GEOLOGIC AGE AND GEOGRAPHIC DISTRIBUTION. 35 
ocean or of bays and swamps, and if they are now thrown into folds, 
raised into mountains thousands of feet above the ocean, and hard- 
ened into stone, it is only because they have been crushed till they 
are hard, pressed laterally into folds, and elevated until they stand 
high above their original positions. Subaerial erosion has subse- 
quently removed much accompanying and surrounding material, and 
in places these shales have weathered into residuary clays, such as 
have been described on page 20. 
The clays of the Paleozoic regions have values peculiarly tneir 
own, for while the clays of the Tertiary area of the State are especially 
well adapted to the manufacture of pottery, those of the Paleozoic 
regions are largely adapted to the production of fire bricks, sewer 
pipes, and paving bricks. 
The Paleozoic rocks have furnished the greater part of the refrac- 
tory material of the world. These clay and clay shales do not as a 
rule possess as high refractory properties as bauxite, steatite, graph- 
ite, or magnesite, but the latter are much less abundant and are for 
this reason much more expensive, while articles made from clays and 
shales are sufficiently refractory for most of the common uses for 
which refractory materials are employed, and the abundance and 
cheapness of such clays insure for them a constant and ever-increasing 
demand. In the United States the clays and clay shales of the 
Carboniferous rocks are extensively used for certain manufacturing 
purposes, while raw clays are shipped to points all over the country 
to be used for the manufacture of retorts and furnace linings of 
special forms. 
The various zinc-smelting works in the United States make their 
own retorts, and the clays used for this purpose are brought to the 
reduction works from well-known Paleozoic beds. And it should not 
be forgotten in this discussion of the character and distribution and 
uses of the clays of this class that their value, other things being 
equal, is determined by their proximity to manufacturing centers and 
to competing lines of railway transportation. These important factors 
have built up the fire-clay industries of St. Louis and Cheltenham. 
The St. Louis, Iron Mountain and Southern Railway extends along 
the eastern edge of the Paleozoic area of Arkansas north of Arka- 
delphia. The southern border of the Paleozoic runs nearly due west 
from a point on Ouachita River a few miles north of Arkadelphia. 
Within this Paleozoic area the clays and clay shales are unevenly 
distributed. In the Ordovician region, lying north of the Boston 
Mountains, shales and clays of value are much less abundant than 
they are in the Boston Mountains and in the Carboniferous rocks 
farther south. Again, in the Silurian region of the great Ouachita 
anticline, running from near Little Rock to the Indian Territory 
line, in Polk County, there are many mountains of novaculite in 
