VOLCANIC ROCKS.

Trap rocks — Name, whence derived — Their igneous origin at first doubted — Their general appearance and character — Volcanic cones and craters, how formed — Mineral composition and texture of volcanic rocks — Varieties of felspar — Hornblende and augite — Isomorphism — Rocks, how to be studied — Basalt, greenstone, trachyte, porphyry, scoria, amygdaloid, lava, tuff — Alphabetical list, and explanation of names and synonyms, of volcanic rocks — Table of the analyses of minerals most abundant in the volcanic and hypogene rocks.

The aqueous or fossiliferous rocks having now been described, we have next to examine those which may be called volcanic, in the most extended sense of that term. Suppose a a in the annexed diagram, to represent the crystalline formations, such as the granitic and metamorphic; b b the fossiliferous strata; and c c the volcanic rocks. These last are sometimes found, as was explained in the first chapter, breaking through a and b, sometimes overlying both, and occasionally alternating with the strata b b. They also are seen, in some instances, to pass insensibly into the unstratified division of a, or the Plutonic rocks.

Fig. 434.

When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost entirely ignorant of the phenomena of existing volcanos. They also found certain rocks, for the most part without stratification, and of a peculiar mineral composition, to which they gave different names, such as basalt, greenstone, porphyry, and amygdaloid. All these, which were recognized as belonging to one family, were called "trap" by Bergmann, from trappa, Swedish for a flight of steps—a name since adopted very generally into the nomenclature of the science; for it was observed that many rocks of this class occurred in great tabular masses of unequal extent, so as to form a succession of terraces or steps on the sides of hills. This configuration appears to be derived from two causes. First, the abrupt original terminations of sheets of melted matter, which have spread, whether on the land or bottom of the sea, over a level surface. For we know, in the case of lava flowing from a volcano, that a stream, when it has ceased to flow, and grown solid, very commonly ends in a steep slope, as at a, [fig. 435.] But, secondly, the step-like appearance arises more frequently from the mode in which horizontal masses of igneous rock, such as b c, intercalated between aqueous strata, have, subsequently to their origin, been exposed, at different heights, by denudation. Such an outline, it is true, is not peculiar to trap rocks; great beds of limestone, and other hard kinds of stone, often presenting similar terraces and precipices: but these are usually on a smaller scale, or less numerous, than the volcanic steps, or form less decided features in the landscape, as being less distinct in structure and composition from the associated rocks.

Fig. 435.