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anorthite. CEEAMIC CHEMISTRY: Crystallographically, the felspars occur in two systems : the potash and potash-baryta spars crystallising in the monoclinic, and the others in the triclinic. But a true soda -potash spar is recognised in anorthoclase. The felspars can* to some extent, be quickly distinguished from each other by their flame tests and by their specific gravi- ties, which have been given in the foregoing table. MICA. The minerals included under the general name of mica vary considerably in composition, but are marked by an extremely perfect cleavage parallel to the basal plane, the thin laminae produced by cleavage being not merely flexible but quite elastic. They crystallise in either the hexagonal or rhombic system. The micas cannot be reduced to any single formula, and many classifications have been accordingly suggested for them. Tschermak, following his procedure in the case of the felspars, suggests that they are isomorphous mixtures of any of the following molecular groups : ll) RO. AI 2 O a . 2 SiOj,, where R may be, singly or in mixtures, K 2 . Na 2 . Li 2 Fe", or H 2 . Hydrous micas, however, have usually undergone alteration. It will be seen from this formula how similar many hydrous micas are in composition to clay-substance, and Hutchings thinks that many fireclays are really hydrated altered micas formed subsequently to sedimentation. (2) 2MgO. SiO 2 , or the equivalent ferrous or manganous compounds.