K S. Dana — New twins of Staurolite and Pyrrhoiite. 385 



and coarse. They occur in a raetamorphic rock which Prof. 

 Bradley states belongs to the Cincimiati group. 



The second locality is in Fannin Co., Georgia, 12 miles 

 southeast of Ducktown, Tenn. The crystals obtained here are 

 of quite uniform size, averaging an inch in length, 

 very perfectly developed, the faces being smo( "" ' 

 ally 



perfectly developed, the faces being smooth, though i 

 ithout polish. They < 



I soft r 



I schist, bek 



ing geologically, according to the same authority, at the base 

 of the Quebec Group. The rock is extensively decomposed 

 so that the crystals are found abundantly loose on the surface 

 of the ground. Large quantities of them may thus be picked 

 up,'of which perhaps one-tenth are perfect crystals. Tney are 

 in general nearly free from the gangue. 



The crystals are with rare exceptions twins. The most 

 common twins are those well known in this species ; that is, 

 (a), those having the composition face |-i, with the vertical axes 

 nearly at right angles to each other ; and {h\ those having 

 the composition-face f-|, with the vertical axes crossing at a 

 angle of about 60°. Under these two types there is a very 

 wide diversity of form arising from the extension or partial 

 suppression of various of the occurring planes. A very com- 

 mon feature in the twins of the first kind mentioned is the 

 almost entire suppression of one pair of the macrodome planes 

 l-t, opposite each other, with the corresponding extension of the 

 other pair ; this same kind of hemihedral development is also 

 extended to the prismatic planes giving rise to forms of \erj 

 oblique appearance. 



Three new forms are shown in figures 1, 2 and 3. Figure 1 

 exhibits a new method of twinning not before observed in this 



species, and making the third type of penetration twins pecu- 

 liar to staurolite. The measured angle of the two brachypina- 

 coids {i-%) for the crystals in their twinning position is 70" 30'. 

 This gives for the composition-face 2-3', which requires for the 

 above angle 70° 18'. In this case the axis of revolution very 

 nearly coincides with the plane i-% and the r-l, of the second 

 individual is very nearly parallel to the prism I of the first. 

 Attention has been called, by Prof Dana, to the fact that if the 



