SIENITE. 
529 
ble quarries of this rock may be opened, if it can be brought into notice and use. It is a 
durable and beautiful stone, and may be procured in large blocks, but it is more labor to 
dress it than many of the granites and sienites sent to market. 
The same rock, east of Verplanck, is changed to a black hornblende rock, with but little 
felspar, and it contains magnetic oxide of iron disseminated in grains. One mile east of Ver¬ 
planck, it is coarsely crystalline and jet black. Beautiful specimens of hornblende may be 
obtained there. The jet black color of this mineral has caused excavations to be made in 
these hills to find coal;* a search that every one, who knows any thing of the associations of 
minerals with each other, would at once know was perfectly fruitless. The great weight of 
the hornblende has induced others to suppose that iron ore would be found here, which is 
possible, though no workable beds have been as yet discovered. In many places there is so 
much magnetic oxide of iron in the hornblende, that the magnetic needle will not traverse 
freely in the vicinity of the beds. Where the hornblende is decomposed, a deep red soil is 
produced, colored by the oxide of iron, and which is highly productive under judicious treat¬ 
ment. 
The sienite rock of the Highlands is of two kinds. One is a coarse granitic aggregate of 
white or reddish felspar and black hornblende, sometimes also containing epidote and grains 
of magnetic oxide of iron, like that at the base of Bull hill one and a half miles north-north¬ 
west of Coldspring village, on the shore of the Hudson; and at the Target rock on Constitu¬ 
tion island, opposite West-Point; the other is composed mostly of felspar of a dark greenish 
or sometimes yellowish and brownish color, with some quartz and hornblende. The latter is 
black or green, and sometimes passes into that described under the name of hornblende rock, 
where the hornblende is arranged in stripes through the rock. The felspar in this kind of 
sienite is occasionally opalescent, but is distinct in characters from that from the north part 
of the State, and which is seen in boulders and blocks on the slopes of the mountains in the 
Highlands. 
3. Gneiss. 
Gneiss is the predominant rock in New-York, Westchester and Putnam counties. It varies 
greatly in external aspect and in composition, in different parts of the tract under investiga¬ 
tion. Its color is dependent upon the relative abundance of its constituents, which are vari¬ 
ously colored in different localities. The felspar is white, reddish, or of a bluish grey; the 
mica is black, brown, yellow, copper-colored, and white ; the quartz is white, grey or smoky. 
In some places mica abounds in the rock, and it approaches to mica slate, but more commonly 
the felspar is most abundant, and gives character to the rock. 
* Another excavation for coal was made near the landing at Peekskill. It was in gneiss in which there was a seam of 
plumbago, or black lead, mixed with pyrites. It would burn for a time, in consequence of the carbon and sulphur, but 
was not adapted for a combustible, even had there been an abundance of it. A great noise was made in the papers for a 
time, but the miners soon lost the vein by sinking the shaft vertically, while the seam of plumbago followed the layers of 
the strata parallel to the dip. 
Geol. 1st Hist. 
67 
