388 PROCEEDINGS OP THE AMERICAN ACADEMY. 



plane of the other. In this case, the ready yielding of the former may 

 have been caused by its being exposed to more active convection currents 

 than the second crystal, due to position in the acid. But this explanation 

 cannot apply to V. 46, where the optimum exposures for cleavage flakes 

 from different ciystals, though from the same hand specimen, varied from 

 two and one half to five minutes, and the pits were of nearly equal size 

 (area and depth) on all the pieces. V. 34 afforded some light on the 

 question in the behavior of two terminated crystals, the (Oil) of the one, 

 less lustrous than the same plane of the other, was the more rapidly at- 

 tacked. The suggestion that the phenomenon is a result of alteration 

 can hardly be avoided. That the alteration may be an almost, if not 

 quite, exclusively physical one and not associated with a serious change 

 in the original chemical molecule of the hornblende, seems clear from 

 the facts observed in an experiment on V. 31. The hand specimen is a 

 " Krystallstock " composed of well defined individuals, tipped with asbes- 

 tus and occasionally showing patches of an asbestiform substance on the 

 sides of the crystals. At two minutes' exposure, one of these gave sharp 

 but relatively few figures on (110) near the point of attachment at the 

 end of the crystal. The other three fourths of the surface of the prism 

 was characterised by the appearance of numerous cleavage cracks which 

 gradually increased in number in the direction of the free (in the druse 

 unattached) end, and there was a simultaneous increase in the number of 

 pits. The development of the latter was independent of the asbestiform 

 patches, and, from the uniformity of color of the general surface, I have 

 concluded that there has not been chemical decomposition sufficient to 

 explain the differing rates of etching. It may be noted that the much 

 attacked end was directed upward in the acid, and any heating by direct 

 contact with the platinum at the bottom of the crucible would tend to 

 dissolve the unaltered extremity of the crystal the more rapidly. 



As a general rule, the crystal-face (110) was observed to be less resist- 

 ant to solution than the parallel cleavage plates of the same individual 

 crystal, but the converse was often true.* Thus at one and two thirds 

 minutes, the cleavage showed much stronger attack than the correspond- 

 ing crystal surface (HO) of a specimen of V. 32, although here again 

 the original face was facing downward, the cleavage plane upward, in the 

 acid solution. The mineral was perfectly fresh in appearance, and be- 

 trayed no alteration to the eye such as seemed best to explain the same 

 relations characterizing other examples: e.g. V.8,V. 14, V. 17,V. 26,V. 42. 



* Cf. Becke's experience with zinc blende, Min. und Petrog. Mitth., 1883, Bd. V. 

 p. 485. 



