104 MINERALOGY AND LITHOLOGY. 



As localities that are of note for black tourmaline, may be noted Graf- 

 ton (small, stellated forms of great beauty, and single fine crystals ; com- 

 municated by Mr. A. Brown), Sullivan, Unity, Newington, Barrington, 

 Bedford, Moosilauke mountain, Hinsdale, Chichester, Goshen, Lyme, 

 Moultonborough, Saddleback mountain. White Mountain Notch (very 

 large), Monadnock mountain, Surry, and Mt. Kearsarge. Pretty speci- 

 mens, formed by needles of black tourmaline piercing white quartz, have 

 been found at Hanover, Gilmanton, and Haverhill, A mineral, thought 

 to be the kind of hornblende called bentonite, which is found at Leba- 

 non, has been shown by Pisani of Paris to be bladed tourmaline. 



The finest tourmalines that are taken from the granitic veins, occur 

 in quartz; but the mineral is also found associated with the feldspar and 

 mica. When in the mica the crystals are usually flattened out into 

 blades, and these blades are often so thin that they are translucent, and 

 can be used as polarizers (see p. 85). I think that good specimens for 

 the making of a polariscope could be obtained from Grafton. 



The difference of color in tourmalines is much dependent on the 

 percentage of iron they contain, which is quite variable. The black 

 tourmalines contain much iron, and the brown tourmalines, little. The 

 composition of our tourmalines is well illustrated by the two following 

 analyses made by Rammelsberg.* The first one is of brown tourmaline 

 from Orford, and the second is of the black tourmaline from Unity : 



Silica, 38.33 36.29 



Boric acid, 9.86 9.04 



Alumina, 33.15 30-44 



Iron protoxide, 2.88 i3-~3 



Magnesia, 10.89 ^-32 



Lime, .77 i-02 



Soda and potash, 1.52 1.94 



Water 2.81 1.72 



100.21 100.00 



These analyses agree with the formula given above; and the great dif- 

 ference in the amounts of iron contained in the brown and black varieties 

 will be noticed. The blue and green varieties, like those at Hinsdale, 



* Ilandiiich tier Mineral Chcmic, 1875, p. 541. 



