The part played by hornblende (i.e. the monoclinie amphiboles) in the constitution of rocks, and the relation of that mineral to angite (including all the monoclinlc pyroxenes), are subjects which, in recent years, have occupied much attention. It has been recognised that the early notions of paragenesis cannot be applied, without many reservations, to rock-forming minerals. Some petrologists have gone so far as to suppose that augite and hornblende "are two different crystallographic forms of essentially the same molecule, of which the former is most stable at high, the latter at ordinary temperatures." If this view receives any extensive confirmation, the well-known conversion of angite into hornblende must be regarded as a purely paramorphic change due to altered conditions.