466 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 52' 



marcasite in a most interesting manner. This forms a sandpaper- 

 like surface, which extends to within 4 mm. of the edges v a, where 

 an even Hne of demarkation separates the stippled part from the re- 

 maining bright portion of the faces. This line runs parallel to v a- 

 to within a few millimeters of the sharp edges, v v' and v^^v^ , and 

 then bends down in a direction roughly parallel to the cleavage, inter- 

 secting the edges at a sharp angle. Also, from the same sharp edges 

 occasional narrow bands, lacking the stippling, extend toward the- 

 blunt edges in a direction parallel to the cleavage. The three back 

 faces, z/", z'"', and z/'^, are entirely wanting in marcasite. This- 

 mineral is confined to the surface of the crystal and must have been- 

 deposited after the growth of the calcite was completed or nearly 

 completed ; yet it is entirely controlled in its distribution by the 

 crystallographic relations of the host crystal. 



The second crystal described bears the Museum number 84435, ^"^^ 

 is shown in its natural development in plate Liii, figure 2. This is 

 made up of the scalenohedron v (2131) and negative rhombohedron 

 e (0112), modified by the rhombohedron / (0445) and the scaleno- 

 hedron t (2134), and is a combination of Farrington's^ type i, 

 composed of v and t, and type 2, composed of v and e. By a parallel 

 shifting of vv^ and corresponding edges, the alternate t faces are 

 distorted into long, narrow planes, which, on account of their small 

 inclination to e and the striations of the latter, are not prominent. 

 The crystal is of a honey-yellow color and in numerous positions is 

 brilliantly illuminated from within by light reflected from a network 

 of cleavage cracks. The v faces are all peculiarly marked, as shown 

 in the drawing. 



The measurements, made by contact, are as follows: 



^Measured (contact). Theoretical. 



v:l= 2131 : 0445 = 44° 44° 6^ 



e : I =0112:0445= 12° 12° 2' 



^: -c' =0112 : 2131 = 45°-52° 5^-° 36' 



z;:/' = 2131 : 3724= 47° 45° 31' 



(2) Calcite with Moving Bubble, from Guanajuato. Mexico 



A calcite twin from Guanajuato, Mexico, bearing the National 

 Museum number 75672, is shown in its true size and development in 

 plate Liv, figure i. The form shows the scalenohedron v (2131), 

 terminated above by the negative rhombohedron e (0112), which, 



* Publ. Field Columb. AIus., Geol. Ser., vol. i (1900), pp. 233-34. 



