OF ARTS AND SCIENCES. 45 



the mineral occurs in several veins or pockets of the serpentine, not 

 more than fifty yards from the quarry where the ripidolite is found. 

 It is associated with oligoclase ; and corundum also occurs in the same 

 serpentine formation, iu Newton township, ten miles distant, on the 

 north edge of the serpentine ridges. The JeiFerisite is found in broad 

 crystals, or ciystalline plates, which frequently have a distinct hexagonal 

 form. On two of these plates sent me by Mr, Jefferis, measuring respect- 

 ively two inches and three-fourths of an inch in diameter, the form is 

 as sharp as on crystals of Muscovite, and the edges make an angle of 

 120", as nearly as it can be measured. Mr. Jefferis writes, — '"In 

 regard to the Jetierisite crystals which you have examined, the large 

 crystals or plates occurred in the vein immediately under the sod, 

 exjiosed to all frost, &c. ; the small crystals were taken out four or five 

 feet below, in the decomposed Deweylite, and were associated with 

 striated oligoclase, of which I sent you specimens." 



The crystals of JeiFerisite cleave like mica, afi^ording thin but un- 

 elastic foliie. The cleavage ])lanes are marked triangularly by lines, 

 crossing at angles of 60° and 120°. In some cases there is a jointing 

 as in crystals of mica, parallel to the shorter diagonal of the rhomb. 

 One crystal sent me by Mr, Jefferis is the half of a rough hexagonal 

 prism, an inch and a quarter high by two inches in diameter. The 

 plane of the optical axes, as in the larger number j,.^ ^ 



of micas, is parallel to one of these lines, as , ^ 



indicated in Fig. 1, coinciding with the shorter 

 diagonal of the rhombic prism, which appears 

 to be the fundamental form in all this class of 

 minerals, and from which the hexagonal form is 

 derived by the truncation of the two acute angles. 

 The double refraction is strongly negative, but ' T? 



the angle between the optical axes varies in the most remarkable 

 manner. I have measured angles on different plates, of 27°, 2-1°, 

 and 10°, and observed many intermediate conditions. Owing to 

 the deep yellow color, the plates become opaque at a very moderate 

 thickness, and for this reason it is impossible to measure the angle 

 with great precision. Some of the plates are apparently uniaxial; 

 but this may result from the blending of the two hyperbolas, due to 

 the thinness of the plate. The dispersion of the axes is but slight, 

 and only perceptible in the thicker laminae when Q <. v. It is obvious, 

 therefore, that the crystallographic characters of the mineral are 

 identical with those of mica. 



The plates are generally, if not invariably, twinned, and the twin- 



