154 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



test of hardness. Experience seems to show that the aver- 

 age jeweler is as ignorant of the properties of the common 

 gems as is the farmer. Most people can with little trouble 

 have the use of either an emery wheel or a little coarse 

 emery, and since diamond is the only natural mineral which 

 can not be scratched by emery, no one need be without the 

 means of determining at once whether a rough stone is 

 diamond. 



EXPLANATION OF PLATES. 



PLATE 4. 



Crystals of calcite from southern Wisconsin, 



Fig. 1. Crystal of type 1 bounded by R 8 and R and having much 

 rounded angles. Fig. 2a. Crystal of type 2a bounded by R 3 alone. 

 Fig. 2b. Crystal of type 2b bounded by |R and R. Fig. 3. Crystal 

 of the somewhat modified type 3. Excluding the small scalenohedrons 

 which bevel the edge re this crystal exhibits the following forms: 

 r,R; e,-|R; v, R 3 ; ft, ftR; *,-ifR; f,-2R; c, 24R; 5, 10R, k, fR. 

 Fig. 4. Crystal of type 4 which has the same modification as type 3 but 

 has rhombohedral instead of scalenohedral habit. Fig. 5. Twinned 

 crystal from the Linden mine with twinning plane the basal pinacoid. 

 The forms developed are R, R 3 , ^R, and OP. One of the individuals 

 has received a secondary growth of darker and yellow green calcite, 

 which tends to give symmetry of development to the twin. Fig. 6. 

 Crystal of type 6 bounded by the forms 24R, R 3 , $ R, and f R. 



PLATE 5. 



Calcite from southern Wisconsin. 



Fig. 1. Parallel growth of calcites of types 1, 2a, 3, and 4. 

 Fig. 2. Stereographic projection of forms observed on calcites from 

 southern Wisconsin. 



PLATE 6. 

 Galena from southern Wisconsin. 



Fig 1. Large crystal from Yellowstone having an apparently hex- 

 agonal symmetry from development in the direction of a digonal axis. 



