96 



E. S. Dana — Ghondrodite from the Tilly-Foster Iron Mine. 



section they admitted of good measurements. The mean of thirty 

 determinations of the angle for red rays gave — 



2Ht>'=88° 48' : the extremes being 88** 36' and 89° O'. 



With a yeHow light (sodium) the angle was essentially the same, 

 but tlie mean was 10' or 15' smaller, which would indicate that the 

 dispersion is /3]>?^, but the matter cannot be considered to be beyond 

 doubt. No other dispersion was observed, that is, none parallel or 

 perjDcndicular to the plane of polarization. 



The index of refraction of the oil employed, as determined by Pro- 

 fessor Wright and myself, was 1"466. 



In conclusion, I have to expi-ess my very great obligations to Prof. 

 Allen for his kindness in giving me free use of all the specimens in 

 his valuable cabinet. Both of the crystals of the third type, as well 

 as several others mentioned, came from his collection ; in fact it was 

 Prof. Allen who first made known the special interest connected with 

 the 'locality. To Mr. Cosgriff, the superintendent of the Tilly- 

 Foster Iron Mine, I am also much indebted for his uniform kind- 

 ness and courtesy to me at the several occasions when I have 

 visited the mine ; as also for the gift of several fine specimens. 



