DALY. ETCH-FIGURES ON AMPHIBOLES. 387 



each case, two minutes, yet the actual amount of material carried off in 

 sohuion from the very minute pits of the anthophylHte is extremely 

 small when compared with that removed in the process of excavating 

 the much larger pits on the monoclinic mineral. While working out 

 optimum exposures, I generally had opportunity to observe the incipient 

 stages of attack on cleavage-cracks and of the roughening of tlie whole 

 surface of the crystal. In this way a general impression of the i-elative 

 attackability of these minerals was gradually made upon me. I give 

 the seiies for what it is worth, beginning with the varieties most resistant 

 to hydrofluoric acid, and naming the others in order of less resist- 

 ance: — 1. The orthorhombic araphiboles. 2. Actinolite. 3. Ti-emolite. 

 4. Glaucophane. 5. The light-colored aluminous monoclinics. G. The 

 common green hornblendes. 7. The basaltic hornblendes. 8. The 

 Richterites. 9. Arfvedsonite. 10. Riebeckite (?). 11. Aenigraatite. 

 It will be seen that the resistance to solution decreases with increase in 

 soda and in sesquioxide of iron. 



But not only are the differences of attackability due to differences in 

 chemical composition and crystal system, they are also strongly affected 

 by physical conditions irrespective of species. The physical influences 

 may entirely mask the attackability resulting from the chemical reaction 

 alone. The theories which have been made to explain the irregular dis- 

 tribution of etch-figures on a given plane by corresponding irregularities 

 in the grouping of the chemically active part of the solvent in use, can 

 have no application to many cases that have come under my notice during 

 the course of tlie present research. They are often rather to be explained 

 as dependent on a loosening of the original molecular structure of the 

 mineral by mechanical action without at the same time being accompanied 

 by chemical decomposition. The presence of submicroscopic cracks or 

 planes of parting (a superficial capillary zone) will necessarily give the 

 acid greater surface by which to attack and permit of a readier dislodg- 

 ment of the molecules from the grip of physical cohesion, A good anal- 

 ogy is found in the hardness of certain pseudomorphs ; manganite with a 

 hardness of 6 forms a pseudomorph after polianite (pyrolusite), but has 

 then an apparent hardness of only 3. 



A few typical examples of this differential resistance to solution will 

 suffice for our present purpose. Two intergrown crystals of V. 42 ap- 

 parently of etjual freshness, each bearing the plane (Oil), were simulta- 

 neously immersed for several periods and examined at the end of each 

 interval for the relative progress of attack. The plane (Oil) of one 

 crystal was seen to have been affected decidedly sooner than the same 



