642 PROCEEDINGS OF THE AMERICAN ACADEMY. 



There is of course more or less gradation between these types. 



Type a. Diaspore of this habit occurs as the filling or inner lining of 

 drusy lenses or veins of corundophyllite in emery. Usually the space 

 is completely filled with bladed diaspore, and when broken open pre- 

 sents an attractive network of long narrow cleavage surfaces of brilliant 

 lustre. Occasionally irregular angular openings are left in which grow 

 the delicate acicular crystals, sometimes quite spanning the cavity, 

 sometimes with one end free and showing terminal planes. They vary 

 in color from amethystine to gray or water-white with brilliant vitreous 

 lustre. Isolated needles were noted with a length of 15 mm. or more, 

 and a diameter of not more than 1 mm., but most of them are shorter 

 and stouter. With them in these cavities are beautiful bipyramidal 

 crystals of pale green amesite, sagenitic rutile, and magnetite crystals, 

 giving a most attractive appearance under a powerful lense. 



The following forms are found on crystals of this habit : 

 b(010), a(100), h (210), m (110), k (230), 1(120), e(011), p(lll), 

 s (212), u (344), x (133), d (455), and g (788). Two of these, d and 

 g, are new forms ; all are discussed below. The prism zone is striated 

 in the direction of its length, as is the zone of pyramids between p and 

 e. Figures 1 and 2 illustrate this habit of crystal. To this type belongs 

 also the crystal described by Dana, 3 on which were the forms b, a, h, 1, 

 e, p, s, u, and v (122). 



Type b. The disc-like diaspores occur in lenticular druses which have- 

 remained partly open, and on the walls of open cracks in emery. The 

 backing of these druses is usually the emery itself with admixed 

 chlorite and without the distinct layer of corundophyllite, as described 

 for the first type. In color the crystals are usually light green, 

 yellowish, or amethystine, and are less brilliant in lustre than those of 

 type a. They are tabular parallel to the brachypinacoid, with maxi- 

 mum dimensions across the plate of 8 to 10 mm. and thickness of 1 to 

 2 mm. ; usually, however, they are much smaller and paper thin. They 

 are ordinarily attached by prism faces to the vein wall and stand out at 

 right angles, exposing both upper and lower terminations ; the disc- 

 like form of the plates is due to the rounded surface, resulting from the 

 oscillation of pyramids and brachydome as shown in Figures 3 and 4. 

 While the crystals are usually implanted separately, they sometimes 

 are in contact to form a drusy surface not unlike that which prehnite 

 ordinarily presents. 



The forms observed on this type are but few: b, h, e, p, and s. 



Crystals of this type were at one time found in considerable abun- 



3 Loc. cit. 



