IGG PROCEEDINGS OF THE AMERICAN ACADEMY. 



and (l.n.T.ll) as negative but left the determination of sign doubtful. 

 The second of these forms is common to all the crystals from Fallon 

 quarry and is certainly positive, so both the others should be likewise 

 so considered. Several negative rhombohedrons new to ilmenite were 

 observed and are based on the following data : 



Form. Calculated. Measured . No. Face limits of p. 



(f> p <f> p 



g (0.3.5.11) 30 00' 23° 34' 30 00' 23^46' 3 23° 28' to 24° 05' 

 k (0. .3.5. 10) " 25 37 " 25 34 1 

 A (0445) " 51 59 " 51 57 2 51 53 to 52 00 



\ (05o2) 30 00' 75 57 30 00' 76 06 1 



Limits of <^ 

 8 (2l50) 10 53 90 00 10 30 90 00 4 10° 36' to 11° 34 



The presence on one crystal of the form f, observed before only by 

 Solly on a Binnenthal crystal, confirms this form. All the forms pres- 

 ent gave angles agreeing very closely with the values calculated from 

 the axial ratio of Koksharow as used by Dana. 



Chemical tests on the ilmenite from both quarries revealed strong 

 qualitative reactions for manganese ; an analysis would be interesting 

 but it was not possible to separate enough of the fresh mineral for this 

 purpose. 



Octahedrite. — Octahedrite is found chiefly in the large central pocket 

 of the Fallon pegmatite, generally in close association with aegirite and 

 often formed posterior to the alteration of that mineral since it is not 

 infrequently seen on the walls of hollow casts of aegirite crystals asso- 

 ciated with fluorite and ilmenite. Isolated crystals were also found 

 implanted on feldspar crystals. The crystals of octahedrite are small, 

 of a deep black color and of very brilliant lustre. They show only the 

 forms c(OOl), m(llO), p(lll), k(112), and z(113), the two last the least 

 common. These crystals are marked by two peculiarities ; they are in 

 large part of prismatic habit with the first-order prism dominant, a habit 

 not before described for this mineral and causing the crystals to be at 

 first mistaken for zircon ; and they occur in cruciform twin groups with 

 the form (101) as twin plane. The twins are sometimes complete inter- 

 penetrations of two equal crystals as shown in the figure ; sometimes 

 but one end of each is developed ; again a larger crystal has a much 

 smaller one in twin relation to it. The groups are exquisitely sharp 

 and leave no doubt as to the definiteness of the twinning since the two 

 upper faces of the unit pyramid of each crystal and the two lower, 

 parallel and opposite faces to these reflect the signal simultaneously in 



