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A1 2 O 3 28.0-38.4, SiO 2 43.5-51.7, H 2 O 1.0-6.2. It will be seen from the indefmiteness of this analysis how variable in composition are the micas, but the formula of muscovite may roughly be given as : KjO. SAl^Oa. 6-16 SiO 2 . 2-6 H 2 O. QUARTZ may be dismissed with less detail. It is silica crystallising in the hexagonal system. Quartz often con- tains enclosures of other substances, sometimes as crystals which may be rutile, chlorite, or kyanite ; sometimes as lacunae containing gas or water, carbonic acid, or sodium chloride in solution, or liquid carbon dioxide. Such are the essential minerals of granite. We are now in a position to understand the formation of clay by the weathering of granite. The important constituent in the making of clay is the felspar, and the best beds of china- clay found in Europe are due to the decomposition of pegmatite, a granite exceptionally rich in felspar. By the action, lasting through countless ages, of water, ice, and carbon dioxide, aided by the fluorides which are always present in small quantities in the rock, and by humic acids and bases from swamps, the spar is gradually deprived of its alkalies and some of its silica, losing in the process, if it be the potash spar, 55 per cent, of its weight and 33 per cent, by volume, although the kaolin often retains the shape of its mother-spar. Lime, when present in the spar, is the first to be removed as soluble bicarbonate ; then the potash and soda are carried off as carbonates or