Chemical Examination of American Minerals. 71 



in which abundance of blue-colored spinell occurs. About a 

 year ago Mr. Nuttall sent me some specimens from the United 

 States of America. Among others there were three or four 

 specimens of a rock composed of feldspar, quartz, and a green- 

 colored substance, partly in grains, and partly in regular 

 octahedrons, which it was easy to recognize as spinell. On 

 one of these specimens there was also a portion of calcareous 

 spar. No information was given me respecting the locality 

 of this rock ; though there could be no doubt from its con- 

 stituents that it was a primitive rock. The number of grains 

 of spiiull in it was exceedingly great. Mo>t of them were 

 very small round grains. But some of them constituted very 

 beautiful and regular octahedrons, though small. The color 

 was a lovely green, and the crystals were all translucent, and 

 some of them transparent. 



Lustre vitreous, and from splendent to shining. 



From the small size of the grains, the fracture could not 

 be easily recognized. It was probably foliated. 



Not scratched by quartz, but easily by sapphire. 



Specific gravity 4 465. This is much higher than that of 

 spinell commonly observed. I do not know to what to as- 

 cribe the difl'erence. The specific gravity was carefully taken; 

 and any foreign matter that might have been accidentally 

 present, would rathei tend to diminish, than increase the spe- 

 cific gravity. 



The constituents, from a careful analysis, were found to be, 



Silica, - . - _ . 5.620 



Alumina, - - - . . 73'308 



Magnesia, - - - _ . 13-632 



Protoxide of iron, - - .. _ 7-420 



Lime, a trace. 



99-980 



