EXPLANATION OF THE PLATE. 



All the figures given in this plate are copied from drawings in Prof. Beck's Min- 

 eralogy of New York. These drawings were made from crystals in the possession 

 of Prof. Emmons, and the crystals are now on exhibition in the Museum. 



The crystallographic nomenclature has been changed to the corresponding sym- 

 bols of Dana's "System of Mineralogy." 



"With the exception of fig. 1, each crystal is selected to illustrate one point only, 

 and the crystal is drawn simply according to its primary form. For the rare planes 

 so common to all of these crystals the reader is referred to the illustration in Dana's 

 " System of Mineralogy." 



Fig. I. Rhombohedron, with its edges replaced by scalenohedral planes. 



Fig. II. Two crystals twinned parallel to i i ; one crystal much larger and more 

 perfectly developed than the other. 

 . Fig. III. Two rhombohedrons twinned parallel to i-i. 



Fig. IV. Rhombohedra in which the O planes are developed. This is frequently 

 carried to such an extent that the crystal is reduced to a thin plate on which the 

 rhombohedral planes are observed as mere bevelments. 



Fig. V. Two crystals twinned parallel to O. Crystals sometimes much distorted 

 through unequal development. 



Fig. VI. Distorted crystal of fig. V. The crystal is probably hemimorphic. 



Fig. VII. Scalenohedral planes developed at the expense of the rhombohedral 

 planes. The reverse of fig. I. 



Figs. VIII and IX. Distorted and twinned crystals. These forms are common 

 though much more complex. 



