494 
ELEMENTS OF GEOLOGY. 
CHAPTER XXVIII. 
VOLCANIC ROCKS. 
External Form, Structure, and Origin of Volcanic Mountains.—Cones and 
Craters.—Hypothesis of “Elevation Craters” considered.—Trap Rocks.— 
Name whence derived.—Minerals most abundant in Volcanic Rocks.—Table 
of the Analysis of Minerals in the Volcanic and Hypogene Rocks.—Similar 
Minerals in Meteorites.—Theory of Isomorphism.—Basaltic Rocks.—Tra- 
chytic Rocks.—Special Forms of Structure.—The columnar and globular 
Forms.—Trap Dikes and Veins.—Alteration of Rocks by volcanic Dikes. 
—Conversion of Chalk into Marble.—Intrusion of Trap between Strata.— 
Relation of trappean Rocks to the Products of active Volcanoes. 
The aqueous or fossiliferous rocks having now been de¬ 
scribed, 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 for- 
Fig. 584. 
a. Hypogene formations, stratified and nnstratified. h. Aqueous formations. 
c. Volcanic rocks. 
mations, such as the granitic and metamorphic ; 5, h the fos¬ 
siliferous strata; and c, c the volcanic rocks. These last are 
sometimes found, as was explained in the first chapter, break¬ 
ing through a and7>, sometimes overlying both, and occasion¬ 
ally alternating with the strata b^h. 
External Form, Structure, and Origin of Volcanic Mountains. 
—The origin of volcanic cones with crater-shaped summits 
has been explained in the “ Principles of Geology ” (chaps, 
xxiii. to xxvii.), where Vesuvius, Etna, Santorin, and Barren 
Island are described. The more ancient portions of those 
mountains or islands, formed long before the times of histo¬ 
ry, exhibit the same external features and internal structure 
which belong to most of the extinct volcanoes of still high¬ 
er antiquity; and these last have evidently been due to a 
complicated series of operations, varied in kind according to 
circumstances; as, for example, whether the accumulation 
took place above or below the level of the sea, whether the 
lava issued from one or several contiguous vents, and, lastly, 
