GENERAL GEOLOGY.


Chyemnical Elements of Rocks. The following elements constitute
nearly the entire rock substance of the State, and, indeed, of the
earth's crust: Oxygen, silicon, aluminum, calcium, magnesium, iron,
carbon, sodium, potassium, hydrogen and sulphur. To these are to
be added, more because of their value than their abundance, lead,
zinc, copper, manganese, gold, silver, chlorine, iodine, bromine,
lithium and titanium. A number of others occur in small quanti-
ties. Probably nearly or quite alf of the remaining known elements
occur in infinitesimal amounts.
Native Elements. Only a few of these substances occur in the
simple elementary state.
Carbon, native copper, silver, gold and sulphur occur in the State,
but, however interesting on account of their valuable properties,
they make up a very insignificant portion of the earth's crust. It
is mainly in the form of compounds that chemical substances enter
into the earth's constitution. It will, therefore, be more serviceable
to consider these compounds than to dwell upon the character of the
elements as such.
Simple, Compounds. By the union of elements, simple compounds
are formed. Thus any of the above elements, united with oxygen,
constitutes an oxide, and is named according to the element so unit-
ing.  Thus sodium, uniting with oxygen, forms sodium    oxide
(oxide of sodium, or soda, old style). A similar combination with
sulphur constitutes a suiphide, as lead, uniting with sulphur, forms
lead sudphide, which is the common lead ore, galena. Of the simple
compounds, the oxides are by far the most important. Among
them, the geologically leading ones are silicic oxide, or silica; ala-
minic oxide, or alumina; calcic oxide, or Ulme; magnesic oxide,
or magnesia; iron (ferric orferroas) oxide, potassic oxide, or potash,
sodic oxide, or soda, carbQnic dioxide or carbonic acid.'
Of these, silica is familiar in the form of quartz, and common
quartzose sand, and does not need further discussion here, than to
remark that chemically it is an acid, though lacking the sensible
acid properties. Alanmina is the essential constituent of corundum,
8sapphire, ruby and emery. It is better known to us as the basic in-
gredient of clay, in which it is united with silica, forming aluminum
silicate. -Lime (calcium  oxide) is familiarly known as common
quick-lime, though in reality the quick-lime used in Wisconsin, with
very few exceptions, is a combination of lime and magnesia, being
1 In the strictest sense this is not regarded as an acid except in combination
with water.


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