500 
ELEMENTS OF OEOLOGY. 
From this table it will be observed that many minerals are 
omitted which, even if they are of common occurrence, are 
more to be regarded as accessory than as essential compo¬ 
nents of the rocks in which they are found.* Such are, for 
example. Garnet, Epidote, Tourmaline, Idocrase, Andalusite, 
Scapolite, the various Zeolites, and several other silicates of 
somewhat rarer occurrence. Magnetite, Titanoferrite, and 
Iron-pyrites also occur as normal constituents of various 
igneous rocks, although in very small amount, as also Apatite, 
or phosphate of lime. The other salts of lime, including its 
carbonate or calcite, although often met with, are invariably 
13roducts of secondary chemical action. 
The Zeolites, above mentioned, so named from the manner 
in which they froth up under the blow-pipe and melt into a 
glass, differ in their chemical composition from all the other 
mineral constituents of volcanic rocks, since they are hydrated 
silicates containing from 10 to 25 per cent, of water. They 
abound in some trappean rocks and ancient lavas, where they 
fill up vesicular cavities and interstices in the substance of 
the rocks, but are rarely found in any quantity in recent 
lavas; in most cases they are to be regarded as secondary 
products formed by the action of water on the other constit¬ 
uents of the rocks. Among them the species Analcime, 
Stilbite, Natrolite, and Chabazite may be mentioned as of 
most common occurrence. 
Quartz Group. —The microscope has shown that pure quartz 
is ofbener present in lavas than was formerly supposed. It 
had been argued that the quartz in granite having a specific 
gravity of 2*6, was not of purely igneous origin, because the 
silica resulting from fusion in the laboratory has only a spe¬ 
cific gravity of 2*3. But Mr. David Forbes has ascertained 
that the free quartz in trachytes, which are known to have 
flowed as lava, has the same specific gravity as the ordinary 
quartz of granite; and the recent researches of Von Rath 
and others prove that the mineral Tridymite, which is crys¬ 
tallized silica of sp. gr. 2*3 (see Table, p. 499), is of common 
occurrence in the volcanic rocks of Mexico, Auvergne, the 
Rhine, and elsewhere, although hitherto entirely overlooked. 
Feldspar Group. —In the Feldspar group (Table, p. 499) the 
five mineral species most commonly met with as rock con¬ 
stituents are: 1. Orthoclase, often called common or potash- 
feldspar. 2. Albite, or soda-feldspar, a mineral which plays 
a more subordinate part than was formerly supposed, this 
name having been given to much which has since been 
proved to be Oligoclase. 3. Oligoclase, or soda-lime feld- 
* For analyses of these minerals see the Mineralogies of Dana and Bristow. 
