498 
ELEMENTS OF GEOLOGY. 
and western parts of Europe, they were almost entirely ig¬ 
norant 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, trap tuff, and 
amygdaloid. All these, which were recognized as belong¬ 
ing 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 it 
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. It was also felt that some general term 
was indispensable, because these rocks, although very diver¬ 
sified in form and composition, evidently belonged to one 
group, distinguishable from the plutonic as well as from the 
non-volcanic fossiliferous rocks. 
By degrees familiarity with the products of active volca¬ 
noes convinced geologists more and more that they were 
identical with the trappean rocks. In every stream of mod¬ 
ern lava there is some variation in character and composi¬ 
tion, and even where no important difference can be recog¬ 
nized in the proportions of silica, alumina, lime, potash, iron, 
and other elementary materials, the resulting minerals are 
often not the same, for reasons which we are as yet unable 
to explain. The difference also of the lavas poured out from 
the same mountain at two distinct periods, especially in the 
quantity of silica which they contain, is often so great as to 
give rise to rocks which are regarded as forming distinct 
families, although there may be every intermediate gradation 
between the two extremes, and although some rocks, form¬ 
ing a transition from the one class to the other, may often be 
so abundant as to demand special names. These species 
might be multiplied indefinitely, and I can only afford space 
to name a few of the principal ones, about the composition 
and aspect of which there is the least discordance of opinion. 
Minerals most abundant in Volcanic Rocks. —The minerals 
which form the chief constituents of these igneous rocks are 
few in number. Next to quartz, which is nearly pure silica 
or silicic acid, the most important are those silicates com¬ 
monly classed under the several heads of feldspar, mica, horn¬ 
blende or augite, and olivine. In the annexed table, in draw¬ 
ing up which I have received the able assistance of Mr. David 
Forbes, the chemical analysis of these minerals and their va¬ 
rieties is shown, and he has added the specific gravity of the 
different mineral species, the geological application of which 
in determining the rocks formed by these minerals will be 
explained in the sequel (p. 504). 
