DALY. — ETCH-FIGURES ON AMPHIBOLES. 



419 



hedral (Groth's '"prismatic") class of the monoclinic system. I ex- 

 amined practically all varieties with this point in mind : figures produced 

 with hydrofluoric acid, caustic soda, caustic potash, and bicarbonate of 

 soda, and the attackability of the different faces, all told the same story. 

 Besides the diagrams so often referred to, I shall introduce another to 

 represent etch-pits on (100) of a hornblende (V. 42) acted upon by 

 caustic soda. (Plate I. Fig. 29.) 



CoaiPARISON "WITH THE PYROXENES. 



Many years of comparative study, chemical and physical, have evolved 

 a vast body of evidence to restore something of the early belief in the 

 identity of the pyroxene and amphibole substance,* but it is, after all, 



alumina furnished a serial correspondence to the variations in c : c. If, however, 

 the doubtful case of Mte. Rossi be rejected and it be noted that tlie extinction c : c 

 for the Siderao occurrence was read on a section not quite parallel to (010), it will 

 be seen that the sum (AI.2O3 + FcoOs) gives better results than any of the variables 

 considered by Doelter. Witness the accompanying table : — 



* Tschermak would find in the relative size of molecule the probable ground 

 for difference in those striking external characters, crystalline development, and 

 cleavage. He hypothecates a size for the amphibole molecule just double that of 

 the pyroxene molecule. Cf. Lehrbuch der Mineralogie, 1894, p. 460. 



