Mendip Hills In some areas the limestone and the dolomitic conglomerate have been mineralised with lead and zinc ores. From the time of Roman Britain until 1908, the hills were an important source of lead. These areas were the centre of a major mining industry in the past and this is reflected in areas of contaminated rough ground known locally as "gruffy". The word "gruffy" is thought to derive from the grooves that were formed where the lead ore was extracted from veins near the surface. Other commodities obtained included calamine (zinc ore), manganese, iron, copper and baryte. The eastern area reaches
Metallic mean Metallic mean The metallic means or silver means (also ratios or constants) of the successive natural numbers are the continued fractions:formula_1 The golden ratio (1.618...) is the metallic mean between 1 and 2, while the silver ratio (2.414...) is the metallic mean between 2 and 3. The term "bronze ratio" (3.303...), or terms using other names of metals (such as copper or nickel), are occasionally used to name subsequent metallic means. The values of the first ten metallic means are shown at right. Notice that each metallic mean is a root of the simple quadratic equation: formula_2, where formula_3 is