HOBBS — MINERALOGY OF WISCONSIN. 119 



The different forms show considerable difference in 

 the character of their markings. The planes of R^ are 

 striated parallel to their combination edge with R. Both 

 R3 and R are often marked by Aetzkandle which incline to 

 follow the direction of cleavage. On R the etched figures 

 have the symmetry and approximately the shape of the 

 artificial figures produced by the action of hydrochloric 

 acid. This face is very frequently vicinal, the facets rep- 

 resenting two scalenohedrons of large indices. Between 

 R and R. a small and undeterminable face occurs which is 

 probably the result of corrosion — Prarosionsfluche of Ham- 

 berg. The new form a, — IfR (0.18. 18. 25) may possibly 

 also be of this character. 



The parallel growths of four of the different types of 

 crystals with one another demonstrate the fact that they 

 represent successive separations from the solutions from 

 which they were formed, their order of age being that rep- 

 resented by the numerals assigned to the types, viz. : 1, 2a, 

 3, 4, type 1 being the oldest. The crystals of type 2b 

 were probably formed at the same time as those of type 

 2a, but in a different situation. Crystals of type 1, though 

 common as a core of the later types, are most rarely found 

 alone. The crystals of type 2a were most easily corroded 

 by solutions, for specimens of parallel growths with the 

 two later types are found in which the core of type 2 has 

 been entirely removed, whereas the enclosing zone of type 

 3 has only been rendered porous and that of type 4 has been 

 comparatively little affected. In one instance the cavity 

 left by the removal of type 2 has been filled with sphal- 

 erite, thus forming a mechanical infiltration pseudomorph. 

 Numerous specimens in the University Collection are 

 parallel growths of the four types in isomorphous layers, 

 and when broken across these types are seen to be sharply 

 separated from one another by the differences in color and 

 transparency. A few parallel growths include less than 

 the full series of types, but no one of the types has been 



