180 PERIDOTITE. 



Troup Co., Georgia; Dudleyville, Alabama; California; near New Idria, 

 Monterey Co., California ; Ural Mountains ; and Euboea, Greece. Another 

 specimen from California had a yellowish and greenish-brown color, while 

 one from Sweden, when very thin, was of a deej)-brown coffee-color. 



The seven following chromites were sent by Dr. Dana. Those from 

 Lancaster Co., Pennsylvania ; Cecil Co., Maryland ; FrankHn, North Carolina ; 

 and Bisersk, Ural Mountains, have a yellowish-brown color. Two specimens 

 from Texas, Pennsylvania, require to be very thin to become translucent, 

 and are of a yellowish-brown to reddish-brown color. One from Jamaica, 

 West Indies, is greenish-yellow in its apparently freshest state, but in the 

 partially changed condition it is reddish-brown. 



A chromite obtained from Mr. Kerr, Commissioner from North Carolina 

 at the New England Fair of 1883, in thin splinters is of a clear coffee-brown 

 color with a greenish tinge. Its fracture is smooth, and presents a surface 

 closely resembling a hardened black gum or pitch — for example, albertite — 

 and has a lustre varying from resinous to vitreous. This was chipped from 

 a large block sent to that exhibition. Most of the chromites examined have 

 a pitchy or resinous look, with a velvet-black color closely resembling solid 

 coal tar; but some are dull. While it may be said that in general more or 

 less translucency exists in the powder of chromite, this apparently resides 

 only in certain portions of the mineral, which is not translucent as a 

 whole. 



So far as the writer is aware, no tests have been made to compare the 

 relative hardness of picotite and chromite, but the former has been assumed 

 to have that of the normal spinel. In the same way the color of the streak 

 of picotite does not appear to -be known for itself ; while that of chromite 

 would seem to be due to its translucency. 



In specific gravity the two minerals bear close relations. Chromite varies 

 in its specific gravity as follows: 4.031, 4.0639, 4.11, 4.115, 4.1647, 4.319, 

 4.422, 4.439, 4.49, 4.50, 4.534, 4.56, 4.566, and 4.568; while the only deter- 

 mination of picotite found, places it at 4.08. 



Both minerals have the same crystallographic form, the same color to their 

 thin sections, and the same color and lustre in the massive state. 



The term coffee-hroimi as used in this text and in the writings of others 

 partakes of the same variability in shade that the infusion of coffee itself does, 

 running from a yellowish or greenish-brown through a reddish-brown to a 

 deep dirty- or muddy-brown. The depth of color, even in the same specimen, 



