O. W. Hawes—Lithia-hearing variety of Biotite. 431 



slit G is the "gitterplatte." P is a prism of 45°, with its re- 

 fracting edge horizontal, and so placed as to bend upward the 

 pencil of rajs from G, T is the observing telescope, with the 

 eye end elevated at an angle of about 35° so as to receive the 

 rays from G after they pass the prism. 



The spectra of the higher orders so overlap, that without the 

 prism, or some analogous contrivance, it is impossible to observe 

 in them any but a few of the strongest lines. By the prism 

 these spectra are separated, one lying above the other ; the red 

 of the 6th order, for instance, falling below the yellow of the 7th, 

 and this underneath the green of the 8th, while above this 

 green lies the blue of the 9th order, and above that the extreme 

 violet of the 10th. Thus the different spectra no longer inter- 

 fere, and it is just as easy to observe the spectrum of the 8th 

 order as that of the 1st, except that the former is fainter on ac- 

 count of the greater dispersion, and the obliquity of the grating, 

 which narrows the transmitted pencil. A direct-vision prism 

 in the eye-piece of the telescope answers the same purpose, 

 but less perfectly. The same plan may have been used before. 

 If so, however, 1 am not aware of it. 



Hanover, N. H., April 19, 18t6. 



Art. LY.— Contributions from the Sheffield Laboratory of Yale 

 College. No. XL. — On a lAthia-bearing variety of Bix)tite ; by 

 George W. Hawes. 



The feldspar quarries about Portland and Middletown in 

 Connecticut have furnished many interesting minerals. The 

 quarries are in the large granitic veins which intersect the 

 gneiss and mica schist of the region. These veins, which have 

 been described by various authors, are remarkable for the simi- 

 larity of their mineral constituents, and the presence in several 

 of rare elements, and it will be shown in this arti<;le that a 

 lithia-bearing biotite is generally present The feldspar is of 

 two kinds, orthoclase and albite ; considerable quartz is found, 

 and with the feldspar it often forms beautiful specimens of 

 graphic granite. Tourmaline, beryl, garnet, columbite, musco- 

 vite and biotite are common, and other species are found more 

 rarely. Plates of muscovite and biotite united by their edges 

 are of frequent occurrence, and sometimes one forms the center 

 of a large crystal or plate of the other. The cleavage lines which 

 are developed by striking the thin plates with a sharp point 

 show that when the two species are thus united, there is a sim- 

 ple relationship between the axes of the muscovite and biotite, 

 as long since found by Prof. G. Rose to be a general fact in the 



