DALY. ETCH-FIGURES ON AMPHIBOLES. 413 



furnished arrow-headed etch-hills on (Til). The direction of the arrows 

 was iu the sense of, though not parallel to, the b axis inwards. On 

 (Oil), likewise, the figures of corrosion were etch-hills of tiiangular 

 shape, in the case of both actinolite and hornblende, but again so ill 

 defined that no certain statement of likeness or unlikeness could be made. 

 V. 16 (actinolite), V. 8 (tremolite)^ and V. 20 (richterite), at respective 

 exposures of two minutes, two minutes, and one minute, show strong 

 attack on the terminal planes and the development of numerous bosses 

 on (Oil) ; the same was true of the hornblendes, V. 33 at two minutes, 

 V. 35 at 1 min., V. 36 at 1^ min., and V. 42 at 1 min. Though charac- 

 teristic, they do not lend themselves to analysis, and I have been able to 

 accomplish nothing toward comparative detailed study. 



Better success was had with the orthopiuacoid. Among the aluminous 

 amphiboles, crystals of V. 33, V. 34, V. 35, and V. 42 gave pits at 1:^^, 

 IJ, 1, and 1 minutes respectively. The result was unifoi'm, — a figure 

 of triangular outline, isosceles, the upper apical angle bisected symmet- 

 rically by the plane of symmetry of the crystal. The figure-faces cor- 

 respond to the edges ; two very steep pyramids, a dome at the base and a 

 bottom-face parallel to (100), which diminishes as the pit matures. Occa- 

 sional figures on V. 42 were notable for the replacement of the pyramids 

 by two pairs of positive and negative pyramids (Plate I. Fig. 28). A 

 much commoner variation is the symmetrical curving of the two lateral 

 edges, probably as an effect of secondary solution as the figure grows 

 older. Simultaneously, the figure tends to grow stouter; thus, a youno-, 

 light figure of V. 33 was found to have a ratio of altitude to base of 

 4:1, while on the same face a dark matured figure possessed a ratio 

 of 3 : 1. I'he stoutest figures measured on any hornblende (V. 34), 

 showed a ratio of 3 : 2. 



Actinolite pits on (100) display one noticeable differonce, and only 

 one, from these last figures; they are more slender (Plate I. Fig. 27). 

 The ratio of altitude to base in the pits of V. 17 (at CA 2h min.) 

 changes from a value of 6 : 1 in the initial figure to a minimum of 3 : 1 

 in a matured pit. Whether this distinction applies to all the non-alumi- 

 nous amphiboles or not is a question that needs for its solving more 

 material than I have had the opportunity of studying. 



Finally, we have to note the corrosion pits on the dome (TOl). 



These are, on the whole, rather difficult to obtain on hornblende by 

 reason of the narrow limits between maximum and minimum times of 

 exposure necessary to bring about their maturing properly. Only three 

 of the species examined afforded good figures, and only one gave those 



