4-60 TRAP ROCKS. rCH.XX.VUL 



CHAPTER XXVni. 



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 ol 

 felspar — Hornblende and augite — Isomorphism — Rocks, how to be studied — 

 Basalt, trachyte, greenstone, porphyry scoria, amygdaloid, lava, tuff— Agglo- 

 merate — Laterite — 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 



bV 



^ . 



^ 





"^ 



IE 



f 



Fig. 

 a 



610. 





xf' 



b 









T in 



a. Hypogene formations, stratified and unstratifled. 

 &. Aqueous fornjations. c. Volcanic roci<s. 



the crystalline formations, such as the granitic and metamorphic ; h h 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 6, 

 sometimes overlying both, and occasionally alternating with the strata b b. 

 They also are seen, in some instances, to pass insensibly into the unstrati- 

 fied 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 entirely ignorant 

 of the phenomena of existing volcanoes. They 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 Bergman n, from trappo^ Swedish for a 

 flight of steps — a name since adoj^ted veiy generally into the nomencla- 

 ture 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 suc- 

 cession of terraces or steps on the sides of hills. This configuration 

 appears to be derived from two causes. First, the abrupt original ter- 

 minations 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 



