PALACHE AND WOOD. — CRYSTALLOGRAPHIC NOTES. 643 



dance in the mine and were much prized by collectors, the broad 

 surfaces, covered with richly colored amethystine crystals, making 

 showy specimens. Such a specimen now in the Harvard Mineral 

 Cabinet, presented by the Ashland Emery and Corundum Company, 

 measures about 20 cm. square and is covered over most of its area 

 with platy crystals, backed by pale green chloritic emery. 



Type c. Crystals of stout prismatic habit characterize the most 

 recent discoveries of diaspore at Chester. The combinations are simple, 

 generally showing only b, h, e, and s, with 1, x, and v less frequently 

 developed. The crystals are always implanted upon a prism plane, and 

 the two developed faces of the prism h are prone to show deep vertical 

 striations without, however, losing their brilliant lustre. Occasionally 

 the brachypinacoid is reduced in size or lacking, and the faces of e 

 more or less curved, giving the crystal a curious lense-shaped form. 

 The crystals are glassy and transparent, with rich colors, ranging from 

 rich brown through wine yellow and green to pure amethystine, often 

 mingled in the same crystal. 



The largest crystal seen was a square prism 1 cm. long with diameter 

 of 5 mm. ; smaller crystals are, however, the rule. They are implanted, 

 singly or in small groups, in cavities in well-crystallized corundophyllite ; 

 a second generation of microscopic crystals of the same type is often 

 present in the cavities, dusting the larger diaspores and chlorite crys- 

 tals with sparkling gem-like points of light. The habit was also found 

 on very brilliant crystals of about 1 mm. size coating cracks of but a 

 few mm. width in solid emery. All veins containing diaspore of this 

 habit seem to have had a final filling of all open spaces by dolomitic cal- 

 cite, the removal of which with acid revealed these very beautiful and 

 unusual crystallizations of diaspore. The habit is illustrated by Figures 

 5, 6, and 7. 



Crystallography : Fifteen crystals were measured, the results of 

 the observations being presented in the annexed table. Besides the 

 two new forms there given a number of measurements were obtained 

 from pyramid forms which, either because of poor quality of the faces 

 or complex indices indicated, did not seem established with certainty. 

 These are recorded at the end of the table. It is to be noted that in 

 all forms the agreement between calculated values and mean observed 

 angles is less close than could be desired, or, from the appearance of 

 the measured crystals, expected. The variation is, however, quite 

 irregular, and because of this no attempt was made to calculate a new 

 axial ratio for diaspore from the measurements. 



Observations on the forms : 



b (010). Natural faces of this form occurred on all but two of the 



