E. S. Dana — Choudrodite from the T'llli/- Foster Iron Jfi/ie. 77 



toward the eye the projection gives it but little width. As a mat- 

 ter of fact the crystals have C uniformly well developed and are 

 generally attached approximately by the extremity of the bracliy- 

 diagonal axis. This having been explained, it will be clear that wliilc 

 tig. 3 is an almost exact rei)roduction of an actual crystal, Hg. 5, by 

 the other method of projection, gives an entirely false idea of its 

 ap])carance. It is certainly true that the latter method shows the 

 hemihedrism in its true light, but this should not weigh against the 

 other more important consideration. 



The crystals from Avhich the partial figures, 7, 10, 16, 17, were drawn 

 were united, along with others quite as diverse, in one small fragment 

 only half an inch in length. It is to be noted that figures 14 and 15 

 are really more different than would appear at first glance. The 

 crystals drawn in figures 16 and 19 also occurred closely conjoined 

 in the same group ; and other examples might be mentioned. One 

 crystal of a very prismatic appearance (when placed in an inverted 

 position) is shown in figure 1 9. 



Presence of minute />/«>* ^^s. — The most remarkable feature of the 

 mineral from this locality is yet to be mentioned. I refer to the mul- 

 titude of minute planes which modify many of the solid angles. 

 One single case will be discussed in detail, as the planes admitted of 

 more than usually exact determination ; it serves well to illustrate 

 the subject. A horizontal projection of a portion of the crystal is 

 shown in fig. 14. The crystal itself was small, and unfortunately so 

 imbedded in dolomite that it was for the most part rough and be- 

 yond even approximate measurements. The part available shoAved 

 G (i-1) faultless; also r^ good; and less satisfactory r^, r^ and 

 r*. On the solid angle between 6', r^ and r^ a large number of 

 miniite planes were observed ; they were so extremely small (all 

 covering a surface not "OS of an inch in breadth) that any exact 

 measurements seemed at first hopeless. They were sharply defined, 

 however, and brilliant, and when the attempt was made it was found 

 that they gave perfectly distinct though fiiint reflections. It may be 

 remarked here that measurements in this case were only made possi- 

 ble by the substitution of a cross, cut in tin foil and illuminated very 

 brilliantly by a gas l)urner, for the ordinary spider lines in the second 

 telescope — a device for Avhich I am indebted to Prof Schrauf of 

 Vienna.* 



The measurements were all made with the greatest care, though, as 

 will be readily ixnderstood, the exact adjustment of planes so small 



* Ber. Ak. Wien, Ixvii, 1873. 



