ALUMINA. 



363 



In an interesting paper on this subject by Dr. Charles T. Jackson, it is shown that there is 

 an insensible gradation from those which have the lowest degree of hardness, to those crystals 

 which have the hardness of andalusite. The following are some of the forms which eire figured 

 by Dr. Jackson, from specimens obtained at Lancaster in Massachusetts.* Figs. 378, 379, 

 and 380. 



Fig. 378. Fig. 379. Fig. 380. 



n u 



Composition. Alumina 60.50, silica 36.50, oxide of iron 4.00 (Bucholz). Alumina 

 60.20, silica 35.30, protoxide of iron 1.32, magnesia 1.00, water 2.03 {Thomson). 



Variety. Chiastolite — Alumina 61 .00, silica 33.00, protoxide of iron 4.00, water 1 .50 

 (Jackson). 



Geological Situation. Andalusite occurs only in primitive strata, especially in mica slate ; 

 also in gneiss and clay slate. 



localities. 



Rockland County. Specimens of this mineral have been found in bowlders of mica slate 

 near Nyack. These bowlders were thought to have been transported from Westchester county. 

 It will undoubtedly be hereafter found in the rocks of Westchester and Rockland, along with 

 staurotide and garnet. 



In Massachusetts, fine specimens have been obtained in the towns of Lancaster and Ster- 

 ling ; and in Connecticut, at Litchfield. 



* Boston Journal of Natural Wttory, I, 55. 



