418 PROCEEDINGS OP THE AMERICAN ACADEMY. 



ents in the actinolitic group, the formula of richterite (after Groth, Dana, 

 and Hintze) would have to be recast to show that the latter species is 

 the result of the mixture of CaMg3(Si03)4 with another metasilicate 

 molecule. It would be highly interesting to determine the figures on 

 grunerite (FeSiOg), cummingtonite [(FeMg)Si03], and dannemorite 

 [(FeMnMg)Si03], for purposes of similar comparison ; many trials with 

 material of the two former from the classic localities failed to produce 

 figures that could be discussed. 



The similarity of the riebeckite pits to those on hornblende may go to 

 show that (if it be true non-aluminous riebeckite that was dealt with 

 during the examination) the strong influence of the alumina molecule 

 may be replaced by that of its common associate, sesquioxide of iron.* 

 In any case, the etch-figures in all thirty of the common and basaltic 

 hornblendes, as well as in the glaucophanes and barkevikite, can hardly be 

 explained except as the effect of an interaction of hydrofluoric acid and a 

 common molecule constituted with reference to one or other or both of 

 the two ses(|uioxides. 



In summary, then, if we accept the law that isomorphic mixtures must 

 have similar etch-figures on corresponding crystallographic planes, we 

 have two divisions among the amphiboles, each of which is isomorphic in 

 itself, but not so related to the other group. If this theory be rejected, 

 we have still the facts remaining of an important difference in the struc- 

 tural plan of each division. 



IIOLOHEDRAL CHARACTER OF THE MONOCLINIC AmPHIBOLES. 



Throughout the whole suite of specimens which I have studied, the 

 evidence is convincing that the family of amphiboles belongs to the holo- 



* Haefcke was so impressed with tlie importance of alumina in the amphibole 

 molecule that lie was led to consider it with the other oxides, to be in combination 

 with orthosilicic acid and thus helps to form salts constitutionally different from 

 the non-aluminous amphiboles, the metasilicates (Inaug. Diss. Gilttingen, Berlin, 

 1890). The masterful influence of alumina is further seen in Wiik's table of ex- 

 tinction angles showing the dependence of the angle of extinction of amphiboles on 

 the percentage of AUOg present. (Zeit. fiir Kryst., 1882-83, Bd. VII. p. 79.) 



That the sum of the sesquioxides as well as the amounts of each should be con- 

 sidered in any comparative study of minerals containing them, is illustrated in an 

 analysis of Doelter's paper on the pyroxenes, in which he traces the influence of 

 alumina and of the sum (Al.^Og -f- Fe.iOg -f FeO) on the optical constant c : c of 

 pyroxene (Neues Jahrb. fiir Min., etc., 1885, Bd. I. p. 43). He finds that FeO alone 

 will not explain the position of c with respect to the vertical axis, nor will AlgOg 

 alone nor Fe.iOs alone ; bat he concluded that both the iron oxides added to the 



