40 
AMERICAN GEOLOGY. 
symmetrical arrangement. Concretions and nodules are sym- 
metrical bodies. The parallel planes of the rhombic forms so 
common in limestones and slaty rocks are due to this force. The 
jointed structure admits of the same explanation. 
§ 35. Composition of the earth’s crust. The mineral king¬ 
dom is composed of a large number of distinct species of sub¬ 
stances, the knowledge of which is highly important. The 
composition, however, of the rocks or masses is represented by 
an extremely small number of simple minerals, which are 
repeated over and over again in the layers of the rocks. The 
rocks are either mechanical mixtures of a few simple minerals, 
or they are made u|5 of a single simple or homogenous mineral 
by itself. Granite, gneiss, sandstones, and conglomerates, are 
examples of the former; and limestone, gypsum, serpentine, 
and hornblende, of the latter. The elementary bodies K are 
extremely rare in nature, or in the mineral kingdom. Sulphur 
is common in volcanic districts, but is a product of decomposi¬ 
tion. Carbon, nearly pure, exists under the form of anthracite. 
The metals, gold, silver, copper, and mercury, may be said to 
be of frequent occurrence, but can not be claimed as component 
parts of the masses geologically considered. 
The minerals which predominate in the earth’s crust are the 
siliceous, aluminous, and calcareous. Silica, as a constituent 
part of the rocks, occurs under two forms: the first and most 
obvious and common is quartz, as it exists in flint, white sand, 
or an aggregation of sand in the form and condition of sand¬ 
stone; the second is an acid, and is combined with one or 
more bases, and forms those bodies which are called silicates: 
feldspar, hornblende, mica, and pyroxene are examples. Some 
rocks contain examples of both forms, as granite, where it is 
in the first form as particles of quartz, and in the second as 
silicates in the feldspar and mica. The aluminous minerals are 
represented by common clay, as it everywhere occurs, or by 
slates which are consolidated by pressure, or baked clay still 
more consolidated and changed by heat. These examples, 
however, are not those of pure alumine; they are mixtures of 
