24 THE CLAYS OF ARKANSAS. 
which they are derived. Wherever these residuary clays are ex- 
posed at the surface they are usually still further modified by weath- 
ering and by organic agencies. When they are washed into streams, 
sorted by the currents, and deposited again they form what are 
known as redeposited clays. 
CLAYS OF MECHANICAL ORIGIN. 
Clays may be produced by grinding to a fine powder any rock that 
will produce clay upon decomposition. When thus pulverized some 
of these rocks, possibly all of them, become more highly hydrated or 
take up more water of crystallization, a change winch is probably 
explicable, in part at least, upon the theory that much more rapid 
decomposition is produced by finely dividing the material so as to 
expose a very large surface to decomposing agencies. The tritura- 
tion of rock fragments dashed against one another either by the 
current of a stream or by the action of waves wears the fragments 
round, forming cobblestones, pebbles, sand, and mud. More mud 
than sand is formed in this operation — in other words, the greater 
part of the material passes almost directly from the condition of a 
solid rock to that of a clay. This clay is carried along by the water 
until the current is checked, when it gradually sinks to the bottom 
and forms a bed or layer. But whether the argillaceous sediments 
carried into the sea by streams are of mechanical or chemical origin, 
they all go to form new beds of clays over the bottom of the sea. 
.All parts of the earth's surface above the ocean's level are being 
gradually cut down by the slow but never-ceasing processes of 
weathering and erosion and are carried away either in chemi#al solu- 
tion or in mechanical suspension. In this process of degradation the 
heavier materials are left behind for further disintegration, while the 
lighter — that is, the clays and fine sands-#-are carried away by 
streams. This mechanical separation by water goes on constantly, 
and during the eons of geologic time the results of such agencies 
have accumulated until they have ultimately produced an impressive 
and almost incredible total. 
KAOLIN. 
The distinction between clay and kaolin, except in a mineralogical 
sense, is an arbitrary one, for the two grade imperceptibly into each 
other. Both kaolin and clay are derived chiefly, either directly or 
indirectly, from feldspathic rocks by decomposition. Kaolin is com- 
posed of silica, alumina, and water, usually with a little impurity, 
while clay has the same constituents, but its impurities form a larger 
percentage of its composition. These impurities are often in the 
aDaubr&>, A., Goologie exp^rimentale, p. 252. 
