122 BULLETIN OP THE UNIVERSITY OF WISCONSIN. 



generally the case, took place along the cleavage cracks 

 parallel to the rhombohedron, thus separating the crystal 

 into partially detached rhombic blocks. Along the chan- 

 nels thus opened the solutions which effected the change 

 found their way and rejjlaced molecule by molecule the 

 residual blocks, the channels remaining unfilled in the 

 completed pseudomorph. 



Numerous specimens in the University Collection indi- 

 cate that the process above described was in some cases 

 varied by the complete solution of the calcite and the fail- 

 ure to refill the cavity. Very beautiful incrustation pseu- 

 domorphs have resulted in this way. The majority of the 

 specimens, however, contain residual portions of unaltered 

 calcite, and clearlj'- indicate that in most cases all stages of 

 the process went on simultaneously in different parts of 

 the crystal, the action beginning at the surface immedi- 

 ately under the incrustation of the "calamine" variety. 



Fig. 4— Smithsonite pseudomorph after calcite, from Mineral Point. 



Such specimens have immediately beneath the incrustation, 

 a layer of the porous "dry bone" retaining the cleavage 

 channels of the calcite. This layer is sharply delimited from 

 the incrustation, but passes insensibly into the layer of 

 porous corroded calcite immediately beneath, which like- 

 wise passes by all intermediate phases into the unaffected 

 calcite nearer the center of the crystal. (See figure 4). 

 The dry bone material of these ^Dseudomorphs is invari- 



