302 Rogers — Persistent Parting in a 



scope, and several other specimens were examined as crushed 

 fragments. All of the slides show the parting to be remark- 

 ably uniform in composition. It is made up to the extent of 

 about 90 per cent of a curious vermicular-like mineral occur- 

 ring in apparently well-formed crystals, the remainder being 

 chiefly carbonaceous matter and quartz in small angular frag- 

 ments. The mineral occurs generally in stout, curved crystals 

 with perfect basal cleavage, as is shown in figure 1. It is 

 rather strongly pleochroic, colorless to brown. The extinction 

 is wavy and is parallel to the cleavage. N g is P56 and JN" P 

 is a little over L55 ; the double refraction is "012 or a little 

 less. The mineral is negative in optical character and is 

 biaxial, but the angle, though generally small, varies. 



These characters all correspond closely with those of the 

 mineral leverrierite,* which was first described by P. Termier 

 in 1890. Termier states that the mineral is commonly asso- 

 ciated or even intergrown with black mica, and that it is 

 embedded in a clay groundmass. In some places it occurs 

 associated with eruptive rocks, in others with black carbona- 

 ceous shale. He believes it to be of metamorphic origin and 

 ascribes it to the action on the shale of heated waters bearing 

 silica, lime, and magnesia. 



A sample of the parting was analyzed by R. C. Wells, of the 

 U. S. Geological Survey. The analysis is given below, and 

 with it, for purposes of comparison, are shown the analyses of 

 two samples of leverrierite. 



1 2 3 



Si0 o ... 46-47 46-40 49-90 



f2&[ -.- 37 ' 03 38 ' 40 'mS 



MgO -44 -30 



CaO__ -13 1-20 tr. 



K 2 0._ _ unclet. 1-13 



PA -50 



Ignition 15-67 15-00 8-65 



99-74 101-50 100-65 



1. Parting. Analyst, R C. Wells. 1913. 



2. Leverrierite. Specimen probably impure. f 



3. Leverrierite. Pure material from Rochebelle, France. J 



It will be noted that the analysis of the parting agrees very 

 closely with that of the first sample of leverrierite but that it 

 differs perceptibly from the second which is said to represent 

 material of greater purity.. 



* Termier, P., Ann. d. Min., xvii, p. 372, 1890. 



+ Op. cit. 



JBiill. Soc. Min., xxi, p. 27, 1899. 



