VI.—The Development of Brown Mica from Augite by Reaction with Felspathic Material

The conversion of augite or hornblende into brown mica appears to take place from three causes:— 1. By crushing of pyroxenic minerals and felspar, as has been observed by Professor Bonney in sundry schists of the Alps. 2. By the intrusion of felspathic material into a rock containing augite or hornblende, as in the Harz, where granite invades diabase, converting the augite into biotite. A similar change has been observed in the gabbro of the Barnavave Mountain, in the Carlingford district, the diallage being wholly or in part replaced by biotite owing to the invasion of granophyric magma. In Sark the intrusion of an aplitic magma into almost pure hornblende has resulted in the formation of biotite.