PART 1. CHAPTER VII. 



89 



Trap Rocks, whence named. 



CHAPTER VII. 



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 analy- 

 ses 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 an- 



Fig. 84. 



a. Hypogene formations, stratified and unstratified. 



b. Aqueous formations. c. Volcanic rocks. 



nexed diagram, to represent the crystalline formations, such as 

 the granitic and metamorphic, b h the fossiliferous strata, and 

 c c the volcanic rocks. These last are sometimes found, as was 

 explained in the first chapter and Frontispiece, breaking through 

 a and &, sometimes overlying both, and occasionally alternating 

 with the strata h h. 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 struc- 

 ture of the northern (^nd western parts of Europe, they were 

 almost entirely ignorant of the phenomena of existing volcanos. 

 They found certain rocks, for the most part without stratification, 

 and of a pecuhar mineral composition, to which they gave differ- 

 ent names, such as basalt, greenstone, porphyry, and amygda- 

 loid. All these, which were recognized as belonging to one 

 family, were called "trap" by Bergmann (from trappa, Swe- 

 dish, 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 sreat tabular masses of 



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