240 Fi—ELD CoLuMBIAN MusrtuM—GEOLocy, VOL. I. 
although the lower part of the crystal is quite incomplete, the bound- 
ary planes are evidently those of a twincrystal. 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 ina 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 reéntrant 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 asa 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 Q’, — 20oR, rests upon its evident 
location in the two zones wv’ and em and upon a very accurate 
measurement with the contact goniometer of the angle e: Q’= 
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.Q, —{R, is quite sporadic in its occur- 

cAmer. joursocie ed per., Vol. 41, paol, 
