462 Prof. B. Silliman on some American Minerals. 



to be easily scratched with a sharp point of hard steel. The 

 crystalline forms of Sillimanite and kyanite are also identical; 

 the one being derived by the simplest modification from the 

 other. The cleavage in both is in the orthodiagonal. 



It may be vi^orthy of remark that " Andalusite " has the 

 same chemical constitution as kyanite, but belongs to the right 

 rhombic form, while kyanite is oblique. Doubtless a case of 

 dimorphism, and perhaps the same may be said with truth of 

 staurotide. 



My pupil, Mr. George J. Brush, afforded me essential aid 

 in the foregoing investigation. 



V. On the Boltonite of Shepard, and Thomson's 

 BisiLiCATE OF Magnesia. 



The mineral named Boltonite by Prof. Shepard *, is found 

 at Bolton in Massachusetts, in a lime quarry, disseminated in 

 irregular masses, seldom showing any traces of crystalline 

 form. The description of Prof Shepard is quoted below f. 



The changes of colour are peculiar ; and often the same 

 mass, which is dark greenish-gray on one end, will have turned 

 light yellow on the other J. Hardness, 5*50 ; specific gravity, 

 3*008 — the same on two specimens, one dark and one light. 



This mineral, when first found, was called Pyrallolite, and 

 is now so labeled in some old collections. Baron Lederer's 

 cabinet of American minerals, now in the Yale College col- 

 lections, contains eight or ten specimens of this mineral from 

 Bolton, under the name Pyrallolite, which were received, as 

 the catalogue indicates, from Robinson, Shepard, Nuttall, 

 Boyd, and other of the early cultivators of American mine- 

 ralogy. 



In his remarks on this mineral. Prof. Shepard says, it is 

 believed to be identical with the substance described by Dr. 

 Thomson § under the name of" bisilicate of magnesia;" and 

 accordingly the analysis of Dr. Thomson is quoted under 



* Shepard's Treatise on Mineralogy, Newhaven, 1835, vol. i. p. 78. 



t Prof. Shepard's description is as follows — " Massive, composition gra- 

 nular : individuals large, cleavage in one direction pretty distinct, in two 

 others oblique to the first, indistinct, but affording indications of a doubly 

 oblique prism, fracture uneven or small conchoidal. Lustre vitreous. 

 Colour bluish gray, yellowish gray, wax yellow to yellowish white. The 

 darker colours change to yellow on exposure to the weather. Hardness, 

 5-0-60. Gravity 2-8-2-9." 



J Mr. Saemann of Berlin, Prussia, in a paper read before the Am. Assoc. 

 for the Promotion of Science at Cambridge, attributes the change of colour 

 in boltonite to minute grains of magnetic iron found disseminated in the 

 substance of the crystals, which, undergoing change by exposure, leave the 

 mineral of a lighter colour than it was when fresh. 



§ Am, Lye. Nat. Hist., New York, vol. ill, p. 60, 



