24:0 Ford — Neptunite Crystals from San Benito Co., Col. 



obtained from the section parallel to (010) might be explained 

 by the fact that often in biaxial crystals what appears to be an 

 interference figure with a large axial angle can be obtained 

 from sections that are cut parallel to the axial plane. 



Attempts were made to measure the indices of refraction of 

 the mineral, but on account of the dark color of the material 

 they failed. The average index of refraction was obtained in 

 the following manner : Small fragments were observed under 

 the microscope immersed in various mixtures of a monobrom- 

 napthalene and methylene iodide until one was obtained which 

 by use of the Becke method was found to be very closely of 

 the same index of refraction as the neptunite. The index of 

 refraction of the liquid was then determined by immersing in 

 it fragments of other minerals with knowm indices of refrac- 

 tion until the same result was obtained. In this way the 

 average index of refraction of the neptunite was found to be 

 close to 1*70. 



An approximate measurement of the axial angle was made 

 by immersing the section cut perpendicular to the acute bisec- 

 trix in a monobrom-napthalene (^=1*6583) and using a small 

 axial angle goniometer placed on the stage of the microscope. 

 The measured angle (2H) was approximately 50°, which w T ould 

 give 2E = 90°. Assuming that /3=1*7, this would give the 

 axial angle 2Y=48°40 / . Dispersion of the optical axes was 

 noted, v >p. 



Chemical. — A qualitative examination of the California 

 neptunite shows its substantial agreement with the analyses 

 given of the Greenland material. It is hoped shortly to be 

 able to make a quantitative analysis and to present its results 

 in a subsequent paper. 



Mineralogical Laboratory of the Sheffield Scientific School of Yale Uni- 

 versity, New Haven, Conn., Jan. 29, 1909. 



