240 Field Columbian Museum — Geology, Vol. i. 



although the lower part of the crystal is quite incomplete, the bound- 

 ary planes are evidently those of a twin crystal. On the reverse side, 

 however, the development is that of a simple crystal. 



The completely formed twins resemble in many respects those 

 from Guanajuato, described by Pirsson.* The twinning plane is the 

 same, the lengthening out in one direction is similar, though not to 

 the extent of the Guanajuato crystals, and the proportional develop- 

 ment of the faces is analogous. The Joplin twins, however, differ 

 notably from those of Guanajuato in their development at the apex of 

 the salient angle. In the Guanajuato twins the scalenohedral planes of 

 the two individuals of the twin are prolonged at the apex of the sali- 

 ent angle until they meet in a point. In the Joplin twins, however, 

 the scalenohedral planes of each individual have only a normal devel- 

 opment, and there is no prolongation to an apex. Some crystals 

 occur in which one individual has grown by the other at the salient 

 angle in the direction of the vertical axis of the individual, this being 

 but another illustration of the curious tendency which has been pre- 

 viously noted. There is, however, no prolongation at the end of the 

 twinning plane. The Guanajuato twins are usually attached at the 

 salient angle, while the reverse is true of the Joplin twins. The 

 attachment of the Joplin twins by the reentrant angle often takes 

 place at the end of a projecting process of calcite, usually more or 

 less bounded by crystal planes, and seeming to serve as a sort of stem 

 of crystal growth. 



As regards the size of the twin crystals, it may be said that the 

 longest noted measures 3 inches (.75 cm.) from end to end of the 

 vertical axes. The lengthening out of the same crystal makes it 

 about 1 inch (.25 cm.) long in the same direction. The combination 

 of forms which make up these crystals has already been indicated. 

 The determination of the new form fl' , - 20R, rests upon its evident 

 location in the two zones v v' and em and upon a very accurate 

 measurement with the contact goniometer of the angle e : 12' = 

 0221:0.20.20.1 = 24°. The calculated value is 23° 59'. The form 

 usually has rounded planes. The determination of the other forms 

 likewise rests upon measurements made with the contact goniome- 

 ter, but these correspond so closely with calculated values, and the 

 analogy of the occurrence of the forms is so similar to that of the 

 other Joplin calcites that there can be little doubt of their being cor- 

 rectly identified. The form/2, -£R, is quite sporadic in its occur- 



"Amer. Jour. Sci. 3d Ser., Vol. 41, p. 61. 



