120 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



observed out of the order stated. The character of one of 

 these interesting parallel growths is represented in plate 

 5, fig. 1. 



The only rejiresentative specimens of type 5 are two 

 large twinned crystals from the Linden Mine. They are 

 interesting both because they are the only specimens in the 

 collection on which I have observed the basal pinacoid, and 

 because they furnish the only observed illustration of twin- 

 ning in the calcites of the region. The specimens are very 

 large (the largest about 7x6x5 inches), and have a gray 

 color and a much corroded surface. The individuals of 

 both specimens exhibit the combination of the fundamental 

 rhombohedron, R (1011), with the basal pinacoid, OP (0001) 

 and the common scalenohedron, R3 (2131). On one of the 

 individuals of the larger specimen the basal plane has a 

 very large development influencing the habit of the crystal, 

 while on the other individual it is quite small. The two 

 individuals have parallel axes, and the twinning plane may 

 be considered either a face of the fundamental prism, ooP 

 (lOlO), or the basal pinacoid, OP (0001). The large triangu- 

 lar area of the basal pinacoid on one individual is divided 

 very accurately into two smaller and perfectly equilateral 

 triangles by the projection above it of the rhombohedral 

 faces of the other individual. On the largest of these tri- 

 angular areas has formed a deep green yellow calcite en- 

 largement. This addition of calcite material has extended 

 the rhombohedral planes which outline two sides of the 

 triangle, and on the third side opposite the rhombohedral 

 face of the other individual it has developed a new rhom- 

 bohedral face, so that the apparent lack of symmetry be- 

 tween the two individuals caused by the greater develop 

 ment of <■ on one of them, is in part removed. The other 

 triangular section of the pinacoid has received no addition 

 of calcite material. To one side of the specimen are at- 

 tached small crystals of barite which will be described be- 

 low. 



