68 Penfield and Ford — Stibiotantalite. 



six times as much as the next largest crystal, which is of the 

 type shown by figure 13. It was found at a considerably later 

 period than the other crystals, and at a different part of the 

 deposit. It was picked up in two pieces, and the fracture sur- 

 faces do not appear fresh, as if recently broken in taking the 

 crystal from the matrix. Originally it must have been quite 

 symmetrical in development and in shape about as shown in 

 figure IT, which, as is also the case with figure 16, is drawn 

 with the a faces to the right and left, instead of front and 

 back, in order to show the form to best advantage. It was 

 attached at the lower left-hand corner to pink tourmaline, bits 

 of which are still adhering to it. The upper and lower edges, 

 and a considerable portion of the lower, right-hand corner 

 have been broken away. The a, a' faces to the right and left 

 are fresh-looking cleavage surfaces ; and whether the pina- 

 coids a and a' were originally present, as shown in figure 17, 

 or whether adjacent g faces came together at the acute edges, 

 cannot be told. 



The habit of this crystal is entirely different from that of the 

 others, and the peculiarities of its surface are also different. 

 The brachydome 8 (043) has been observed only on this crys- 

 tal. The prism g is not striated vertically. The surfaces are 

 all etched, so that only measurements with a contact goni- 

 ometer may be had, but these all agree with those obtained 

 from other crystals. The best-formed etchings on the prism g 

 are rather deep depressions, shaped about as shown in figure 16, 

 though the majority are much more rounded : in places they 

 join one-another, giving rise to furrows running irregularly 

 over the surface. The etchings on the brachydome 8 are long 

 depressions, some of them quite deep, while those on the pyra- 

 mid w are also long, but somewhat comma-shaped, the tails 

 pointing away from the edge w /\ 8. The crystal is light yel- 

 lowish-brown in color, more transparent than any of the others, 

 and has a specific gravity of 6*69. 



The crystal is a polysynthetic twin, though the outward 

 form gives no evidence of it. The two large fragments which 

 make up the specimen do not of themselves exhibit pyroelec- 

 tricity, probably because of numerous cracks running through 

 the material, but a small homogeneous fragment, when tested, 

 exhibited alternating bands of positively and negatively excited 

 material, remarkably uniform and not over 2 mni in width. 



Throughout a portion of its interior the crystal is curiously 

 cavernous, although the exterior is firm and consists of remark- 

 ably pure, transparent material. The cavity looks as though 

 it had been eaten out b} 7 some solvent, and is lined with some 

 secondary material which without endangering the specimen 

 could not be gotten out in sufficient quantity for a satisfactory 



