[Illustration: Burley clay. Note the oolitic structure, the “burls.”
From a diaspore pit near Swiss.]
Burley clay is a fire clay intermediate in alumina content between flint
clay and first quality diaspore. It takes its name from the oolites
(rounded pellets of diaspore) which are scattered through a flint clay
base and which were called “burls” by the early clay miners. As the
relative number of diaspore oolites increase, an otherwise flint clay
becomes burley-flint, then typical burley, and finally grades into
second quality and first quality diaspore. Clay in any stage of the
variation may be found in some part of the diaspore region or pits. Most
of the remarks written on diaspore apply as well to burley clay.