CHAPTER II. 
CHARACTER, ORIGIN, OCCURRENCE, AND USES OF 
ARKANSAS CLAYS." 
CLASSIFICATION OF THE CLAYS. 
No sharply defined classification of the common clays is altogether 
satisfactory, for clay is not a definite compound nor a well-defined 
mineral species, and the qualities and values of clays depend upon 
physical and chemical properties which vary within wide limits, so 
that the various kinds of clay grade imperceptibly into one another. 
The clays of this State may be classified, however, with reference to 
their mode of formation, as: (1) Residual clays, (2) clays of mechan- 
ical origin, (3) clays and kaolins of chemical origin, and (4) bauxite. 
They will be briefly discussed in this order. 
RESIDUAL CLAYS. 
Clays and kaolins are chiefly secondary products derived either 
directly or indirectly from feldspathic rocks — that is, from rocks com- 
posed either entirely or largely of the mineral feldspar, such as granite, 
syenite, and gneiss. The feldspathic rocks may be either eruptives — 
that is, rocks injected in a molten condition from below into or 
through crevices in the earth's crust — or they may be the crystalline 
rocks that underlie our oldest sedimentary rocks. Whenever and 
wherever these rocks occur near the surface they decay — many of 
them very slowly — and form kaolins or clays containing more or less 
impurities. When the rocks contain little else than feldspar, or when 
the minerals that would stain or otherwise injure kaolin are removed 
in solution, they form, on decomposition, kaolin of more or less value; 
but when they contain considerable quantities of iron, manganese, 
or other discoloring impurities they form clays of various colors. 
Such clays and kaolins are said to be derived directly from feldspathic 
rocks. 
The precise process of the alteration of the solid syenites of the 
Fourche Mountain region to kaolins and clays is illustrated by the fol- 
lowing analysis. The column marked "blue syenite" shows the com- 
position of the original unaltered rock, the column marked "decom- 
a Persons wishing to consult other works on clays will find a bibliography of the subject in Bull. 
U. S. Geol. Survey No. 143, 1896; a second edition of that bibliography was published by the Amer- 
ican Ceramic Society at Columbus, Ohio, in 1906. 
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