206 TRANSACTIONS OF THE [ApR. 24 



niicroperthite have been described by Klockmann*, Lebmannf, 

 Lacroix|, Komberg§, Hobbs||, and others. The fact that the 

 secondary feklspar fills cracks in the rocks shows that at least 

 part of the microperthite has been formed subsequent to the 

 period of dynamo-metamorphism. On the other hand, no 

 instance has yet been found where the secondary feldspar has 

 itself been fractured. Thus, so far as it goes, the evidence is 

 in favor of the supposition that the formation of microperthite 

 is a chemical process, which goes on in great part subsequent 

 to fracturing and crushing of the rock. But whenever formed, 

 microperthite is now so abundant that its development must be 

 regarded as one of the most important factors in the history of 

 the rock. 



Like the quartz, the feldspar gives evidence of mechanical 

 deformation in a more or less complete shattering, in undula- 

 tory extinction, and in the bending and breaking of the twinning 

 lamella: of the plagioclase. 



Quartz and feldspar together make up a large part of the 

 ordinary gneiss, the other constituents being present in much 

 smaller proportions. Of the ferro-magnesian minerals biotite 

 is rather the most abundant in the slides examined, with horn- 

 blende approaching it in quantity. The biotite forms very 

 irregular masses, with the usual strong pleochroism. It is 

 characterized by an abundance of pleochroic halos, which are 

 usually quite independent of perceptible inclusions. It shows 

 a strong tendency toward alteration, the color changing to 

 green, while the surface becomes diisty. A complete alteration 

 to chlorite is, however, rarely apparent. 



Hornblende forms irregular masses, sometimes prismatic, of 

 dark green color with decided pleochroism. Except in one or 

 two sharply defined cases, there is nothing to indicate that the 

 hornblende may be uralitic. It is evidently one of the oldest 

 of rock constituents. 



In three or four sections of the ordinary gneiss pyroxene 

 occurs in irregular grains, but is not of sufficient importance to 

 be regarded as an essential mineral. 



Iron oxides are represented by finely divided hematite in the 

 red feldspars; by limonite in cracks and fissures ; by magnetite 



* Zeits. Deutsch. Geolog. Geselk XXXIV., p. 381. 



I " Untersuehungen ueber die Entstehung d. Altkrystal. Schiefer," p. 217. 

 t Bull. Soc. Min. Fr. IX., p. 131. 



§ Neues Jahibuch fuer Mineralogie, etc., B. B. YIII., p. 300. 



II Bull. Geol. Soc. America, IV.. p. 171. 



