DALY. 



ETCH-FIGDRES ON AMPHIBOLES. 



417 



from the other, that, od etching (110), (010), or (TOl), one could tell 

 with ease whether he be dealing with an amphibole of the first class, 

 devoid of a sesquioxide, or with one of the second, probably aluminous, 

 but possibly one whose iron represented the total content of sesquioxide. 

 The testimony of the etch-pits, then, would be to establish perfect iso- 

 morphism in each of the two groups, but to refute the idea that there is 

 isomorphism between the two groups themselves. Whether or not this 

 hypothesis be justified by future investigation, the facts of the case seem 

 to have an important bearing on the theory of Retgers. 



Arfvedsonite figures present relationships to the hornblende type, but 

 that species cannot be asserted from them to be in the same isomorphic 

 series with pargasite, for example. 



It was thought to be of interest to test further the etching properties 

 of actinolite and hornblende by using caustic alkalies as the solvent in- 

 stead of hydrofluoric acid. The results were confirmatory of the division 

 just made in the amphiboles. A crystal of Zillerthal actinolite was im- 

 mersed in molten caustic soda for 15 seconds. A large number of sharply 

 outlined pits were produced on both (110) and (010). Plate I. Figs. 32 

 and 33 and Fig. 11, are diagrammatic 

 representations of them oriented on their 

 respective planes. Now a cleavage piece 

 of AVolfsberg basaltic hornblende (V. 54) 

 furnished splendid figures on (110) after 

 35 seconds' exposure in the same reagent, 

 and these were of the varieties shown in 

 Plate I. Figs, 34a and 345, and Photograph 

 16. Another crystal of the Kafveltorp 

 hornblende (V. 42) gave the pits of Plate 

 I. Figs. 35fl!, 35b, 35c, and Photo. 17, on 

 (110) and pits represented by Figure 12 

 on (010). Still a third aluminous horn- 

 blende (V. 33) from Arendal afforded 

 good figures at 30 seconds, this time prac- 

 tically identical with those of V. 42. 

 Without further analyzing these types, it 

 appears to be a legitimate conclusion that 

 the differing habit of the two may be 

 regarded as significant, not accidental, but indicative of a fundamental 

 difference in the two kinds of substance. 



If the tremolite molecule represents one of the fundamental ingredi- 

 VOL. XXXIV. — 27 



Figure 12. 



Figure 11. 



