468 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. $2 



American calcite. r is dull and G: is fairly brilliant, though its 

 signal is not well defined. The measurements upon which the iden- 

 tification is based are as follows: 



Measured. Theoretical. 



G: : G 

 G:: G 



r : G 



' = (7295) : (7925) = 78° 16^ 78° 3' 



'' = (7295) : (9275) = 21° o' 20° 44^ 



= (io7i) : (7255) = 17° 8' 17° 36' 



Type 2, an average crystal of which, enlarged, is shown in ortho- 

 graphic projection in plate Liv, figure 3, and in clinographic projec- 

 tion in figure 4, is the common type. It occurs very symmetrically 

 developed and is composed of the scalenohedron y (3251) and the 

 negative rhombohedron e (0112), modified by the positive rhombo- 

 hedrons r (ion) and k (5052), and the rare scalenohedron G: 

 (7295). e is deeply striated parallel to rr' ; r, k, and G: are dull, 

 and y only slightly lustrous. The crystals were measured by the 

 reflection goniometer, but as direct reflections could not be obtained, 

 the measurements were made by bringing the faces into parallel 

 alignment with the vertical cross-hair. This method gave readings 

 only slightly more accurate than those obtained by contact on larger 

 crystals, but such were sufficient to identify the forms. The identifi- 

 cation of G: was strengthened by its more accurate determination on 

 type I. The faces of the scalenohedron 3; have a tendency toward 

 rounding, so that the edges between the upper and lower faces are not 

 always well developed ; hence the crystals have a barrel-shaped ap- 

 pearance. 



The specimens described have been placed in the National Museum 

 collections under the number 86574. 



EXPLANATION OF PLATES 



Plate LIH, Fig. i. — Joplin calcite, showing peculiar stippling of marca?ite. 

 Natural size. Nat. Mus. No. 84435. 

 Fig. 2. — Joplin calcite, combination of v, c, and /. Natiwal size. 

 Nat. Mus. No. 84435. 



Plate LIV, Fig. i — Calcite twin, Guanajuato, Mexico. Path of moving bubble 

 outlined by dots. Natural size. Nat. Mus. No. 75672. 



Fig. 2. — Crystal of type i, Virgilina calcite. Enlarged. Nat. Mus. 

 No. 86574. 



Fig. 3. — Crystal of type 2, Virgilina calcite. Orthographic pro- 

 jection. Enlarged. Nat. Mus. No. 86574. 



Fig. 4. — Crystal of type 2, Virgilina calcite. Clinographic pro- 

 jection. Enlarged. Nat. Mus. No. 86574. 



