Some clays, however, such as fireclays, contain very little potash or soda, while they are rich in alumina; and it is a fair inference that hydrated aluminous silicates, such as kaolin, are well represented in these rocks.
Potassium bisulphate is useful in the preliminary treatment of refractory aluminous ores.
Thus Strabo states that in his time a process was employed for refining and purifying gold in large quantities by cementing or burning it with an aluminous earth, which, by destroying the silver, left the gold in a state of purity.
His account of the aluminous district of Tolfa and adjacent hills, published in 1786, gained for him the notice of the king of Naples, who invited him to inspect the mines and similar works in that kingdom, and appointed him professor of mineralogy to the royal artillery.
Others are rich in pyrites, which, on oxidation, produces sulphuric acid; this attacks the aluminous silicates of the clay and forms aluminium sulphate (alum shales).
Wide, whose south-eastern extremity is the intersection of the 32nd meridian with the Alabama boundary, is characterized by beds of aluminous grey and white sandstone, aluminous and siliceous clay-stone, quartzitic sandstone, and green sand and marls.
Throughout much of the Piedmont Plateau and Mountain regions the decomposition of felspar and of other aluminous minerals has resulted in a deep soil of clay with which more or less sand is mixed.