MINERALOGY AND PETROLOGY. 953 



Augite.] 



In the oldest rocks (the Kawishiwin igneous rocks) augite is found with its 

 optical characters usually destroyed, but its ophitic relation to the associated feld- 

 spars still preserved by the resultant uralite (Nos. 1011, 1078, 1758, 1759, 1760). In 

 the Kawishiwin clastic greenstones it is uniformly converted to some form of horn- 

 blende (Nos. 356, 997, 1003, 1014, 1071), except where for some favorable conditions 

 it has been partly preserved as augite. In the Upper Keewatin, about Kekequabic 

 lake, where it is markedly affected by aagyrine characters, whether in the green 

 schist or in the granite and porphyry (Nos. 1399, 1767) it is sometimes well preserved 

 (Nos. 1094, S6G) and is sometimes partly converted into hornblende (Nos. 1400, 1047, 

 1060) or disseminated in the alteration form of actinolite spicules throughout the 

 schist (Nos. 1419, 1421). In most places in the clastic greenstones of the Upper 

 Keewatin it is wholly lost by alteration to hornblende and to chlorite (Nos. 1345, 

 1788, 1799) and this is universally true of the acid elastics which frequently embrace 

 hornblende that may be supposed to have been derived wholly or partly from augite. 



There are some hornblendic intrusives in the Keewatin which contain pheno- 

 crysts that afford curious and characteristic phenomena, presenting alliances with 

 camptonyte (Nos. 872, 877, 915, 1318, 1786). Such are found about Vermilion lake 

 and at Ely (No. 1786) and indicate that even in such conditions the original augite 

 grain, or fragment, has been through such mechanical and chemical stress that it 

 has taken on the hornblendic crystalline form, but has retained an impress of its 

 prior state. Such ancient augitic areas seen in hornblendes are indicated by the 

 greater absorption which appears at the centres of the hornblendes, or which spread 

 irregularly through them (Nos. 872, 1047, 1786). This feature appears not only in 

 the distinct dikes at Vermilion lake and at Ely, but also in much of the intrusive, 

 recrystallized rocks of Kekequabic and Snowbank lakes. Such alteration and intru- 

 sion, when seen in the Upper Keewatiu, was probably pre-Animikie. 



In the Keweenawan augite is a more abundant mineral than in the Archean, 

 owing probably to later origin and to less of mountain-making vicissitude, as well as 

 to the preponderance of basic igneous rock in that formation. In the Keweenawan 

 also, as a rule, it is nearly unchanged. Its clearness and purity are evinced in every 

 thin section taken from such portions of the interior of the rock where it has received 

 only the normal influences. In other words, time has had no noticeable effect, denoting 

 metasomatic alteration (Nos. 639, 820, 1137). It is only where the Keweenawan 

 rocks containing augite have experienced an unusual history, either before or after 

 consolidation, and usually prior to final cooling, that augite has been altered. It is 

 then sometimes changed to chlorite, or magnetite and chlorite, or its elements are so 

 scattered in the production of new secondary minerals and mingled with those from 

 other sources that they cannot be traced to their present resting places. In such 



