22 ANNALS NEW YORK ACADEMY OF SCIENCES 



spar, which makes up perhaps two fifths, is generally orthoclase, with 

 varying amounts of microcline and plagioclase. The extinction angles 

 of the latter show it to be albite and oligoclase. In all of the sections 

 examined, the feldspar was considerably altered to kaolin, this being 

 much more striking than in the other members of the series. Epidote 

 also occasionally forms in small amount through the feldspar. In the 

 four thin sections of granite from the quarries, the alteration of the 

 plagioclase was especially apparent, as was also the case in the rock from 

 the drill core ; in the surface specimens, however, the orthoclase seems to 

 be the more kaolinized. Several cases of zonal alteration in orthoclase 

 were noticed. Muscovite occurs in typical habit ; the grains vary consid- 

 erably in size but are seldom as large as the quartzes. It is altered only 

 in the surface specimens; in these, it is hydrated and appears to yield 

 damourite. It is usually much more abundant than the biotite. The 

 latter is seldom entirely fresh and is often altered to chlorite. Horn- 

 blende is rare, and when found is generally altered. Magnetite is pres- 

 ent in very small quantity; apatite and zircon are practically lacking. 



In the diamond drill hole above referred to, which was sunk to 425 feet 

 for water at a point half a mile east of Jacob's Hill, the ordinary white 

 granite was first encountered ; this changed gradually to a somewhat finer 

 grained bluish gray-white granite carrying a larger percentage of quartz. 

 Through this, several streaks of a basic hornblendic segregation, a foot 

 or so across, were found. 



At several points, the granite is quarried as a building stone. The 

 three cuts to the northeast are known as the Mohegan Quarry, the one 

 to the south as the Peekskill or Cornell. The latter is the largest; from 

 it was taken the rock out of which the bulk of the Cornell Dam across 

 the Croton Eiver was constructed. The granite from the Mohegan 

 quarry is used as the chief building stone in the Cathedral of St. John 

 the Divine, in ISTew York City, which when completed will be the fourth 

 church in size in the world. The stone has also been employed in several 

 other edifices in the city. While the alteration of the granite is consider- 

 able, as above described, it is not serious in character ; and the rock com- 

 mends itself on account of the paucity of the dark minerals. The altera- 

 tion of the small amount of biotite has proceeded as far as it is likely 

 to; the muscovite and orthoclase are fairly stable minerals; and the 

 plagioclase is not abundant, while the large amount of quartz insures 

 the rock against serious alteration. Pyrite and other notably deleterious 

 constituents are entirely lacking. The rock is firm and easily worked, 

 although the abundance of joints of all sizes militates against the ex- 

 traction of large blocks. The actual amount of granite taken out varies, 



