206 TRANSACTIONS OF THE [Apr. 24 
microperthite have been described by Klockmann*, Lehmannf, 
Lacroix}, Romberg§, Hobbs||, and others. The fact that the 
secondary feldspar 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 
lamelle 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 dusty. 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. Gesella XXXIYV., p. 381. 
t “ Untersuchungen ueber die Entstehung d. Altkrystal. Schiefer,” p. 217. 
+ Bull. Soe. Min. Fr, IX., p. 131. 
§ Neues Jahrbuch fuer Mineralogie, ete., B. B. VIII., p. 300. 
| Bull. Geol. Soc. America, IY., p. 171. 
