344 Dr B. Silliman, Junior's Optical Examination 



particularly in northern New York, in Canada, and in Morris 

 and Sussex counties in New Jersey. One of the most noted 

 localities of this species is Edwards, in St Lawrence county, 

 N. Y., where it is found both colourless, of an eminent silvery 

 lustre, and also of a rich brownish-yellow colour. 



4. Biotite. — This species includes the uniaxial or hexagonal 

 micas. Most of the varieties of this species are of a dark 

 colour, — often black or greenish-black, and transparent only 

 in very thin laminae. Owing to this prevalent dark colour, 

 it is often difficult or quite impossible to obtain satisfactory 

 evidence of the optical character ; and there is little doubt 

 that some localities quoted in this article as furnishing uni- 

 axial micas, should be in fact classed among the phlogopites. 

 Only one American variety of this species has yet been ana- 

 lyzed, viz., that from Monroe, N. Y., by Von Kobell. They 

 are generally magnesian micaSj and have for the oxygen ratio 

 of their protoxides, alumina and silica, the ratio 1:1:2 = 

 R^ Si + Il Si. This species, and those anomalous specimens 

 which are classed under it in the present article, but which 

 probably belong elsewhere, offer interesting subjects for 

 chemical examination. 



Beside the phlogopites and biotites, properly so called, there 

 are several micas which have fallen under my observation in 

 this research, which are anomalous in character. These 

 present under the influence of polarized light an elliptical 

 coloured image, in which, however, it is not possible to bring 

 out clearly the two poles of a binaxial mica, nor, on the 

 other hand, the symmetrical cross of a uniaxial crystal. The 

 divergence is too constant and too regular to allow the sup- 

 position that the ellipticity is due to a mal-position of the 

 laminae, or to a separation between the thin plates (remarked 

 on as a cause of irregularity in certain crystals by Biot). 

 The divergence of the axes in these exceptional cases is too 

 decided not to attract the attention of the experienced ob- 

 server, and still these specimens would probably, by most 

 persons, be set down as uniaxial, especially in thin plates. 

 Indeed, many phlogopites, when viewed in thin plates, ap- 

 pear so nearly uniaxial as scarcely to excite attention to their 

 binaxial character, while in plates of suitable thickness they 



