HOBBS MINERALOGY OF WISCONSIN. 



149 



adjusted on the goniometer. Owing to the minute size of 

 the crystals, the face a is the only one which furnishes even 

 a fair image of the signal. The angle a : m was roughly 

 measured twice, using the front lens of the telescope, and 

 found to be about 39, the usual value given being 37 50'. 



THE DRUSE MINERALS OF THE HAMILTON CEMENT ROCK 

 AT MILWAUKEE. 



Calcite. The Hamilton cement rock which occurs at Mil- 

 waukee carries cavities in which good crystals of calcite 1 

 may be found. A number of specimens have been collected 

 by Professor Edward Kremers, the head of the School of 

 Pharmacy of the University , and generously presented to 

 the University Mineral Collection. I am therefore largely in- 

 debted to him for the material which is here described. The 

 crystals are translucent and either colorless or yellowish. 



FIG. 9. CALCITE FROM MILWAUKEE. 



They vary in length from 8-12 mm , and in diameter from 

 3-5 inm Their habit is short columnar, conditioned by the 

 combination of a steep positive rhombohedron and ^ R 

 (Oll2) (see figure 9). The former is much rounded but can be 

 approximately determined to be 18 R by measurement of 

 the convergence of the long sides of its faces. One crys- 

 tal in the collection was attached by this face and is doubly 



1 The occurrence of calcite, pyrite, and rarely also sphalerite in the'cavities of the cement 

 rock has been mentioned by Chamberlin (Geology of Wisconsin, Vol. II, p. 393, 1878). 



