The potter's clay of Ischia served for the potteries of Cumae and Puteoli in ancient times, and was indeed in considerable demand until the catastrophe at Casamicciola in 1883.
Other minerals found here are graphite, alum, potter's clay and roofing-slate, and, besides, famous silvermines were worked at Iglau during the middle ages.
The soil is mostly sand, clay (brick-clay and potter's-clay are not uncommon), and peat-bogs,with a few patches of "black earth."
There are also salt-works, and some deposits of potter's clay.
Antimony, quicksilver, stone, marble, slate and potter's clay are also worked, and there are brine springs in the Hellweg and mineral springs at Lippspringe, Oynhausen, &c.