leucite and amnte are the chief constituents of Amphi-genyte (or Leucitophyre), the rock of the esuvian lavasi n st alluded to.