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PROCEEDINGS OP THE AMERICAN ACADEMY. 



Figure 13. 



which are nearly parallel to the trace of the second cleavage may run 

 into one another, forming a more or less straight edge in that direction 

 (Plate I. Fig. 37^). The small differences in the shape and orientation 



of the pits on (110) and (ITO) are 

 such as to form another testimony 

 to the fact expressed by Forstner 

 that cossyrite (aenigmatite) closely 

 approximates a monoclinic habit.* 

 (See Figure 13.) 



It is possible that the moot ques- 

 tion as to whether kolbingite is a 

 distinct species, or (after Brogger) 

 an intergrowth of arfvedsonite 

 and aenigmatite, might be settled 

 by etching cleavage pieces of the 

 mineral. 



Zinc-bearing rhodonite (fowlerite, P. 72) is much nearer the mono- 

 clinic pyroxenes, as shown by etch-figures, than is aenigmatite like the 

 monoclinic amphiboles. Fowlerite at 45 seconds' exposure gave tri- 

 angular pits on (110) and (ITO), which are elongated in the sense of 

 the edge 110 : ITO, and are strongly suggestive of the pyroxene figures 

 on cleavage plates. 



Summary of Conclusions. 



Reviewing the ground over which we have come, we may make the 

 following brief resume of results : — 



(1) It seems to be clear that, for the group of the amphiboles, a 

 special method of etching must be adopted, if a comparative study of 

 the etch-figures derived from the different species is to be instituted. A 

 universal solvent must, of course, be used, but its temperature, degree of 

 concentration, and facility of convection at the time of each attack must 

 be attended to if a strict control over the effects of corrosion be possible. 

 It is only by observing this principle, that the measurement of the out- 

 lines of pits and their elements of form will lead to the most valued con- 

 clusions ; and we have seen that measurement, i. e. discussion of the 

 figures by quantitative methods, serves this purpose much more perfectly 

 than does a mere statement of the kind of pit or etch-hill to be seen on 

 any given specimen. It has further been shown, with respect to the pits 



* Zeit. fur Kryst., Bd. V. p. 350. 



