94 E. S. Dana— Chondrodite from the TUhj-Foster Iron Mine. 



when unbroken it must have been nearly an inch in length. In the 

 condition in which it was found it showed only the brachydomes e' 

 ande^, with the pyramids ?^^, n^, and m^ ; it had the deep garnet-red 

 color of crystals of the second type, and with the exception of the uni- 

 versally i^resent fractures was perfectly clear and transparent. 



On the optical jDroperties of the mineral hi question, we have, as 

 far as I have been able to find, no information except what has been 

 giveu by DesCloizeaux, Manuel de Mineralogie, 1862, p. 141. He 

 says : " Double refraction energetic ; positive bisectrix normal to ^ * ; 

 optic-axes situated in the jdane parallel to the base ; divergence in 

 oil for red and yellow rays, 82° 14'. Dispersion almost nothing; 



Guided by the above, a section was cut from the crystal described, 

 which was pai-allel to C', i. e., perpendicular to the brachydomes pres- 

 ent. The examination of this section showed : i st, that the acute 

 bisectrix is normal to G (i-i, 010) ; 2d, that this bisectrix is positive ; 

 3d, that the optic-axial angle is large, the axes being seen only when 

 oil is used ; but 4th, that the axes do not lie in the hasal jjlane, but in 

 a plane making an angle of about 15 t° with it. This last point was 

 so unexpected and anomalous that every effort was made to explain 

 the measiirements in soine other way, but with no success. The 

 planes on the crystal had been carefully measured, before the slicing, 

 and the angles agi-eed perfectly with those of type II for the planes 

 mentioned, so that it was impossible to assume that the crystal had 

 not been correctly put into position. By means of a staiiroscope, 

 made by Fuess in Berlin, after the excellent pattern of Groth,* the posi- 

 tion of the two axes of polarization, as referred to e^, and also to e^ in 

 plane C, were carefully determined. The measurements were repeated 

 twenty times, the error arising from an imperfect adjustment of the 

 Nicols being eliminated in the usual mannei*. The result was as 

 follows : 



Supplement angle made by the plane of the axes — 



with e^(|-^=r203), 18° 9' ; hence with the basal plane, (J, 25" 50'. 

 withe^(2-^=201), 45° 9'; " " « " 25° 46'. 



In order to confirm these results, other crystals were sought, which 

 would admit of like determinations. None could be found which 

 would serve for measm-ing the axial angle ; but two small ones, on 

 which the plane C was naturally developed, proved to be clear 

 enough to allow of measurements with the stauroscope. The first 



*See Pogg. Ann., cxliv, 34, 1870. 



