24 Diorites and Granites of Sivift's Creek, 



the field suggests that which the microscopic analysis confirms 

 — namely, that they are nodular argillaceous schists and 

 quartz schists ; in fact, those members of the regional meta- 

 morphic series which are found in many localities near 

 Omeo in the outer verge of the regionally metamorphosed 

 formations. 



On the one hand, they are found, as at Swift's Creek, to 

 pass by increasingly altered examples into the true mica 

 schists and gneiss, and on the other by less altered examples, 

 as would be the case at Swift's Creek did the intrusive 

 masses and their contact zones not intervene, into the 

 normal Silurian slates and sandstones. 



(c.) The intrusive igneous Bocks — their component 

 Minerals. 



The rocks of this class form, as a whole, a great intrusive 

 mass of about 30 square miles in area. They are surrounded 

 by a zone of contact schists, are flanked in the east, north, 

 and west by the regional metamorphic, and in the south 

 by the more or less normal Silurian formations. These 

 intrusive igneous rocks are compounded of various minerals, 

 and it is in accordance with their co-occurrence that the 

 rock masses must be classed. I now proceed to examine 

 their individual mineral constituents, and to record my 

 observations. 



The Felspars. 



It becomes of great importance, in classifying the 

 felspars, to attempt, if possible, to fix upon some optical 

 characters of sufficient constancy to serve as standards of 

 reference. Such constant characters have been indicated 

 by Des Cloizeaux,* and are frequently referred to by 

 Kosenbusch.-f* The former states that in thin plates of 

 albite, parallel to the basis, the plane of the maximum 

 obscuration forms an angle with the composition face of 

 3° 50' to 4° 50'; in oligoclase that direction is almost parallel 

 to the composition face ; in labradorite, the angle is 5° 17' to 

 6° 58', and in anorthite 20° to 40°, when the plate- is 



* Comptes Bendus, Ixxxii, 1876; and abstract Nenes Jahrluch fur 

 Miner alogie, 1876, p. 658. 



f MihrosTiopiscTie Physiographie der Massigen Gesteine, 1877, p. 14 et 

 infra. 



