i 



W. E. Ford — Ilerderite Crystals from Auburn, Maine. 285 



and narrow truncations. The prism m (110) was usually seen 

 as an extremely narrow face. The pinacoid b (010) was 

 observed on only one or two crystals. The negative pyramid 

 n (331) was commonly small but of good quality. The new 

 negative pyramid h ((534) was ordinarily much etched and 

 curved and in most cases quite minute in size. The other 

 negative pyramids, #(313) and r(121), were of good size, but 

 characteristically striated and curved. 



Figures 4 and 5 show the twin crystals. Figure 4 shows 

 the various forms in characteristic development, but it has 

 been drawn with ideal symmetry. Figtire 5 attempts to show 

 one of the twin crystals according to its actual development. 

 Most frequently, as is shown in figure 5, one of the twin indi- 

 viduals is larger and better developed than the other. The 

 faces in the reentrant angle are commonly etched and more or 

 less curved. Oscillation from one pyramid to another is 

 common. 



The new forms were determined by a comparison of the fol- 

 lowing calculated and measured angles, the theoretical values 

 having been derived from the constants given by Penfield. 

 The measured angles given were derived from the measure- 

 ment of the best crystal. 



Calculated. Measured. 



c(001) : h (634) = 46° 58' 46° 38' 



«(331) : h (634) =23° 44' 23° 51' 



h (634) : h! (034) = 25° 1' 24° 48' 



g (343) : g' (343) = 50° 0' 50° 29' 



The agreement between calculated and measured angles is 

 not as complete as could be desired, but considering the poor 

 quality of the faces is reasonably close. In the case of h (634) 

 the evidence of the three different angles given leaves no 

 doubt but that the correct symbol has been assigned to the 

 form. In the case of g (343) the one angle taken together with 

 the fact that it clearly lies in the zone with b (010) and r. (121), 

 was thought to establish its identity. The rare form, r (121), 

 was established by the following measurement: 



Calculated. Measured. 



r(l21) : r' (121) =69° 56' 69° 56' 



The quality of the crystal faces was variable. In the case 

 of certain forms and on particular crystals the character of the 

 signals obtained on the reflection goniometer was excellent. 

 With other forms, however, on account of their vicinal develop- 

 ment and the striations upon them, the measured angles could 

 only be considered approximate. When the more reliable 

 angles were studied, however, it was found that in general they 

 were intermediate between the theoretical values derived in 



Am. Jour. Sci.— Fourth Series, Vol. XXXII, No. 190.— October, 1911. 

 21 



