OF ARTS AND SCIENCES. 61 



It will be seen from the above analyses that, although the atomic 

 ratio between all the basic radicals and the silicon is the same as in 

 Culsageeite, Jefferisite, and Biotite, the ratio between the protoxide 

 and sesquioxide radicals is very ditterent. In this respect the mineral 

 resembles the phlogopite micas, in which also the protoxide radicals 

 preponderate ; and the symbol given above for Hallite, less the water, 

 is identical with that given by Professor Dana as the more probable 

 formula of the phlogopites. 



The opacity produced by the interspersed material made it difficult 

 to determine the optical characters of the mineral, as the rings produced 

 with polarized light could only be seen with very thin plates, and the 

 cross was therefore ill defined ; so that, although in some cases there 

 appeared to be a separation of the hyperbolas, the plates could not be 

 distinguished from uniaxial. On one specimen the hexagonal form was 

 very perfect, and the crystal presented the planes of a rhombohedron 

 having an angle over the basal edge of about 122°, resembhng the 

 crystals of Biotite from Greenwood Furnace. Mr. Hall informs me 

 that these more perfect crystals have only been found in one pocket of 

 the serpentine. 



The distinction, however, between the phlogopites and the biotites is 

 not fundamental, either chemically or physically. Chemically, both 

 species are orthosilicates ; that is, the atomic ratio between the silicon 

 and the sum of the basic radicals is 1:1. The species differ in com- 

 position only in the relative proportion of the sesquioxide and protoxide 



II VI 



radicals. In the phlogopite the ratio of i? to R is probably normally 

 2:1; but of the published analyses the value varies between that ratio 

 and the ratio 3:2. In the biotites the same ratio is probably normally 

 1:1; but here, again, the different analyses which have been made give 

 values varying between 5 : 3 and 1:2. In hke manner the optical 

 distinction between the phlogopites and biotites, of which so much has 

 been made, is equally indefinite. Between a so-called phlogopite, like 

 that from Jefferson County, N. Y., with an angle of about 15", and the 

 apparently uniaxial plates of biotite from Vesuvius, there is every possible 

 gradation — sometimes, as I have shown, on one and the same mica plate ; 

 and I have endeavored in this paper to explain the cause of this varia- 

 tion. With the Vesuvian biotites themselves, — if the specimens in the 

 miueralogical cabinet of Harvard College are fair representatives of 

 the mineral from this locality, — it is only occasionally that we find a 

 perfectly uniaxial plate. More commonly there are distinct evidences 

 of twinning, and on the borders of the hexagonal plate may be dis- 



