332 



University of California. 



[Vol. 2. 



Figure 4 shows the apparent positions, as viewed under the micro- 

 scope, of the figures on the four faces of a crystal, the greater of 

 the prismatic angles lying between A:B and C:D. It is to be 

 noted that the most acute angle of the figure points either up or 

 down according to whether the face is to the front or back (normal 

 ■orientation) of the crystal. In this respect the figures agree with 

 Daly's statement that the "claw" of the figure points towards the 

 positive hemipyramids. Daly deduces the latter statement from 

 an analogy between the etch-figures of actinolite and those of 

 other varieties of hornblendes of known orientation, and Wright* 

 deduces the same relation from a study of a hornblende from 

 Vesuvius. Accepting the above as the correct correlation, the true 

 orientation of the four planes would be as follows: A = (iio), 

 B = (uo). C = (iio), D=(iio). It is to be noted, however, that 

 these results do not represent the actual position of the figures on 

 the crystal, since in my own figures, and presumably in those of 

 Daly and Wright, no account is taken of the fact that right and left 

 are interchanged. 



Clicmical Properties. — The mineral is scarcely affected by strong 

 hydrochloric acid and was only partially decomposed by heating 

 in a sealed tube with boiling sulphuric acid. Analyses of two 

 carefully-selected and inclusion-free samples are" given in Table I. 

 Sample No. 1 was obtained from the boulder that furnished the 

 material used in the determination of the optical properties; No. 2, 

 from a boulder collected at locality B. The two analyses show in 

 their general features no marked variations, the differences being 

 such as might easily arise from the substitution of isomorphous 

 bases. Compared with typical actinolite from Zillerthal (see 

 analysis No. 3) and with other analyses of glassy actinolite (see 

 analyses 4, 5, and 6) there is still a general agreement, the note- 

 worthy differences being the lower percentage of magnesia, and the 

 presence of considerable amounts of alumina, of the alkalies, and of 

 combined water. The low percentage of magnesia is largely com- 

 pensated for by the higher content of ferrous oxide and lime. The 

 alumina appears to be higher than in any of the analyses of this 



-Tschermak's Min. u. Pet. Mitth. Band 19, S. 317. 



