MINERALOGY AND PETROLOGY. 943 



Andularia. OUpoolase, andesine, etc.) 

 Albite. Labradorite. 



able rock masses, belonging norrnally to the basic series, have been so permeated by 

 the acid elements and stained by hematite that they appear to belong with the red- 

 rock series, and the cause of such an error is attributable to the orthoclastic aspect 

 of the red feldspars, more than to any other feature. 



Adularia, a form of orthoclase, occurs at the old Minong mine, on Isle Royale, 

 where it is associated with calcite, forming a crystalline coating on metallic copper 

 and lining geodes, the adularia being of later date than the calcite (No. 583). The 

 origin of this mineral is probably due to solfataric action during the eruptive activity 

 of the Keweenawan. 



Oligoclase, andesine, etc. These intermediate lime-soda feldspars have a wide 

 range but not great frequency of occurrence. They have been found in numerous 

 acid and intermediate rocks. Oligoclase favors the granites, quartz-porphyries and 

 syenytes. and the acid metamorphic rocks, but andesine is more allied to the basic 

 series (No. 300) . The former is more frequent in the acid mica schists and the latter 

 in hornblendic. They occur in some of the conglomerates, especially those of Keke- 

 quabic lake (Nos. 1061, 1062), and in the esterellyte of that vicinity (Nos. 1094, 1399). 

 Oligoclase is very often associated with orthoclase. Andesine has been found in the 

 clastic greenstones (No. 1367C). 



Albite. The pure soda feldspar has been identified but rarely (Nos. 403, 872, 

 2102 and 2243). Some part of the white feldspar in a coarse pegmatyte (No. 1997) 

 is albite or oligoclase-albite. It is there associated with microcline. In all cases it 

 seems to be a secondary mineral. It takes part in the formation of epidosyte 

 (No. 842). 



Labradorite in its broader sense is the prevalent feldspar of the basic igneous 

 rocks, especially of the Keweenawan. Indeed, it may be said that in normal con- 

 ditions it is the sole plagioclase. When other feldspars have been identified in the 

 diabases or gabbro of the Keweenawan (No. 222) it has been in nearly all instances 

 under circumstances warranting the presumption of endomorphism resulting from 

 contact on the elastics, and this is especially true of the occurrence of andesine or of 

 oligoclase and orthoclase. In several instances the feldspar of the Keweenawan 

 diabases has been identified as bytownite (Nos. 810, 814) or labrador-bytownite, 

 these determinations having been indicated by the extinction angles and not by 

 chemical analyses. In such determinations throughout this work great reliance has 

 been placed on the extinction angles in sections cut perpendicular to the bisectrices 

 as established by Dr. Fouque of the College de France.* An unequivocal occurrence 

 of anorthite in the Keweenawan has been so rare that it produces little or no effect 

 on the proper description of the Keweenawan basic rocks. 



"Contribution 4 l'tude des feldspaths des roches volcaniques. Bulletin tie la finciflf Fram.nix, ,!< tfhlfraiogie. Tome 17, p. 

 283,1894. 



