of the Trimetric System. 



53 



axes, angles (measured over the extremity of the other axis) 

 nearly alike, this other axis is the true vertical, and the vertical 

 prism is near 90° in angle. 



Polymignite is near Wohlerite in its crystallization. With 

 the fundamental form adopted, the known octahedron is 22 

 (2P2), and the occurring prisms are IS = 109° 46', 21 = 70° 50', 

 4a = 39° 9'. 



Polycrase affords angles in three directions near 90°, which- 

 ever position be taken. In the figure annexed, 

 the position and lettering correspond to the 

 dimensions given in the table. Should we 

 change it, and make the brachydiagonal the 

 vertical axis, then : 



1 = 93° 52', U = 91° 29', IS = 95° 2'. 



And, again, if we make the macrodiagonal 

 the vertical axis : 



/= 91° 29', 11 = 84° 58', It = 86° 8'. 



The symbols of these planes in these three positions (which 

 we may call A, B, C,) are as follows : 



A (figure) 1 13 21 il iS ii 



B 1 3 ^ : 2 (base) Si il 



C 1 13 hi il \% O(base) 



The form is near to Bournonite. It is also related dis- 

 tantly to the Columbite species, the prominent difference being, 

 jive degrees in the angle of the vertical prism. 



The species of Table II. fall into four sections, depending'on 

 the angles of the unit domes. 



MARCH, 1854. 5 



Ann. Lyc. Nat. Hist. Vol. VI. 



