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And clays are sometimes found with abnormal losses on ignition. To take a single example, a kaolin found at Passau, in Bavaria, results from the decay of ekebergite, a scapolite, of which it often retains the external forms. Its composition is: Si0 2 46.4, A1 2 3 35.0, ttjO 18.6 per cent. It is possible that bacterial agencies have been at work in clay formation, but though some experimenters have thought they were able to increase the plasticity of clay by inoculation, there is no real evidence yet that the weathering of felspar is promoted by microbe or enzyme. Before leaving the geological origins of pottery materials, the more frequently occurring silicates should be mentioned. Comparatively few of the metallic elements are found naturally as silicates. Thirteen sodium, potassium, lithium; calcium, magnesium, barium, strontium, man- ganese; iron and aluminium; and (of the heavy metals) zinc, copper, and bismuth exhaust the list. Many of them occur together with others, and it is easier for the mineralogist to group the silicates under general formulae. The augite or pyroxene species, R"Si0 3 , contains varie- ties diopside, alaite, salite, malacolite, coccolite, diallage, hedenbergite, jeffersonite, fassaite, omphacite, pyrgom, achmite, segirine, breislakite. Wollastonite is CaSi0 3 ; pectolite, 4CaSiO 3 . Na 2 Si0 3 . H 2 Si0 3 ; spodumene, Li 2 0. A1 2 3 . 4Si0 2 .