84 MINERALOGY. . 
important are those belonging to the family of feldspar. Feldspar is an 
essential ingredient of entire rocks, as granite and gneiss; minerals allied 
to this are important constituents also of other rocks, as labradorite and 
syenite. F'eldspathic minerals, as constituents of the soil, are of the utmost 
value. Most of them disintegrate readily under atmospheric influences, and 
yield up to the piants in the soil those inorganic ingredients so necessary to 
their growth. 
On examining certain minerals, as granites, which are rich in feldspar, 
we shall find that they gradually lose their compactness by exposure to the 
weather, crumbling into a fine sand. On investigation this is found to Ze 
rich in alumina and poor in alkali. In ali probability it is the mechanical 
agency of the water, with the unceasing alternations of temperature, 
together with the dissolving action of the carbonic acid contained in the 
water, that wears away the stone; the carbonic acid also extracts the 
stronger bases, and thus contributes, in a great measure, to the disintegration 
of the rock. It is also to be observed, that the silica, in the condition in 
which it is separated from alkaline silicates, by aqueous influence, is very 
soluble ; also, that the feldspar, with its alkalies, communicates to water a 
proportion of that dissolved silica so necessary to the sustenance of certain 
plants, especially the grasses, and which is deposited in their tissues. It has 
already been mentioned that purer beds of feldspar furnish, in many 
localities, by their decomposition, masses of the finer porcelain earth or 
kaolin... And it is not only this naturally-formed kaolin that is made use of 
in the manufacture of different kinds of porcelain and stone-ware; the 
mixtures of the constituents must frequently be varied. Thus, in the 
manufacture of porcelain, the coarser qualities of feldspar are ground, and 
mixed up with the finer porcelain earth, in the proportions required for the 
particular kind of ware. 
The class of the salts is no less important. Carbonate of lime, in the 
form of limestone, furnishes an excellent building material, and is one of the 
most important constituents of a fertile soil. Besides this general utility of 
carbonate of lime, certain varieties, forming entire rocks, find valuable 
applications. The granular crystalline form of compact carbonate of lime 
is known as marble, the different varieties being produced by the fineness 
of the grains, or the varying character and distribution of the coloring 
matters. The coarse blocks of marble, after being extracted from the 
quarries, are taken to marble works, and there cut up into slabs and other 
forms, by means of broad saw-like steel blades, stretched in frames. The 
lithographic stone, used so extensively at the present time, is also a 
carbonate of lime. Chalk is another of these carbonates of lime, differing, 
however, from the rest mentioned, in consisting largely of the calcareous 
shells of minute infusoria and other animals, thus more nearly related to the 
muschelkalk. Many useful applications of carbonate of lime depend on the 
circumstance that the carbonic acid is driven off at a strong red heat, the 
lime retaking the acid from the atmosphere when exposed to the latter. 
Burnt or quick lime, when water is poured over it, takes up a certain 
amount, forming a definite chemical combination, a hydrate of lime, which, 
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