56 Dr B. Silliman, Junior's, Optical Examination 



Moriah, of Sterling, of Franklin, and of Gouverneur, have 

 successively been thrown out of the list of biotites into the 

 binaxial species. Others remain in doubt as those . of 

 Topsham, Me., and Easton, Pa. 



Greenwood Furnace, Monroe, Orange County, N. Y., is the 

 locality of a very remarkable and perfectly crystallized 

 biotite. It is figured by Beck,* and fully described also by 

 Dana,t and it appears to have been analyzed by von Kobell, J 

 as quoted by both the authors just cited. The remarks made 

 on a previous page respecting the equivocal optical character 

 of some of the micas apply to this variety also. It is opaque 

 in plates over one-twelfth of an inch thick, but of rich olive 

 green in thin plates. 



The figure of its rings under polarized light is decidedly 

 elliptical, but not so much so perhaps as to require any other 

 explanation than the remark of Biot, before quoted, respect- 

 ing the power of thin plates to disturb the ray and produce 

 the effect of binaxial structure in a uniaxial crystal. The 

 author has a new analysis of this variety in progress, the 

 results of which he will present on a future occasion. The 

 chemical constitution of very few American biotites has been 

 examined : indeed the locality just referred to is the only one 

 cited. Von Kobell's result corresponds with the formula 

 given on page 375. 



Mica of Monroe, N.Y, — This mica from the same town as 

 the last is also uniaxial and gives a figure almost entirely 

 circular. It is distinguished by its bronze lustre and dark 

 greenish olive colour inclining to grey or black. It is im- 

 perfectly transparent, having by transmitted light a dusky 

 or hazy appearance. It occurs in plates of immense size, 

 which are marked on the cleavage surfaces with rhombic and 

 triangular figures (no distinct lateral planes have, however, 

 been found), as well as with transverse cleavage lines. It is 

 slightly elastic but very tough. One specimen which the 

 author has seen in the cabinet of Mr C. M. Wheatley in New 

 York is nearly two feet in diameter. Mr Horton of Monroe 

 has also furnished the writer with numerous very large speci- 



* Min., N. Y., p. 37. t Min. (1850), 360. J Kastn. Archv., xii., 29. 



