. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 592 TEAP ROCKS. [Ch. XXVIII. CHAPTER XXVIII. VOLCANIC ROCKS. Trap EocksâName, whence derivedâTheir igneous origin at first doubtedâTheir general appearance and characterâVolcanic cones and craters, how formedâ Mineral composition and texture of volcanic rocksâVarieties of felsparâ Hornblende and augiteâIsomorphismâRocks, how to be studiedâBasalt, trachyte, greenstone, porphyry, scoria, amygdaloid, lava, tuffâAgglomerateââ LateriteâAlphabetical fist, and explanation o
. Elements of geology, or, The ancient changes of the earth and its inhabitants as illustrated by geological monuments. Geology. 592 TEAP ROCKS. [Ch. XXVIII. CHAPTER XXVIII. VOLCANIC ROCKS. Trap EocksâName, whence derivedâTheir igneous origin at first doubtedâTheir general appearance and characterâVolcanic cones and craters, how formedâ Mineral composition and texture of volcanic rocksâVarieties of felsparâ Hornblende and augiteâIsomorphismâRocks, how to be studiedâBasalt, trachyte, greenstone, porphyry, scoria, amygdaloid, lava, tuffâAgglomerateââ LateriteâAlphabetical fist, and explanation of names and synonyms, of volcanic rocksâTable 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,. a. Hypogene formations, stratified and unstratified. b. Aqueous formations. c. Volcanic rocks. to represent the crystalline formations, such as the granitic and meta- morphic; 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 5, 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. When geologists first began to examine attentively the structure of the northern and western parts of Europe, they were almost en- tirely ignorant of the phenomena of existing volcanoes. They found certain rocks, for the most part, without stratification, and of a pecu- liar 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
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