302 B. K. Emerson—Dyke of Elwolite-syenite in New Jersey. 
they appear as a tangent to the curved surface, and this pierces 
the planes of the small ellipses, passing through each at its cen- 
ter. Any inaccuracy in either drawing interferes with these 
results and is at once manifested in the binocular picture. 
40 W. 40th st., New York, Feb. 25, 1882. 
REFERENCES. 
! This Journal, Nov. and Dec., 1881. 
® Edinburgh Transactions, vol. xv, Part III, p. 360. 
* Brewster on the Stereoscope, London, 1856, pp. 50-100, 
4 This Journal, IT, vols. xx and xxi. 
° Am. Journal of Photography, vol. v, p. 114. 
® Helmholtz, Optique Physiologique, p. 827. 
7 This Journal, Dec., 1881, p. 447. 
. 824. 
* Brewster on the Stereoscope, p. 91. 
‘0 The same, p. 95. 
ely Vow, SD ISL 
© Elementary Physiology, Macmillan & Co., p. 280. 
Art. XXXII.—On a great dyke of Foyaite or Elcolite-syenite, 
cutting the Hudson River Shales in Northwestern New Jersey ; 
by Ben. K. Emerson. 
west of Libertyville, without interruption, to its pouleninae 
uster. The hypersthene is of a greenish black color, of a 
bright metallic luster; it occurs in small and slender crystals 
more or less perfect. : 
“At the southwestern extremity the dyke presents a peculiar 
and striking appearance. It does not occupy a long and high 
hill, with nearly perpendicular slopes, like the northeastern 
part, but, owing to a powerful and rapid disintegration, it has 
