92 JiJ. 8. Dana — Chondrodite from the Tilly-Foster Iron Mine. 



were, of course, destitute of all luster, but they were mostly smooth 

 and large enough to allow of the convenient use of the hand gonio- 

 meter. 



R"^ on 7^" (behind) gave measurements varying, in a series of trials, 

 from 78° to 79", required 79° 4'. 



7t" on R' (behind) gave 62^, required 63° l'. 

 i?" on R^ (l)ehind) gave 72 , required 71 174- 

 Zil' on R" (behind) gave 72 , required 71 17^. 



These angles on both crystals were identical within the allowed 

 error of observation (say 30'). The above are the best angles 

 afforded by any of the planes. 



It is entirely impossible to refer these angles to any of the forms of 

 the second type. When compared with the third type, it is seen 

 that on making the supposition that R' and W (front) are p^ and p* 

 respectively, and R"^ and R^ (behind) are p* and p^, we obtain for: 



p\\ p' (behind)=77° 12'; pV p' (behind)~65'^ 6'; 

 p'^ p' (behind)=:70° 32'; pV p' (behind)=:7l° 37'. 



It will be seen, by comparing these with the ])reviously given angles, 

 that the measured angles correspond much better with the iirst type, 

 and my confidence in them is so great that this would alone be re- 

 garded as sufficient to establish the point ; and that, without refer- 

 ence to the fact that tlie supposed method of occurrence of the third 

 tyj^e planes is contrary to all the laws of the species. 



The decisive proof is derived froTU the fact that both crystals are 

 certainly holohedral, the planes on both sides being similar with the 

 exception of R\ and there is nothing of the obliquity which is ob- 

 served in the hemihedral forms. 



The measured angles of C on _Z?\ right and left, were identical, 



though not obtainable with exactness ; the measurements gave 152^° 



-154° : this is also true for C on R\ right and left,= 140i°-142^°. 



3 



In the first crystal e/'::^ i (035) occurs, and in the other J'^ 



o 



(l-7=r01 1). The occurrence of C is also to be noticed, as it is rare on 

 humite; in fig. 23, the oscillatory combination of 7^"* and R" will be 

 also observed. 



The following table includes the most important angles for the oc- 

 curring })lanes, calculated from the fundamental form of the second 

 type on the assumption that the lateral axes are equal, and the vertical 

 axes have the ratio of 14 : 15. The measured angles are also added 

 thouo"h only approximate ; in the form given they were obtained imme- 

 diately from the measurements over the top of the crystals (see above). 



