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



larger portion as untrustwortliy, retaining those which had given the 

 best angles. It is not pretended that the symbols deduced are, even 

 in the majority of cases, correct beyond question ; for the angles, 

 while perfectly reliable in some cases, are in others somewhat uncer- 

 tain, and for reasons already explained this throws still greater doubt 

 over the indices which calculation may produce. 



Table VIII. 



675 

 5-11-6 

 14-15 -4 

 4-17-3 

 2-10-11 



641 



259 

 579 

 689 



C= w(OlO). 

 Meas. Calc. 



A= (001). 

 Meas. Calc. 



65° 25'65° 51' 



72 13 ,72 15 



■i 



82 45 i82 40 



83 55 



83 44 



55 44 55 28 



84 37 



84 40 



The group of planes clustered about ^^ has already been de- 

 scribed. It is interesting to note that in two instances analogous 

 groups were observed, of which ^(l-i = Oil) was a member (see fig. 

 15), and in two other cases the common and prominent planes ^i^ (fig. 

 15) and vi^ (fig. 17) were found in the same relations. The angles 

 obtained for these planes show conclusively the degree of dependence 

 to be placed on those measured for the other planes. It will be 

 remembered that in all cases these planes were exceedingly minute. 



The fact already mentioned, that all of these planes belong to the 

 macrodiagonal series, may possibly be explained in part by the fact 

 that it is uniformly that portion of the crystal (i. e., near 6', i l) 

 which is exposed and well developed. There still remains the fact, 

 wliich will be noticed by a glance at the spherical projection (plate 

 xiii) and which does not allow of an analogous explanation, that 



