DALY. — ETCH-FIGURES ON AMPHTBOLES. 401 



doubtless correlated with a special chemical constitution. It is not difficult 

 to believe that the molecular substance of the crystals may be put into a 

 condition of strain by varying rates of expansion and contraction in the 

 ditferent zones due to temperature changes. Again, there is also possible 

 a massive stress set up in the crystal caused by its contortion in the rock 

 from which it was derived. In this case, we are dealing with a druse- 

 mineral ; nevertheless, I have observed that at least two of the individu- 

 als are greatly twisted so that the plane of the cleavage is replaced by a 

 curved surface with from 8° to 10° of curvature. Attempts were made 

 to etch this jjarticular crystal on (110), without marked success, although 

 the mineral was strongly attacked ; yet the few pits actually obtained 

 seemed to be of the distorted types. May this massive distortion not be 

 the result of differential stress in the zones ? * 



The Philipstad (cleavage) sub-type of etch-pit is thus analogous to the 

 other sub-types peculiar to hornblendes, but it is similar to no one of 

 them. It bears the same relation to the second principal kind of pit 

 observable on this mineral ; namely, that on the zone occurring on all the 

 individuals so far examined, just underlying the crystal face (Photographs 

 4 and 5). Most commonly, they are of the shape illustrated in Photo- 

 graph 4 ; that is, while the general habit of the figure is very like that 

 of the Wolfsberg sub-type, tliere is here a more pronounced blunting of 

 the lower end of the pit by an edge nearly as long as the transverse edge 

 of the upjier end. The rarer pits with a sharpened lower end are 

 portrayed in Photograph 5. The photographs show with sufficient 

 clearness the contrast existing between these pits and those character- 

 istic of the inner zones exposed on cleavages. There is never any inter- 

 mixture of the two kinds ; the former is confined to a light colored 

 exterior zone, which appears to be less strongly charged with iron than 

 the inner zones, which are always darker in color. Examination proved 

 that these pits on the outside zone could not be explained as natural 

 etchings ; they are manifestly the result of an interaction between the 

 acid and the mineral, aud probably differing from the figures on the 

 other zones on account of the fact that, while the latter agree in chemical 

 composition fairly closely among themselves, there is a strong chemical 

 peculiarity adhering to the outer zone. That the phenomenon is not 

 confined to the reagent used is evident from the comparative study of 



* Sudden twisting of crystals does not seem to affect the form of the etch- 

 figures. Thus, I etched a cleavage plate of gypsum which had been bent through 

 an angle of at least 15°. The pits produced had perfectly normal characters, as 

 those recently described by Viola. 

 VOL. XXXIV. — 26 



