1921 [ NATURAL SCIENCES OF PHILADELPHIA. 453 



saxonite, composed of olivine and enstatite. A pseudo-porphyritic 

 type is found consisting of a light grayish-green serpentine, with 

 small patches rich in magnetite, the particles of which are arranged 

 in definite patterns, indicating a possible derivation from a fer- 

 riferous pyroxene, probably hypersthene. 



The williamsite is a compact, green, translucent, jade-like ser- 

 pentine, which under the hand lens shows a fibrous structure. 

 Under polarized light the rock is seen to be composed of aggregates 

 of fibres polarizing in first order gray and yellow colors (fig. 3). 

 Rarely minute particles of a silver-white undetermined nickel 

 sulfide may be found in the williamsite. Mesh structure is absent 

 and no traces of the parent mineral from which the williamsite is 

 derived occurs. The writer considers the mineral to have been 

 derived from enstatite by hydro thermal metamorphism. 



Analyses of the williamsite are given below, showing it to be a 

 pure serpentine. 



Analyses of Williamsite 



Line Pit, Lancaster County, Penna. Smith and Brush; Am. J. Sci. (2) 15 



213, 1853. 



A B 



SiOj 41.60 42.60 



Al.Oa tr. tr. 



XiO 0.50 0.40 



FeO 3.24 1.62 



MgO 39.71 41.11 



H2O 12 . 70 12. 70 



99.15 99.22 



Chromite. The chromite forms granular black masses, and 

 contains some disseminated williamsite. In very thin sections the 

 mineral shows a reddish translucency, and evidence of fracturing 

 during the period of ssrpentinization (fig. 4). 



- Origin of the Line Pit deposit. It is at once apparent from the 

 form and position of the Line Pit deposit that it was formed later 

 than the surrounding rock. Unfortunately but little is known re- 

 garding the behavior of chromite in igneous melts, beyond the 

 fact that its occurrence is limited to the ultrabasic rocks and that it 

 begins to crystallize early, and the period of its crystallization 

 may overlap that of the enstatite. 



That the chromite deposit had been formed previous to the ser- 

 pentinization is indicated by the evident fracturing of the chromite 

 grains (fig. 4) which occurred during this process. 



