CHROMITE AND PICOTITE. 181 



appears to be due in part to the thickness, and in part to alteration. For 

 example, in some the yellowish-brown and reddish-brown shades are mingled 

 in such a manner that it is evident that the latter shade is due to change in 

 the state of the iron, and marks a stage in the progression towards opacity. 

 In other cases the two tints are so related that the shade is seen to be due 

 to the wedge-shaped form of the fragment examined. 



The translucency is better observed in the powder than in the thin sec- 

 tion, if the mineral tends to be at all opaque, since thinner edges are obtained 

 by the process of fracture. The writer has found the quickest and simjilest 

 way to prepare the powder for microscopic examination, — to place a minute 

 fragment, less than a pin's head in size, on a glass slide,* and then crush it on 

 this slide imder a clean knife-blade. The scattering of the powder can be pre- 

 vented by placing a finger over the blade at the point xmder which the grain 

 lies. In this way, by using a small blade, the finger projects over both sides 

 and serves as a cushion to prevent the broken particles from flying off, gives 

 a more uniform pressure, and saves the production of so much fine dust as to 

 obscure our observations. Thus the powder can be directly examined on the 

 slide on which it was crushed. 



It does not appear practicable at the present time to enter upon any 

 satisfactory discussion of the chemical relations existing between picotite and 

 chromite ; yet, as a contribution towards that desired end, a list of analyses 

 of the two minerals has been arranged in order of the relative percentage 

 of chromic oxide, and it will appear in the list of tables as Table I. One of 

 the difficulties in the way of a satisfactory determination of the relations 

 of chromite and picotite is the absence of analyses made from material care- 

 fully studied microscopically, as well as a like absence of analyses made from 

 material of intermediate grades between the typical picotite and the typical 

 chromite. For it will not escape the observer's attention that the analyses 

 naturally group about these two poles, since typical specimens arc selected 

 for analysis. Another difficulty is the fact that only five analyses of picotite 

 and one of chrompicotite are known, out of the one hundred and twenty 

 analyses here collected. Hence it is that the series of analyses is far from 

 being so continuous as it would probably be found to be if more attention 

 had been paid to the question of the relations of these two minerals. 



The percentage of chromic oxide begins as low as 4.7\ per cent in one 

 chromite, and in the picotites does not rise higher than 8 per cent, while the 



* Any window-glass, if cut of proper size for the stage of the microscope, will do. 



