IIOBBS — MINERALOGY OF -WISCONSIN. 



149 



adjusted on the goniometer. Owing to the minute size of 

 the crystals, the face a is the ODly 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.—l^YiQ Hamilton cement rock which occurs at Mil- 

 waukee carries cavities in w^hich good crystals of calcite' 

 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 yellow^ish. 



Fig. 9.— Calcite from Milwaukee. 



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

 3_5mm. Tj^eir habit is short columnar, conditioned by the 

 combination of a steep positive rhombohedron and — i R 

 (0112) (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 



J The occurrence of calcite, pjTite, and rarely also sphalerite in the cavities of the cement 

 rock has been mentioned by Chamberlin (Geology of Wisconsin, Vol. II, p. 395, 1878). 



