638 
GEOLOGY OF THE FIRST DISTRICT. 
attention was called to this fact by Prof. Bailey of West-Point. The veins, interlaminations, 
beds and masses of granite and sienite, and also of augite and hornblende, with the effects 
they have produced on the contiguous rocks near West-Point, are well worth the attention of 
the geologist, as well as the student of geology. Although I have spent twelve years at 
West-Point, and during most of that time was an ardent explorer of the mineralogy and 
geology of the Highlands for ten miles around, every visit of exploration reveals new and 
interesting facts. 
Quarries, 
The location recommended for a quarry on Stony point, above Coldsprings, which, as 
distinguished from Stony point in Rockland county, has been called Quarry point (Vide pp. 
527 and 528), has been opened, and furnishes a beautiful building stone. The locations 
recommended on Stony point in Rockland county, on Waggon’s islands, on the north shore 
of the Horserace, and other points in the Highlands, will yield also an inexhaustible supply 
of beautiful and indestructible building materials. 
On the variation in the length op our day, : 
DIAMETER. 
A VARIATION IN THE EARTH S 
The times of rotation, velocities, etc., may easily be calculated approximatively from the 
proportions, — : — : : v : v' : : t : f 
f 
r* r'^ 
whence — — L. and f = — 
tr^ 
r'i 
V and v' being the velocities of rotation, 
r and r' “ radii of the sphere, 
t and t' “ times of rotation : 
If r = 3956 miles, r' = 3955 miles, and i = 24 hours; then, 
_ 24 X (3956)^ _ 24 x 244920496804096 __ 5878091923298304 
(3955)^ 
244672945705125 
244672945705125 
= 24 r 12^'42^^'.2. 
The variation of the radius of the earth to the amount of one mile, would, according to 
the above, cause a variation of the time of a revolution on its axis, or of the length of the day, 
of about IL minutes, or, more exactly, of V 12^' 42^^'.2 -f . 
The above formula is not strictly an accurate one to express the conditions of the earth as 
a revolving body; for it is not a sphere, and by an alteration of its radius, its density would 
not remain the same, and some other conditions would cause some slight variations ; but the 
above expression is believed to be the most simple that gives an approximatively true result 
of such changes as have been considered. 
