Summary of the Granitic Structure. 207 
tion from slight bendings of cleavage planes to a complete 
breaking down of the crystal into mosaic structure or lenticular 
augen. Peripheral granulation is the most common type of 
change. Sericite is developed wherever shearing has taken 
place, though when the feldspar is broken up without shearing, 
muscovite usually takes the place of sericite. Chlorite is rare; 
in many instances titanite occurs abundantly, in the feldspar 
especially, though it is found in other minerals as well. The 
biotite, which occurs very sparingly, is almost invariably altered 
to chlorite. When unchanged it is a green color, the pleochro- 
ism parallel to a is yellowish green, parallel to b a deep green. 
The mineral occurs in irregular fragments without crystal faces 
or angles; it is very rich in apatite inclusions, which in turn 
rarely present crystalline development but occur as round grains. 
The cleavage planes of the biotite are filled with iron oxide, 
which stains the mineral a rusty yellow, and makes careful 
study difficult. 
In addition to the three principal minerals mentioned, there 
are a number of accessory minerals, magnetite, ilmenite, titanite, 
rutile, zircon and garnets. The two latter are rare, when the 
garnet does occur, it is usually quite perfectly crystalline, show¬ 
ing sections of the dodecahedron; it occurs in the disturbed 
areas of the granite and is clearly of secondary origin. Rutile 
in fine needlelike crystals occurs very abundantly, some sec¬ 
tions are so filled with it that they have a cloudy appearance. 
The remaining minerals occur only at rare intervals and are 
not sufficiently prominent to require special description. 
SUMMARY OF THE GRANITIC STRUCTURE. 
The granite as a whole is a holocrystalline granular aggre¬ 
gate of quartz, feldspar and biotite, with some secondary mus¬ 
covite, and a number of accessory minerals. The granite is 
porphyritic, plagioclase usually occurring in this form. The 
ground mass consists of quartz and feldspar of medium sized 
grains; these in part interlock, in part are cataclastic in origin, 
arising from the granulation and breaking up either of the 
porphyritic crystals or the larger grains of the ground mass. 
The size of the grains varies greatly, portions of the ground 
