78 E. S. Dana — CJiondrodite fro^n the Tilly -Foster Iron Mine. 



was not an easy task. The symbols were calculated from the angles 

 thus obtained. But as even then some doubt remained as to the de- 

 gree of dependence which was to be placed in them, the measure- 

 ments were all repeated with the same care as before. Tlie result 

 was perfectly satisfactory, as the variation in no case exceeded tlie 

 probable error of observation given to each angle when measured for 

 the first time. This variation in most cases did not exceed ±2'. 

 The following supplement angles were obtained for the more promi- 

 nent planes: C /^r'^ gave 33° 33' (required 33° 32f ) ; and C/^r* 

 gave 25° 53' (required 25° 58'); and r^^r^ gave 31° 31' (required 31° 

 38'). The following table contains the angles for each of these mi- 

 nute planes as measured on 6', and r"^ and also on ^-, itself one of 

 this group. 



Table VI. 



