Constituents of various Minerals. 247 



Mr BOWER'S analysis approaches pretty nearly to that of Mr 

 Mum, only he confounded together the alumina and zirconia. 

 He obtained 



Silica, . ,. v V . . . 42.666 



Alumina, 54.111 



Oxide of iron, . . . 1.999 



Water, 0.510 



99.286 



The constituents found by Mr MUIR give us for the chemical 

 constituents of the mineral, 



15 atoms silicate of alumina, 

 3 atoms silicate of zirconia, 

 1 atom silicate of iron. 



Were the silicate of iron to be considered as accidental, Sil- 

 limanite would be a compound of 



5 atoms silicate of alumina, 

 1 atom silicate of zirconia. 



2. Cummingtonite. 



This mineral, likewise, I owe to the kind attention of Mr Nu- 

 TALL. It was found at Cummington, Massachussets, where it 

 occurs in a rock composed of quartz, garnet, and Cummingtonite. 

 Mr NUTALL expressed his suspicion that it would prove merely 

 a variety of Sillimanite. But it is much softer, and in its che- 

 mical constitution is quite different. 



It occurs in fine needles, constituting tufts of crystals, in 

 which the needles diverge slightly from each other. 



Colour greyish-white. Lustre silky. 



