592 



TEAP ROCKS. 



[Ch. XXVIII. 



CHAPTER XXVIII. 



VOLCANIC ROCKS. 



Trap Eocks — 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, 

 trachyte, greenstone, porphyry, scoria, amygdaloid, lava, tuff — Agglomerate — ■ 

 Laterite — Alphabetical fist, 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, 



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 — a name since 

 adopted very generally into the nomenclature of the science ; for i+ 

 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 flow 



