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may roughly be divided into ball clay, china clay, and fire olay ; or, in other words, a plastic clay, a non-plastic clay, and a refractory clay. CHINA CLAY, or kaolin, is a friable, non-plastic substance showing six-sided scales under the microscope. Its specific gravity is 2.2. The proportion of kaolin in granite is generally from 10-30 per cent. J. M. Coon has recently (1911) made the mechanical analyses of the St. Austell deposits. The average of five samples gave the figures : Per Cent. Coarse quartz 37.4 Medium quartz 18.9 Fine quartz, tourmaline 2.8 Very fine quartz, coarse mica, tourmaline 2.0 Fine mica, coarse clay, tourmaline 2.6 Very fine mica, medium clay 3.3 Marketable china clay 31.5 Another sample yielded 49 per cent, of marketable clay, but this is quite exceptional. The clay as dug up is pumped with water into narrow channels called 'micas,' which are fitted with traps that can be raised or lowered at will, so that the clay is allowed to deposit a certain amount of its heavier material, and then run into fresh micas. The mica sand deposited in the first set is waste, but that from the others is re-washed and sold as mica-clay. From the micas the clay water passes,