4() THE CLAYS OF ARKANSAS. 
of it. The rich colors of the Jonesboro-Paragould bricks therefore 
seem to be due to these two substances. Clays that are deficient in 
both these substances, or that contain one of them in lumps — as the 
soil from the buckshot land at Harrisburg — will make bricks of poor 
color, which, however, may be modified or improved by the addition 
of soils containing iron or manganese. 
The colors of bricks depend also on the degree of heat to which the 
kilns are raised and the chemical combination of the iron or man- 
ganese. Careful manufacturers of bricks keep close watch on these 
important factors and produce bricks of uniform color and hardness. 
In the Crowleys Ridge region the refinements of brick manufacture 
have not been attempted, although the loess soils found there are 
preeminently suited to the production of fine brick. 
